Faunal and algal crusts on exposed to moderately wave-exposed circalittoral rock

14-11-2002
Researched byDr Harvey Tyler-Walters Refereed byThis information is not refereed.
EUNIS CodeA4.214 EUNIS NameFaunal and algal crusts on exposed to moderately wave-exposed circalittoral rock

Summary

UK and Ireland classification

EUNIS 2008A4.214Faunal and algal crusts on exposed to moderately wave-exposed circalittoral rock
EUNIS 2006A4.214Faunal and algal crusts on exposed to moderately wave-exposed circalittoral rock
JNCC 2004CR.MCR.EcCr.FaAlCrFaunal and algal crusts on exposed to moderately wave-exposed circalittoral rock
1997 BiotopeCR.MCR.GzFa.FaAlCFaunal and algal crusts, Echinus esculentus, sparse Alcyonium digitatum and grazing-tolerant fauna on moderately exposed circalittoral rock

Description

Moderately exposed circalittoral rock in slight tides with a rather barren appearance (reminiscent of a brittlestar bed after the brittlestars have moved elsewhere - brittlestars Ophiothrix fragilis recorded in moderate abundance). Can be sand/sediment scoured or grazed. Usually small Alcyonium digitatum, some Abietinaria abietina and sparse Nemertesia spp. present. Also Urticina felina, often associated with patches of muddy shell gravel and sand, or on North Sea coasts Urticina eques. Most of rock surface with coralline or non-coralline red algal crusts as well as patches of bryozoan crusts such as Parasmittina trispinosa. Echinus esculentus common in some areas and Pomatoceros triqueter found throughout, especially on vertical faces. The richer examples of this biotope also have Caryophyllia smithii, Antedon bifida, delicate hydroids, ascidians such as Ascidia mentula and holothurians such as Aslia lefevrei and Pawsonia saxicola, which may appear seasonally, in more cryptic habitats. Regional variants occur - e.g. with Thuiaria thuja and Bolocera tuediae on North Sea coasts. Under-boulders and crevices often have Pawsonia saxicola, Galathea spp., encrusting sponges, terebellids, Pododesmus patelliformis and Munida rugosa. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

Recorded from Shetland, Orkney, south east Scotland and north east England, Youghal Bay, Ireland and the west coast of Scotland.

Depth range

-

Additional information

None entered

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Habitat review

Ecology

Ecological and functional relationships

This biotope is dominated by species able to tolerate intense sea-urchin grazing, such as red coralline encrusting algae or rapid growing species which colonize space and grow quickly before they are removed by grazers or predators. The fauna is relatively sparse in comparison to other faunal turf communities (see CR.Bug and MCR.Flu), but the epifauna is more developed on vertical surfaces, under overhangs or boulders, in crevices inaccessible to grazing sea urchins and in temporary escapes or predation refuges. Most of the epifauna are sessile, permanently fixed, suspension feeding invertebrates that are, therefore, dependant on water flow to provide: an adequate supply of food and nutrients; gaseous exchange; remove metabolic waste products; prevent accumulation of sediment, and disperse gametes or larvae. Little is known of ecological relationships in circalittoral faunal turf habitats (Hartnoll, 1998) and the following has been inferred from studies of other epifaunal communities (Sebens, 1985; 1986).
  • Large brown laminarians may be found on the tops of bedrock ridges in the photic zone, giving way to foliose and filamentous red and brown algae (e.g. the red algae Delesseria sanguinea, Cryptopleua ramosa, Lomentaria spp. and Plocamium cartilagineum, and the brown algae Dictyota dichotoma). But large foliose algae are relatively uncommon within the biotope. The dominant macroalgae are grazing tolerant encrusting corallines (e.g. Lithothamnion spp. and Phymatolithon spp.) or non-coralline encrusting algae, which may cover large areas of the rock surface giving it a pink appearance (Sebens, 1985; Hartnoll, 1998; JNCC, 1999).
  • Active suspension feeders on bacteria, phytoplankton and organic particulates and detritus include sponges (e.g. Pachymastia johnstonia, and Halichondria panicea), the soft coral Alcyonium digitatum, encrusting bryozoans (e.g. Parasmittia trispinosa, Bicellaria ciliata and Crisia eburnea), occasional erect bryozoans (e.g. Bugula species and Flustra foliacea), barnacles (e.g. Balanus crenatus), porcelain crabs (e.g. Pisidia longicornis), and sea squirts (e.g. Ascidia spp., Ascidiella spp., Clavelina lepadiformis, Ciona intestinalis, and Botrylloides leachi). However, the water currents they generate are probably localized so that they are still dependant on water flow to supply adequate food.
  • Passive suspension feeders on organic particulates, plankton and other small animals include, hydroids (e.g. Abietinaria abietina, Halecium halecium, Kirchenpaueria pinnata and Nemertesia antennina), soft corals (e.g. Alcyonium digitatum), feather stars (e.g. Antedon bifida) and brittlestars (e.g. Ophiothrix fragilis).
  • Larger prey are taken by passive carnivores such as anemones and cup corals (e.g. Caryophyllia smithii and Urticina felina) (Hartnoll, 1998).
  • Sea urchins (e.g. Echinus esculentus) are generalist grazers, removing young algae, barnacles, ascidians, hydroids and bryozoans and potentially all epifauna, leaving only encrusting corallines and bedrock. Sea urchins were shown to have an important structuring effect on algal and epifaunal communities and succession (Jones & Kain, 1967; Sebens, 1985; 1986; Hartnoll, 1998) and are no doubt important in this biotope (see seasonal/temporal change below).
  • Other grazers include top shells (e.g. Gibbula cineraria), small crustaceans (e.g. amphipods) and Calliostoma zizyphinum, which grazes hydroids, while chitons (e.g. Tonicella marmorea) and the tortoise-shell limpet Tectura testudinalis graze encrusting coralline algae.
  • Specialist predators of hydroids and bryozoans include the nudibranchs (e.g. Doto spp., and Onchidoris spp.) and pycnogonids, (e.g. Nymphon brevirostre), while the nudibranch Tritonia hombergi preys on Alcyonium digitatum, and some polychaetes also take hydroids.
  • Starfish (e.g. Asterias rubens and Crossaster papposus, Solaster endeca), crabs and lobster are generalist predators feeding on most epifauna, including ascidians and sea urchins
  • Scavengers include polychaetes, small crustaceans such as amphipods, starfish, brittlestars, and decapods such as hermit crabs (e.g. Pagurus bernhardus) and crabs (e.g. Cancer pagurus and Necora puber).
  • Mobile fish predators include gobies (e.g. Pomatoschistus spp.), wrasse (e.g. Ctenolabrus rupestris and Labrus bergylta) and butterfish Pholis gunnellus feeding mainly on small crustaceans, while species such as flounder (Platichthys flesus) are generalists feeding on ascidians, bryozoans, polychaetes and crustaceans (Sebens, 1985; Hartnoll, 1998)
Competition
Intra and interspecific competition occurs for food and space. Sebens (1985, 1986) demonstrated a successional hierarchy, in which larger, massive, thick growing species (e.g. large anemones, soft corals and colonial ascidians) grew over low lying, or encrusting growth forms such as halichondrine sponges, bryozoans, hydroids and encrusting corallines. In reduced or absent grazing, the epifauna of vertical rock walls became dominated by large massive species, depending on the degree of predation, especially by sea urchins. Sebens (1986) noted that Lithothamnion out-competed Phymatolithon for space, often overgrowing the thinner Phymatolithon. But the thicker, raised and often knobbly Lithothamnion suffered more from sea urchin grazing, so that a competitive equilibrium existed between the two encrusting coralline species in heavily grazed communities (Sebens, 1986).

Seasonal and longer term change

Seasonal change
Many of the species within the community demonstrate seasonal changes in abundance and reproduction. Many species of algae show marked seasonal variation in growth with a peak in summer (see Hiscock, 1986c), although in this biotope additional growth merely provides more food for sea urchins. In temperate waters, most bryozoan species tend to grow rapidly in spring and reproduce maximally in late summer, depending on temperature, day length and the availability of phytoplankton (Ryland, 1970). Several species of bryozoans and hydroids demonstrate seasonal cycles of growth in spring/summer and regression (die back) in late autumn/winter, over wintering as dormant stages or juvenile stages (see Ryland, 1976; Gili & Hughes, 1995; Hayward & Ryland, 1998). For example, the fronds of Bugula species are ephemeral, surviving about 3-4 months but producing two frond generations in summer before dying back in winter, although, the holdfasts are probably perennial (Eggleston, 1972a; Dyrynda & Ryland, 1982). Similarly, Bicellaria ciliata produces 2 generation per year, larvae from the second generation producing small over wintering colonies (Eggleston, 1972a; Hayward & Ryland, 1998). The uprights of hydroid Nemertesia antennina die back after 4-5 months and exhibit three generations per year (spring, summer and winter) (see reviews; Hughes, 1977; Hayward & Ryland, 1998; Hartnoll, 1998). Many of the bryozoans and hydroid species are opportunists (e.g. Bugula flabellata) adapted to rapid growth and reproduction (r-selected), taking advantage of the spring/summer phytoplankton bloom and more favourable (less stormy) conditions (Dyrynda & Ryland, 1982; Gili & Hughes, 1995). Some species such as the ascidians Ciona intestinalis and Clavelina lepadiformis are effectively annual (Hartnoll, 1998). Therefore, the biotope is likely to demonstrate seasonal changes in the abundance or cover of the dominant bryozoans and hydroids. Winter spawning species such as Alcyonium digitatum may take advantage of the available space for colonization. Seasonal storms probably affect the community removing the uprights of hydroids, erect bryozoans, some ascidians, starfish, sea urchins and mobile epifaunal species, especially where the biotope occurs on boulders or stones that may be mobilized by extreme water movement.

Succession
Sebens (1985, 1986) described successional community states in the epifauna of vertical rock walls. Clear space was initially colonized by encrusting corallines, rapidly followed by bryozoans, hydroids, amphipods and tube worm mats, halichondrine sponges, small ascidians (e.g. Dendrodoa carnea and Molgula manhattensis), becoming dominated by the ascidian Aplidium spp., or Metridium senile or Alcyonium digitatum. Sea urchins (e.g. Echinus esculentus in Britain and Ireland) most likely have a significant effect on community structure and succession and their grazing trails can often be seen through bryozoan turfs, leaving bare rock or encrusting corallines behind (Keith Hiscock pers comm.). Sebens (1985, 1986) noted that high levels of sea urchin predation resulted in removal of the majority of the epifauna, leaving encrusting coralline dominated rock. Reduced predation allowed the dominant epifaunal communities to develop, although periodic mortality (through predation or disease) of the dominant species resulted in mixed assemblages or a transition to another assemblage (Sebens, 1985, 1986). Similarly, removal of sea urchins from a 10m wide strip of the Port Erin breakwater allowed macroalgae sporelings, including the kelp Laminaria hyperborea, to colonize the experimental area within a year and only survived within the experimental area (Jones & Kain, 1967). This biotope probably represents an early successional community dominated by encrusting red algae and rapidly colonizing hydroids and tubeworms due to intense grazing pressure. Sebens (1985) noted abrupt changes in the invertebrate communities between horizontal rock faces (as dominate in this biotope) and vertical surfaces and overhangs, which sea urchins find more difficult to traverse. Vertical surfaces and overhangs, and under boulders, exhibited a more developed epifaunal community (Sebens, 1985).

Community stability
Long term studies of fixed quadrats in epifaunal communities demonstrated that while seasonal and annual changes occurred, subtidal faunal turf communities were relatively stable, becoming more stable with increasing depth and substratum stability (i.e. bedrock and large boulders rather than small rocks) (Osman, 1977; Lundälv, 1985; Hartnoll, 1998). Many of the faunal turf species are long-lived, e.g. 6 -12 years in Flustra foliacea, 5-8 years in Ascidia mentula, over 20 years in Alcyonium digitatum, 8-16 years in Echinus esculentus and probably many hydroids (Stebbing, 1971a; Gili & Hughes, 1995; Hartnoll, 1998). However, Bugula dominated communities recorded from the west Anglesey in 1996 were reported to be 'silted and ragged' in the same season the following year, suggesting some inter-annual variation may occur (Brazier et al., 1999). Lundälv (1985) reported long term stability in presence but short term variation in population density of Ascidia mentula, Ciona intestinalis, Boltenia echinata and Protanthea simplex on rocky sublittoral communities over a 12 year period in the Skagerrak. It was suggested that variations in population density were due to physical disturbance of the communities by storms or grazing by sea urchins, variation decreasing with depth. Sebens (1996) also demonstrated that while epifaunal communities were dominated by the same set of species over a period of years the relative frequency of the different species varied. For example, the sea squirt Aplidium spp. showed a two year cycle of decline and re-growth, consistent with recovery after removal by sea urchin grazing. Therefore, the relative abundance of the epifaunal components of the community are likely to vary with the abundance and long term changes in sea urchin abundance and grazing pressure.

Habitat structure and complexity

This is a rock biotope in which the main habitat complexity is provided by crevices, fissures and overhangs rather than epibiota. Fluctuations in sea urchin grazing will allow rapid colonizing species to develop epifaunal turfs, with occasional escapes of slow colonizing or slow growing species such as sponges and soft corals (e.g. Alcyonium digitatum), providing some epifaunal complexity in an otherwise sparse community. Well developed epifaunal communities may occur in grazing refuges such as crevices or under hangs.
  • The bedrock is dominated by a layer of encrusting corallines, with encrusting bryozoans, and sometimes barnacles e.g. Balanus crenatus
  • Where present, encrusting epifauna may be overgrown by erect bryozoans and hydroids (e.g. Bugula species and Nemertesia antennina) interspersed with encrusting sponges, ascidians (e.g. Ascidia spp. ) and Alcyonium digitatum.
  • Rock with patches of muddy shell gravel and sand may support Urticina felina (Connor et al., 1997a).
  • The faunal crust and sparse turf provides interstices and refuges for a variety of small organisms such as nemerteans, polychaetes, gastropods, and amphipods, while the erect species provide substrata for caprellid amphipods.
  • Where present, underboulders and crevices may support the sea cucumber Pawsonia saxicola, squat lobsters (e.g. Galathea spp. and Munida rugosa), encrusting sponges, terebellids, the jingle shell Pododesmus patelliformis (Connor et al., 1997a).
  • Fissures and crevices provide shelter for mobile species including decapods crustaceans such as shrimp, crabs and lobsters, sea urchins, starfish and fish. Gobies, wrasse and butterfish probably utilize available rock ledges and crevices, while large species such as flounder and cod probably feed over a wide area, albeit in low numbers.
The biotope may show spatial variation in community complexity and exhibit a mosaic of different species patches (Hartnoll, 1998). For example, with areas recently cleared by predation, disease or physical disturbance in the process of re-colonization, together with areas dominated by Bugula species, sponges, or ascidians, or areas at intermediate stages of succession. The upper edges or boulders or rocky outcrops, most directly in water flow, tend to exhibit the most species rich and abundant faunal turfs with for example hydroids and perhaps feather stars (e.g. Antedon bifida). Where sediment scour and abrasion occur (e.g. the sediment/ rock interface), only a small range of species, such as the sponges Polymastia spp. or the anemone Urticina felina are able to survive (Stebbing, 1971b, Eggleston, 1972b; Sebens, 1985, 1986; Connor et al., 1997a; Hartnoll, 1998; Brazier et al., 1999).

Productivity

Any macroalgae (in shallower examples of this biotope) and encrusting coralline algae provide primary production that enters the food chain indirectly in the form of detritus, algal spores and abraded algal particulates, or directly as food for grazing gastropods, chitons, sea urchins or fish. However, circalittoral faunal turf biotopes are dominated by secondary producers. Food in the form of phytoplankton, zooplankton and organic particulates from the water column together with detritus and abraded macroalgal particulates from shallow water ecosystems are supplied by water currents and converted into faunal biomass. Their secondary production supplies higher trophic levels such as mobile predators (e.g. starfish, sea urchins, and fish) and scavengers (e.g. starfish and crabs) and the wider ecosystem in the form of detritus (e.g. dead bodies and faeces). In addition, reproductive products (sperm, eggs, and larvae) also contribute to the zooplankton (Hartnoll, 1998). However, no estimates of faunal turf productivity were found in the literature.

Recruitment processes

Most of the species within this biotope produce short-lived, larvae with relatively poor dispersal capacity, resulting in good local recruitment but poor long range dispersal. Most reproductive propagules are probably entrained within the reduced flows within the faunal turf or in turbulent eddies produced by flow over the uneven substratum, resulting in turbulent deposition of propagules locally. Many species are capable of asexual propagation and rapidly colonize space. For example:
  • Hydroids are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). For example, the hydroids Kirchenpaueria pinnata and Abietinaria abietina lack a medusa stage, releasing planula larvae (Cornelius, 1995b). Planula larvae swim or crawl for short periods (e.g. <24hrs) so that dispersal away from the parent colony is probably very limited (Sommer, 1992; Gili & Hughes, 1995). But Nemertesia antennina releases planulae on mucus threads, that increase potential dispersal to 5 -50m, depending on currents and turbulence (Hughes, 1977). Few species of hydroids have specific substrata requirements and many are generalists capable of growing on a variety of substrata. Hydroids are also capable of asexual reproduction and many species produce dormant, resting stages, that are very resistant of environmental perturbation (Gili & Hughes, 1995). Hughes (1977) noted that only a small percentage of the population of Nemertesia antennina in Torbay developed from dormant, regressed hydrorhizae, the majority of the population developing from planulae as three successive generations. Rapid growth, budding and the formation of stolons allows hydroids to colonize space rapidly. Fragmentation may also provide another route for short distance dispersal. However, it has been suggested that rafting on floating debris (or hitch hiking on ships hulls or in ship ballast water) as dormant stages or reproductive adults, together with their potentially long life span, may have allowed hydroids to disperse over a wide area in the long term and explain the near cosmopolitan distributions of many hydroid species (Cornelius, 1992; Gili & Hughes, 1995).
  • The brooded, lecithotrophic coronate larvae of many bryozoans (e.g. Flustra foliacea, and Bugula species), have a short pelagic life time of several hours to about 12 hours (Ryland, 1976). Recruitment is dependant on the supply of suitable, stable, hard substrata (Eggleston, 1972b; Ryland, 1976; Dyrynda, 1994). However, even in the presence of available substratum Ryland (1976) noted that significant recruitment in bryozoans only occurred in the proximity of breeding colonies. For example, Hatcher (1998) reported colonization of slabs, suspended 1 m above the sediment, by Bugula fulva within 363 days while Castric-Fey (1974) noted that Bugula turbinata, Bugula plumosa and Bugula calathus did not recruit to settlement plates after ca two years in the subtidal even though present on the surrounding bedrock. Similarly, Keough & Chernoff (1987) noted that Bugula neritina was absent from areas of seagrass bed in Florida even though substantial populations were present <100m away.
  • Echinoderms are highly fecund, producing long lived planktonic larvae with high dispersal potential. However, recruitment in echinoderms is poorly understood, often sporadic and variable between locations and dependant on environmental conditions such as temperature, water quality and food availability. For example, in Echinus esculentus recruitment was sporadic and Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations between 1980-1981 (Nichols, 1984). Bishop & Earll (1984) suggested that the population of Echinus esculentus at St Abbs had a high density and recruited regularly whereas the Skomer population was sparse, ageing and had probably not successfully recruited larvae in the previous 6 years. However, echinoderms such as Echinus esculentus, and Asterias rubens are mobile and widespread and are likely to recruit be migration from other areas.
  • Sponges may proliferate both asexually and sexually. A sponge can regenerate from a broken fragment, produce buds either internally or externally or release clusters of cells known as gemmules which develop into a new sponge, depending on species. Most sponges are hermaphroditic but cross-fertilization normally occurs. The process may be oviparous, where there is a mass spawning of gametes through the osculum which enter a neighbouring individual in the inhalant current. Fertilized eggs are discharged into the sea where they develop into a planula larva. However, in the majority development is viviparous, whereby the larva develops within the sponge and is then released. Larvae have a short planktonic life of a few hours to a few weeks, so that dispersal is probably limited and asexual reproduction probably results in clusters of individuals.
  • Anthozoans, such as Alcyonium digitatum and Caryophyllia smithii are long lived with potentially highly dispersive pelagic larvae and are relatively widespread. They are not restricted to this biotope and would probably be able to recruit within 2-5 years (refer to the Key Information reviews; Sebens, 1985; Jensen et al., 1994). Juvenile anthozoans are susceptible to predation by sea urchins or overgrowth by ascidians (Sebens, 1985; 1986).
  • Ascidians such as Molgula manhattensis, Ciona intestinalis and Clavelina lepadiformis have external fertilization but short lived larvae (swimming for only a few hours), so that dispersal is probably limited (see MarLIN reviews). Where neighbouring populations are present recruitment may be rapid but recruitment from distant populations may take a long time.
  • Mobile epifauna will probably recruit from the surrounding area as the community develops and food, niches and refuges become available, either by migration or from planktonic larvae. For example, Hatcher (1998) noted that the number of mobile epifaunal species steady increased over the year following deployment of settlement panels in Poole Harbour.
Recruitment is partly dependant on the availability of free space, provided by grazing, predation, physical disturbance or seasonal die back on some species. The presence of erect species may interfere with recruitment of others, e.g. the dense stands of the hydroid Obelia longissima inhibited settlement by Balanus crenatus cyprid larvae but encouraged settlement by ascidian larvae (Standing, 1976). In addition, filter feeding hydroids and anthozoans probably take the larvae of many organisms. Once settled the slow growing species may be overgrown or devoured by predator/grazers, e.g. juvenile Alcyonium digitatum are highly susceptible to being smothered or eaten when small but can survive intense sea urchin predation when large (Sebens, 1985, 1986). Overall, rapid growth and reproduction secures space in the community for many species e.g. hydroids and bryozoans while ascidians and anthozoans are better competitors but more susceptible to predation (Sebens, 1985; 1986).

Time for community to reach maturity

The recolonization of epifauna on vertical rock walls was investigated by Sebens (1985, 1986). He reported that rapid colonizers such as encrusting corallines, encrusting bryozoans, amphipods and tubeworms colonized within 1-4 months. Ascidians such as Dendrodoa carnea, Molgula manhattensis and Aplidium spp. achieved significant cover in less than a year, and, together with Halichondria panicea, reached pre-clearance levels of cover after 2 years. A few individuals of Alcyonium digitatum and Metridium senile colonized within 4 years (Sebens, 1986) and would probably take longer to reach pre-clearance levels.

The addition of sea urchins to vertical rock wall communities previously devoid of urchins resulted in removal of the ascidian Aplidium, the mats of tubiculous amphipods and tubeworms leaving only bare rock within 2-3 months. Only a few large Alcyonium digitatum and fleshy red encrusting algae remained (Sebens, 1985). Overall a reduction in or absence of sea urchin grazing would allow opportunistic, bryozoans, hydroids, tubeworms and ascidians to grow and colonize space rapidly, probably developing a faunal turf within 1-2 years. Mobile epifauna and infauna will probably colonize rapidly from the surrounding area. However, slow growing species such as some sponges and anemones, will probably take many years to develop significant cover, so that a diverse community may take up to 5 -10 years to develop, depending on local conditions. But on their return, grazing by sea urchins could probably restore the biotope to bare rock dominated by encrusting algae within a few months.

Additional information

None entered.

Preferences & Distribution

Recorded distribution in Britain and IrelandRecorded from Shetland, Orkney, south east Scotland and north east England, Youghal Bay, Ireland and the west coast of Scotland.

Habitat preferences

Depth Range
Water clarity preferences
Limiting Nutrients Data deficient
Salinity Full (30-40 psu)
Physiographic Open coast
Biological Zone Circalittoral
Substratum Bedrock, Large to very large boulders, Small boulders
Tidal Very Weak (negligible), Weak < 1 knot (<0.5 m/sec.)
Wave Exposed, Moderately exposed
Other preferences None known

Additional Information

This biotope has been recorded from wave exposed to moderately wave exposed coasts, although it probably occurs at greater depth with greater surface wave exposure. It is recorded from moderately strong to very weak tidal streams. Water movement, either wave or current induced, is essential for suspension feeders such as hydroids, bryozoans, sponges, amphipods and ascidians to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Therefore, records in very weak tidal streams are probably in areas of moderate to strong wave exposure.

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

-

Additional information

The MNCR recorded 497 species within this biotope although not all species occurred in all records of the biotope.

Sensitivity reviewHow is sensitivity assessed?

Explanation

This biotope corresponds to the early successional epifaunal communities, with intense sea urchin grazing, dominated by encrusting algal crusts and faunal crusts (e.g. tubeworms and bryozoans) described by Sebens (1985, 1986). Sebens (1985, 1986) demonstrated epifaunal succession to faunal turf and eventually ascidian, Alcyonium or Metridium dominated communities after the removal of sea urchins. In similar experiments, Echinus esculentus was shown to control the lower limit of Laminaria species in the Isle of Man (Jones & Kain 1967; Kain 1979). Therefore, Echinus esculentus has been suggested as a key functional species, since its loss would result in major changes in the community. Alcyonium digitatum has been included as important characterizing since it occurs in most records of the biotope. Lithophyllum incrustans has been included to represent the encrusting coralline algae characteristic of the biotope. Parasmittina trispinosa and Pomatoceros triqueter are included as characteristic faunal crusts. In the absence of detailed research on Parasmittina trispinosa, reference was made to Electra pilosa and other bryozoans to represent its sensitivity.

Species indicative of sensitivity

Community ImportanceSpecies nameCommon Name
Important characterizingAlcyonium digitatumDead man's fingers
Key functionalEchinus esculentusEdible sea urchin
Important otherLithophyllum incrustansEncrusting coralline algae
Important otherParasmittina trispinosaAn enrusting bryozoan
Important otherPomatoceros triqueterA tubeworm

Physical Pressures

 IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
High High Moderate Major decline Low
Removal of the substratum would result in removal of the community and its associated species, therefore an intolerance of high has been recorded. Most of the species in the biotope, including the sea urchins would probably recover within less than 5 years (see additional information below).
Intermediate High Low Minor decline Low
Smothering by 5 cm of sediment will prevent feeding and reduce growth and reproduction, interfere with respiration and potentially cause localized anoxia, and interfere with larval settlement. Tall erect species, e.g. Nemertesia antennina, may survive due to their size, while some hydroids may survive as dormant stages. But encrusting sponge species and ascidians are likely to be damaged or killed by smothering, while vertical surfaces and overhangs will provide refuges from the effects of the factor.

Echinus esculentus is slow moving and unlikely to escape smothering by 5cm of sediment but even if immobilized is unlikely to starve within a month. But juveniles and small individuals may be adversely affected. Large specimens of Alcyonium digitatum may be up to 20cm tall and probably unaffected but the smaller colonies more typical of this biotope may be smothered, impairing respiration, and hence adversely affected (see MarLIN reviews). Encrusting corallines and encrusting bryozoans are unlikely to be affected since they were reported to survive being overgrown by other species and hence smothering (Gordon, 1972; Sebens, 1985; Todd & Turner, 1988)

Overall, smothering may result in loss of small or juvenile urchins, potentially resulting in a reduction in the grazing intensity. Therefore, an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).
Low Very high Very Low No change Low
Suspension feeding organisms may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. Animal dominated communities develop preferentially on steep surfaces and under overhangs, e.g. bryozoan larvae preferentially settle under overhangs, presumably to avoid smothering and siltation (Ryland, 1977; Hartnoll, 1983). Wendt (1998) noted that Bugula neritina grew faster on downward facing surfaces than upward facing surfaces, presumably due to siltation and reduced feeding efficiency on upward facing surfaces. But where water flow is sufficient to prevent siltation, Bugula turbinata may colonize upward facing surfaces (Hiscock & Mitchell, 1980). Large massive sponges tend to favour fast flowing waters that are free of silt while encrusting species can tolerate more turbid conditions, (e.g. Halichondria panicea), although the response to suspended sediment loads varies with species (Morton & Miller, 1968; Moore, 1977). The tolerance of ascidians to suspended sediment varies with species, e.g. Clavelina lepadiformis and Morchellium argus are probably relatively tolerant (see species reviews) whereas Aplidium pallidum and Botrylloides leachi may be more sensitive.

Moore (1977) suggested that Echinus esculentus was unaffected by turbid conditions. Similarly, Comely & Ansell (1988) recorded this species in the presence of suspended material up to 5-6 mg/l. Echinoderm pedicellariae keep the test clear of settling larvae, spores and presumably sediment particles. Similarly Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucus, although mucus production incurs an energetic cost.

This biotope occurs in moderately wave exposed to wave exposed sites with weak to very weak tidal streams. Where water flow is adequate to prevent excessive siltation, an increase in suspended sediment at the benchmark level is likely to reduce feeding efficiency and hence growth and reproduction in suspension feeders, which may be important for species with several generations per year (e.g. Bugula spp. or Nemertesia spp. In areas of weak water flow and increased depth (reduced effects of wave action), an increase in suspended sediment will increase siltation to the detriment of several members of the community, especially the hydroids and bryozoans. Therefore, an overall intolerance of low has been recorded to represent the additional energetic costs associated with increased suspended sediment (e.g. mucus production), although Echinus esculentus, Alcyonium digitatum and encrusting corallines are unlikely to be adversely affected. Recovery is likely to be rapid (see additional information below).
Tolerant High Not sensitive* Decline Low
A decrease in suspended sediment may decrease food availability to suspension feeders within the biotope for the duration of the benchmark (one month) but otherwise not adversely affect the biotope. Therefore, not sensitive has been recorded.
Not relevant Not relevant Not relevant Not relevant Not relevant
Encrusting algae, sea urchins, bryozoans, sponges, soft corals, and hydroids are probably highly intolerant of desiccation. However, this biotope is circalittoral, occurring below 5-10m depth (most records between 10-30m depth) and unlikely to be exposed to the air and desiccation.
Not relevant Not relevant Not relevant Not relevant Not relevant
An increase in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action may be increased (see water flow rate below).
Not sensitive* Not relevant
A decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action may be decreased (see water flow rate below).
High High Moderate Minor decline Low
This biotope was recorded from weak to very weak tidal streams (Connor et al., 1997a; JNCC, 1999). Water movement is essential for suspension feeders such as hydroids, bryozoans, sponges, amphipods and ascidians to supply adequate food, remove metabolic waste products, prevent accumulation of sediment (siltation) and disperse larvae or medusae. Most hydroids utilize a narrow range of water flow rates for effective feeding, and feeding efficiency decreases at high water flow rates (Gili & Hughes, 1995). Similarly, water flow rates affect filter feeding efficiency in bryozoans, the preferred ranges depending on species. An increase in water flow from weak to strong (see benchmark) is likely to adversely affect some members of the community due to drag. For example, species tolerant of strong water flow, e.g. Tubularia indivisa, Halichondria panicea, Alcyonium digitatum and Flustra foliacea may increase in abundance, while species that are less tolerant of strong water flow, e.g. Nemertesia spp., Caryophyllia smithii, Ophiothrix fragilis and Ascidia mentula may be excluded (see Hiscock, 1983). In addition, very strong water flow may interfere with larval settlement and recruitment.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 1 knot. Outside the beds specimens were occasionally seen being rolled by the current (Comely & Ansell 1988), which may have been up to 2.6 knots. Urchins are removed from the stipe of kelps by wave and current action and are also displaced by storm action. However, Echinus esculentus also occurs in strong tidal streams (e.g. A3.2122) and even in shallow areas where wave action is extremely strong such as Rockall (Laffoley & Hiscock, 1988). But in the above examples the sea urchins occurred in developed macroalgal communities. Jones & Kain (1967) demonstrated that monthly removal of Echinus esculentus allowed a macroalgal community to develop within two to three years. Experimental removal significantly reduced the density of the sea urchin in the experimental area, and Jones & Kain (1967) concluded that the lower limit of the kelp beds on Port Erin breakwater were controlled by sea urchin grazing intensity. Therefore, any factor that reduces the density of the sea urchin population will reduce grazing intensity. An increase in water flow from weak to strong (see benchmark) is unlikely to result in removal of the Echinus esculentus population. But an increase in water flow may reduce the abundance of Echinus esculentus and reduce grazing intensity. Sebens (1985) demonstrated that the experimental addition of sea urchins to vertical rock faces removed the epifaunal community back to bare rock and encrusting corallines but on their subsequent removal the epifaunal community had partly recovered within a year.

Overall, a reduction in grazing intensity, together with an increase food supply due to increased water flow, is likely to result an increase in the abundance of suspension feeders and other epifauna and major changes in the community leading to loss of the biotope as described. Therefore, an intolerance of high has been recorded. Loss of several intolerant species is likely to reduce species richness in the short term, while subsequent development of a faunal turf communities will increase species richness in the long term. The biotope may become replaced by epifaunal turf communities characteristic of strong water flow e.g. CR.Bug or MCR.Flu. Recovery is likely to be high (see additional information below).

Tolerant Not sensitive* Minor decline Low
This biotope occurs in areas of weak to very weak tidal streams. While wave induced water movement is probably of more importance in theses sites, a further reduction in water flow is unlikely.
Intermediate High Low Minor decline Low
Most species within the biotope are recorded to the north or south of the British Isles and are unlikely to be adversely affected by long term increases in temperature at the benchmark level. For example, the hydroid Abietinaria abietina occur from the Arctic south to Madeira, the sponge Pachymatisma johnstonia occurs south to Spain, and Alcyonium digitatum occur from Norway to Portugal. Growth rates were reported to increase with temperature in several bryozoans species but zooid size decreased, possibly due to increased metabolic costs at higher temperature (Menon, 1972; Ryland, 1976; Hunter & Hughes, 1994). Temperature is also a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). The bottle-brush hydroid Thuiaria thuja may occur in northern records of this biotope, and is thought to be susceptible to climate change, probably retreating further north in response to long term increases in temperature.

Echinus esculentus is recorded from the north and south of the British Isles, experiencing for example temperatures between 0 -18 °C in the Limfjord, Denmark (Ursin 1960), and is unlikely to be affected by long term changes in temperature at the benchmark level. But Bishop (1985) noted that prolonged exposure to 19 ° C disturbed gametogenesis and suggested that Echinus esculentus could not tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Therefore, Echinus esculentus may be intolerant of short term acute increases in temperature.

Overall, an intolerance of intermediate has been recorded to represent the potential effects of short term temperature shock on Echinus esculentus. The biotope has only been recorded from northern areas suggesting that a long term increase in temperature may cause a retreat northwards. Recoverability is likely to be high (see additional information below).
Low Very high Moderate No change Low
Most of the hydroid and bryozoan species within the biotope are recorded to the north or south of the British Isles and are unlikely to be adversely affected by long term increases in temperature at the benchmark level. Temperature is a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). Sebens (1986) reported that growth rates of most species were higher in the warmer months, except in Alcyonium spp. and Spirorbis spp. which showed little seasonal differences.

Echinus esculentus is recorded from the north and south of the British Isles, experiencing for example temperatures between 0 -18 °C in the Limfjord, Denmark (Ursin 1960), and is unlikely that this species will be adversely affected by a long term temperature change in British waters. Similarly, Alcyonium digitatum occurs from Iceland in the North, to Portugal and was also reported to be apparently unaffected by the severe winter of 1962-1963 (Crisp, 1964).

Therefore, a decrease in temperature at the benchmark level is unlikely to adversely affect the biotope but an intolerance of low has been recorded to represent the effects on growth rates.
Tolerant Not relevant Not relevant Not relevant Not relevant
The only species likely to be affected by a reduction in light intensity due to increased turbidity are the encrusting algae. But encrusting coralline algae are amongst the deepest water species of macroalgae occurring in the circalittoral, at great depths, and a light levels as low as 0.05 -0.001% of surface incident light (Lüning, 1990). A reduction in light intensity may reduce their growth rates, especially in the deepest examples of the biotope. However, their extremely slow growth rates mean that the corallines will probably not be adversely affected for the duration of the benchmark. Therefore, not sensitive has been recorded.
Tolerant Not sensitive* Rise Very low
A decrease in turbidity, and hence increased light, will increase the growth rates of macroalgae within the biotope (e.g. Delesseria sanguinea) especially in shallower examples of the biotope. This may allow macroalgae to increase in abundance, depending on grazing intensity. However, this biotope is characterized by grazing, so that although periodic escapes of macroalgae may be favoured , the biotope will probably not be significantly changed.
Intermediate High Low Minor decline Low
This biotope was recorded from wave exposed to moderately exposed habitats, with a few examples from very wave exposed habitats (JNCC, 1999). Many of the epifaunal species in the biotope are likely to tolerate an increase in wave exposure from moderately exposed to very exposed, for example, Alcyonium digitatum, Bugula species, the sponges Halichondria panicea and the hydroid Kirchenpaueria pinnata. However, less flexible or weaker hydroids and bryozoans may be removed, e.g. Nemertesia antennina and Nemertesia ramosa. Increased wave action at the benchmark level in the shallow extent of the biotope may decrease sea urchin and starfish predation, allowing larger, massive species (e.g. sponges, Alcyonium digitatum, anemones and ascidians) to increase in dominance, becoming a different successional community (see Sebens, 1985). For example, the similar biotope IR.AlcByH occurs in wave exposed conditions in the infralittoral (less deep) and is dominated by Alcyonium digitatum.

Echinus esculentus may be removed form kelp stipes by the action of currents and wave action or the effects of storms. Increase wave action is likely to depress the upper extent of shallow sublittoral populations. But Echinus esculentus occurred in significant numbers as shallow as 15m below low water level in the most wave exposed location in the British Isles at Rockall (Keith Hiscock pers comm.).

Hence, it is likely that some species within the biotope, especially delicate hydroids may be lost, and the community structure change in favour of more massive species. Therefore, a proportion of the biotope is likely to be lost or changed (especially in shallow examples) and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Tolerant Not sensitive* Minor decline Low
This biotope is found in moderately wave exposed to wave exposed habitats with a few records from sheltered or very wave sheltered habitats. However, species adapted to strong water movement may be reduced in abundance or lost e.g. Flustra foliacea and Pachymatisma johnstonia, while species more tolerant of sheltered conditions may increase in abundance e.g. Ascidia mentula (see Hiscock, 1983) and hydroids. But wave sheltered conditions are likely to encourage sea urchin grazing, which may reduce the abundance of epifaunal species including hydroids and bryozoans yet further, to leave only bare rock or encrusting coralline. However, given the grazed nature of the biotope, little significant difference in the biotope is likely to occur, so that not sensitive has been recorded.
Tolerant Not relevant Not relevant No change High
Hydroids, bryozoans, sponges, sea urchins and ascidians are unlikely to be intolerant of noise or vibration at the benchmark level. Mobile fish or decapod species may be temporarily scared away from the areas but few if any adverse effects on the biotope are likely to result.
Tolerant Not relevant Not relevant No change High
Many of the species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators, and day length in their behavioural or reproduction. However, their visual acuity is probably very limited and they are unlikely to be intolerant of visual disturbance at the benchmark level.
High Very high Low Rise Low
Erect epifaunal species, e.g. soft corals, bryozoans and hydroids are thought to be especially intolerant of physical disturbance, and epifaunal assemblages show reduced biomass and abundance in areas impacted by fishing (e.g. trawls and dredges). For detail see MarLIN reviews e.g. CR.Bug, MCR.Flu or A5.621. But, in this biotope, the epifaunal and algal crusts probably experiences greater levels of physical disturbance in the form of grazing, and are either tolerant (e.g. encrusting corallines) or rapidly recolonize available space (e.g. hydroids and tubeworms). However, urchins themselves may be sensitive.

Abrasion by lobster pots, anchor chains, or mobile fishing gear will probably result in displacement, loss of spines and some damage to tests of adult Echinus esculentus. Species with fragile tests, such as Echinus esculentus and Echinocardium cordatum were reported to suffer badly as a result of impact with a passing scallop dredges (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000a). Adults can repair non-lethal damage to the test and spines can be re-grown but most dredge impact is likely to be lethal. Therefore, physical abrasion due to a passing anchor or dredge is likely to kill or remove a proportion of the sea urchin population. Any activity that significantly reduces the sea urchin population and hence grazing intensity it likely to result in major changes in the community. Therefore, an intolerance of high has been recorded. Recoverability is likely to be very high as sea urchins are likely to migrate into the area before any significant growth of epifauna can occur (see additional information below).

High Very high Low Minor decline Not relevant
Echinus esculentus is probably regularly displaced to deeper water by storms. Displaced specimens are able to move up the shore after displacement (Lewis & Nichols 1979b). Large erect epifaunal species, e.g. Alcyonium digitatum and hydroids can not reattach if removed and would be lost. Encrusting species such as encrusting corallines would probably be killed if removed although removal is unlikely. Mobile scavengers and epifauna, e.g. crabs and brittlestars will probably survive displacement and return to suitable substrata. Overall, any activity that significantly reduces the sea urchin population and hence grazing intensity it likely to result in major changes in the community. Therefore, an intolerance of high has been recorded. But displacement of the sea urchins is likely to be temporary as they are likely to return to the biotope quickly, so that little long term effect on the biotope is likely and a recoverability of very high has been recorded.

Chemical Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
High High Moderate Major decline Very low
Tri-butyl tin (TBT) has a marked effect on numerous marine organisms (Bryan & Gibbs, 1991). The encrusting bryozoan Schizoporella errata suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l. Copepod and mysid crustaceans were particularly intolerant of TBT while crabs were more resistant (Bryan & Gibbs, 1991), although recent evidence suggests some endocrine disruption in crabs. The adverse effect of TBT on reproduction in Nucella lapillus and other neogastropods is well known (see review), and similar effects on reproduction may occur in other gastropod molluscs, including nudibranchs. Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. Rees et al. (2001) suggested that TBT inhibited settlement in ascidian larvae. This report suggested that epifaunal species (including bryozoans, hydroids and ascidians) may be at least inhibited by the presence of TBT.

Smith (1968) reported large numbers of dead Echinus esculentus at between 5.5 and 14.5 m depth in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area after Torrey Canyon oil spill (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999).

Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). But Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Crisia spp. and Cellaria sp. did not occur less than 600m from the effluent source and noted that Bugula flabellata did not occur within the bay.

The species richness of hydroid communities decreases with increasing pollution (Boero, 1984; Gili & Hughes, 1995). However, Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth.

Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove (Pedu-men-du, Cornwall) died resulting from the offshore spread and toxic effect of detergents e.g. BP 1002 sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (Smith, 1986). Possible sub-lethal effects of exposure to synthetic chemicals, may result in a change in morphology, growth rate or disruption of reproductive cycle.

Therefore, hydroids crustaceans, gastropods, and ascidians are probably intolerant of TBT contamination while sea urchins and bryozoans are probably intolerant of chemical pollution. Therefore an intolerance of high has been recorded, albeit at low confidence. A recoverability of high has been recorded (see additional information below).

Heavy metal contamination
High High Moderate Rise Low
Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001). Bryozoans are common members of the fouling community and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995). Heavy metals caused reproductive anomalies in the starfish Asterias rubens (Besten, et al., 1989, 1991). Gastropod molluscs have been reported to relatively tolerant of heavy metals while a wide range of sublethal and lethal effects have been observed in larval and adult crustaceans (Bryan, 1984).

Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were intolerant of heavy metals. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu). Sea-urchins, especially the eggs and larvae, are used for toxicity testing and environmental monitoring (reviewed by Dinnel et al. 1988). Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) and showed developmental abnormalities in the skeleton (Gomez & Miguez-Rodriguez 1999). Therefore, sea urchins may be intolerant of heavy metal contamination.

Bryozoans and hydroids may only manifest sublethal effects due to heavy metal contamination. But Echinus esculentus and its larvae are probably highly intolerant. Loss of sea urchin grazing is likely to result in major changes in the community and a biotope intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Hydrocarbon contamination
Intermediate High Low Decline Low
Circalittoral communities are likely to be protected from the direct effects of oil spills by their depth. However, the biotope may be exposed to emulsified oil treated with dispersants, especially in areas of turbulence, or may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates. Invertebrates groups vary in their sensitivities, e.g.:
  • Species of the encrusting bryozoan Membranipora and the erect bryozoan Bugula were reported to be lost or excluded from areas subject to oil spills. (Mohammad, 1974; Soule & Soule, 1979). Soule & Soule (1979) reported that Bugula neritina was lost from breakwater rocks in the vicinity of the December 1976 Bunker C oil spill in Los Angeles Harbour, and had not recovered within a year. However, Bugula neritina had returned to a nearby area within 5 months (May 1977) even though the area was still affected by sheens of oil. Houghton et al. (1996) also reported a reduction in the abundance of intertidal encrusting bryozoans (no species given) at oiled sites after the Exxon Valdez oil spill.
  • The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate hydroid Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995).
  • Suchanek (1993) reported that the anemones Anthopleura spp. and Actinia spp. survived in waters exposed to spills and chronic inputs of oils. Similarly, one month after the Torrey Canyon oil spill, the dahlia anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968).
  • Smith (1968) reported dead colonies of Alcyonium digitatum in the locality of Sennen Cove (Pedu-men-du, Cornwall) resulting from the combination of wave exposure and heavy spraying of dispersants along the shoreline to disperse oil from the Torrey Canyon tanker spill (see synthetic chemicals).
  • Crude oil from the Torrey Canyon and the detergent used to disperse it caused mass mortalities of echinoderms: Asterias rubens, Echinocardium cordatum, Psammechinus miliaris, Echinus esculentus, Marthasterias glacialis and Acrocnida brachiata (Smith, 1968).
  • Halichondria panicea survived in areas affected by the Torrey Canyon oil spill, although few observations were made (Smith 1968).
  • Crump et al. (1999) described "dramatic and extensive bleaching" of 'lithothamnia' following the Sea Empress oil spill. Observations following the Dona Marika oil spill (K. Hiscock, pers. comm.) were of rock pools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was rapidly affected by oil during the Sea Empress spill, the oil only destroyed about one third of the thallus surface thickness and regeneration occurred from thallus filaments below the damaged area so that recovery occurred within about a year.
If the physiology within different animals groups can be assumed to be similar, then bryozoans, echinoderms and soft corals may be highly intolerant of hydrocarbon contamination, while hydroids may demonstrate sublethal effects and anemones and some species of sponge are relatively tolerant. While the direct affects of oil spills will be ameliorated by depth in this circalittoral habitat, the abundance of sea urchins is likely to reduced, resulting in community change and possibly, change to a different biotope. Several other members of the community may be lost or damaged as a result of acute hydrocarbon contamination, and an intolerance of intermediate has been suggested, albeit at very low confidence. Recoverability is likely to be high (see additional information below).
Radionuclide contamination
No information Not relevant No information Insufficient
information
Not relevant
No information found
Changes in nutrient levels
High High Moderate Decline Low
An increase in nutrient levels from e.g. sewage sludge, sewage effluent or riverine flooding, may result in an increase in inorganic and organic suspended particulates (see above), increased turbidity (see above) and increased phytoplankton productivity. An increase in nutrient levels may stimulate macroalgal growth, competing with the faunal crusts especially in its shallower examples, although increased turbidity due to phytoplankton abundance may offset the effect of nutrient enrichment (Hartnoll, 1998) while increased macroalgal growth may benefit sea urchins. Moderate nutrient enrichment may increase the food available to the community in the form of phytoplankton, zooplankton or organic particulates. Sea urchins are unlikely to be directly affected since Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age. Comely & Ansell (1988) also suggested that Echinus esculentus could absorb dissolved organic material. However, eutrophication may result in indirect effects such as deoxygenation (see below) or algal blooms.

Whilst the biotope is unlikely to be directly affected by algal blooms, the biotope may be adversely affected by toxins from toxic algae that accumulate in zooplankton, or smothered by dead 'bloom' algae and deoxygenation resulting from their subsequent decay (see below). For example, death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of invertebrates, including Echinus esculentus and Marthasterias glacialis, while sessile bryozoans, sponges and Alcyonium spp. appeared moribund, presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al. 1979). A bloom of the toxic flagellate Chrysochromulina polypedis in the Skagerrak resulted in death or damage of numerous benthic animals, depending on depth. The red algae Delesseria sanguinea lost pigmentation, and ascidians exhibited high mortalities even at 17m depth, while in shallow water all dominant species (including Ciona intestinalis, Halichondria panicea and Asterias rubens) were killed. The toxic effects of the algal bloom resulted in a marked change in the community structure (Lundälv, 1990).

The potential toxic effects of the algal blooms and the siltation and anoxia caused by death of an algal bloom may result in loss of several members of the community, especially sea urchins, and an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Not relevant Not relevant Not relevant Not relevant Not relevant
This is a circalittoral biotope, occurring in fully saline conditions and unlikely to experience increased salinity.
High High Intermediate Decline Low
The majority of species within the biotope are only found in full salinity.
Echinoderms are generally unable to tolerate low salinity (stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). At low salinity urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. Echinoderm larvae have a narrow range of salinity tolerance and develop abnormally and die if exposed to reduced or increased salinity. While Stickle & Diehl (1987) noted that local adaptation can occur, echinoderms are generally considered to be intolerant to reduced salinity. For example, Lawrence (1996) reported that sudden river discharge in 1977, killed Strongylocentrotus droebachiensis in Newfoundland and Labrador fjords and at one locality 60-75% of the population was killed to a depth of 12-15m.

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata Bryozoa were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna. Similarly, the majority of hydroids are subtidal and, although some brackish water species exist (Gili & Hughes, 1995), they are probably intolerant of prolonged decreases in salinity. Similarly, most sponges are subtidal and probably intolerant of reduced salinity. Halichondria panicea occurs in damp areas of the intertidal but probably only experiences short periods of reduced salinity due to rainfall.

Gessner & Schramm (1971) summarized the effects of salinity changes on marine algae and noted that most sublittoral red algae cannot withstand salinities below 15 psu. The maerl forming coralline algae, Phymatolithon calcareum and Lithothamnion corallioides are found only in fully saline waters (between 30-40 psu) and the growth of some maerl species is impaired below 24 psu. However, other species such as Lithophyllum incrustans are intertidal and tolerate fluctuating salinities.

Overall, the majority of the species in the community are likely to be intolerant of a reduction in salinity, resulting in loss of many species within the community. Loss of or marked reduction in the sea urchin population will probably result in major changes in the community. Therefore, an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).

High High Moderate Decline Low
Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans were unaffected by periods of moderate hypoxia (ca 0.35 -1.4 ml/l) and short periods of hypoxia (<0.35 ml/l) in the York River, Chesapeake Bay, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation.

Diaz & Rosenberg (1995) reported that the abundance of crustaceans and echinoderms decreased in hypoxic conditions. Echinoderms are probably intolerant of hypoxia (see reviews). For example, death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of invertebrates, including Echinus esculentus and Marthasterias glacialis, while sessile bryozoans, sponges and Alcyonium spp. appeared moribund, presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al. 1979). Mobile fauna are also likely to begin to leave the habitat once the oxygen fall below ca 2.8 mg/l (Diaz & Rosenberg, 1995).

This biotope occurs in moderate water movement (see water flow and wave exposure) and is unlikely to experience low oxygen levels, except due to algal blooms (see nutrients), smothering or decrease wave action. Therefore, hypoxia at the benchmark level will probably result in the loss of a proportion of both sessile and mobile species, and a decrease in species richness. Loss of or marked reduction in the sea urchin population will have a significant effect on the community and an intolerance of high has been recorded. Recoverability is likely to be rapid (see additional information below).

Biological Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
High High Moderate Rise Very low
Alcyonium digitatum acts as the host for the endoparasitic species Enalcyonium forbesi and Enalcyonium rubicundum (Stock, 1988). Parasitization may reduce the viability of a colony but not to the extent of killing them but no further evidence was found to substantiate this suggestion. Sebens (1985, 1986) noted that Lithothamnion species were susceptible to boring polychaetes, which increased their susceptibility to grazing damage. But the result susceptibility was probably one and important factor in the competitive equilibrium between Lithothamnion and Phymatolithon species. The sea urchin Echinus esculentus has been reported to suffer mass mortalities due to 'bald urchin disease' (see review), although not in the British Isles. Populations of the sea urchin Strongylocentrotus droebachiensis have also been reported to undergo large fluctuations in numbers, with increased numbers forming swarms that denude areas, followed by mass mortalities due to disease. Diseases in sea urchins may be an important natural controlling factor in sea urchin population dynamics. Periodic fluctuations in sea urchin populations will probably affect succession, and the dominant epifaunal (see Sebens, 1985; Hartnoll, 1998). A decrease in sea urchin grazing due to disease induced mass mortality may allow epifaunal succession to proceed to more epifaunal turfs and massive growth forms, e.g. ascidians, sponges and anemones. Therefore, although no mass mortalities of sea urchins in British waters have been observed or attributed to disease, disease could potentially result in major changes in the community and an intolerance of high has been recorded, albeit with very low confidence. Recoverability is likely to be high (see additional information below).
No information None No information Insufficient
information
Not relevant
No information found.
Intermediate High Low Rise Low
Echinus esculentus is the key functional species within this biotope. Collecting of Echinus esculentus for the curio trade was studied by Nichols (1984). He concluded that the majority of divers collected only large specimens that are seen quickly and often missed individuals covered by seaweed or under rocks, especially if small. As a result, a significant proportion of the population remains. He suggested that exploited populations should not be allowed to fall below 0.2 individuals per square metre. But in this heavily grazed biotope, any reduction in grazing pressure may significantly affect the community, probably allowing increased escapes of erect epifauna, and possibly macroalgae in shallow examples of the biotope. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Not relevant Not relevant Not relevant Not relevant Not relevant

Additional information

Recoverability The development of this community is probably dependant on the abundance and density of the sea urchin population. The majority of the epifaunal and algal crust species were shown to re-colonize cleared areas quickly. For example red crustose algae, encrusting bryozoans, tubeworms, tubicolous amphipods and worms, erect hydroids and bryozoans were reported to cover cleared areas within 1-4 months in spring, summer and autumn (Sebens, 1986). Colonial ascidians re-appeared and achieved significant cover within a year. Sponges (e.g. Halichondria panicea) recovered cover within >2 years while only a few individuals of Alcyonium and Balanus balanus established after 4 years, probably requiring longer to achieve prior cover (Sebens, 1985; 1986). Encrusting coralline algae (e.g. Lithothamnion and Phytomatolithon took 1-2 years to recolonize cleared areas (Sebens, 1985; 1986) and with their slow growth rates probably take many years to recover their original abundance.

In the absence of sea urchin grazing the community will probably develop into a faunal turf community, following a similar successional sequence to that demonstrated by Sebens (1985). But Sebens (1986) demonstrated that addition of sea urchins removed the majority of species, leaving only bare rock and encrusting algae within 2-3months. Therefore, biotope recovery is likely to be rapid once sea urchin abundance returns to prior levels after disturbance.

Echinus esculentus produces long-lived planktonic larvae with high dispersal potential. Settlement is thought to occur in autumn and winter (Comely & Ansell, 1988). But recruitment is sporadic or variable depending on locality, e.g. Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols studies between 1980-1981 (Nichols, 1984). Lewis & Nichols (1979) found that adults were able to colonize an artificial reef in small numbers within 3 months and the population steadily grew over the following year. Bishop & Earll (1984) suggested that the population of Echinus esculentus at St Abbs had a high density and recruited regularly whereas the Skomer population was sparse, ageing and had probably not successfully recruited larvae in the previous 6 years. But Echinus esculentus is a widespread, mobile species so that recovery is probably improved by migration from neighbouring areas.

Overall, most of the species within the biotope could probably recolonize bare rock, and regain significant cover within 5 years, with the exception of sponges and anemones. Loss of sea urchins would allow faunal turf communities to develop, however, on their return the biotope would probably be recognizable within 2-3 months. Sea urchin recruitment is sporadic and dependant on location but populations would probably recover within 5 years, except in locations isolated by geography or hydrography.

Importance review

Policy/Legislation

- no data -

Exploitation

Collecting of Echinus esculentus for the curio trade was studied by Nichols (1984). He concluded that the majority of divers collected only large specimens that are seen quickly and often missed individuals covered by seaweed or under rocks, especially if small. As a result, a significant proportion of the population remains. He suggested that exploited populations should not be allowed to fall below 0.2 individuals per square metre.

Additional information

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Citation

This review can be cited as:

Tyler-Walters, H., 2002. Faunal and algal crusts on exposed to moderately wave-exposed circalittoral rock. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/337

Last Updated: 14/11/2002