Bugula spp. and other bryozoans on vertical moderately exposed circalittoral rock

20-05-2002
Researched byDr Harvey Tyler-Walters Refereed byThis information is not refereed.
EUNIS Code EUNIS Name

Summary

UK and Ireland classification

EUNIS 2008
EUNIS 2006
JNCC 2004
1997 Biotope

Description

Vertical rock faces in the circalittoral (often at same depth as lower infralittoral biotopes as well as deeper) with a dense turf of Bugula spp. and Scrupocellaria spp. and the sponges Tethya aurantium, Pachymatisma johnstonia, Hemimycale columella and occasionally Dercitus bucklandi in crevices are often present. Also patches of Nemertesia antennina and Crisia eburnea. Most surfaces also with a thin cover of Cryptopleura, Rhodophyllis 'spiky' and Plocamium. Some areas may have large patches of Clavelina and a few areas with Perophora, Polycarpa scuba and Ascidia mentula. Antedon bifida also occurs in crevices. Bugula turbinata tends to predominate in shallower records of this biotope, whereas deeper records have a mixture of at least three Bugula spp., dominated by Bugula plumosa. Many of the records with this biotope have been recorded as parts of other habitat records despite the clarity in which this biotope occupies vertical faces of almost any size in some parts of the country, particularly in Wales and further south in the Irish Sea. Softer rock faces bored by Hiatella arctica (IR.AlcByH.Hia) tend to be more species-rich, reflecting the large number of niches and holes inhabited by small cryptic species. (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 only a few sites in the British Isles, including Orkney, Portland Harbour, Fowey estuary, the Lleyn Peninsula, Bardsey and west Anglesey. However, the biotope has been recorded as parts of other habitats and is probably, therefore, under recorded.

Depth range

-

Additional information

None entered.

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Habitat review

Ecology

Ecological and functional relationships

This biotope is dominated by 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).
  • A few plants are found in this biotope, primarily in the upper reaches of the biotope. 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 reds Delesseria sanguinea, Cryptopleua ramosa, Lomentaria spp. and Plocamium cartilagineum, and the browns Dictyopteris membranacea and Dictyota dichotoma) and eventually articulate corallines and then only encrusting corallines with increasing depth (Sebens, 1985; Hartnoll, 1998; JNCC, 1999).
  • Active suspension feeders on bacteria, phytoplankton and organic particulates and detritus include sponges (e.g. Pachymastia johnstonia, Clathrina coriacea and Halichondria panicea), the soft coral Alcyonium digitatum, erect and encrusting bryozoans (e.g. Flustra foliacea, Chartella papyracea, Bugula species, Scrupocellaria reptans, Bicellaria ciliata and Crisia eburnea), barnacles (e.g. Balanus crenatus), caprellid amphipods, porcelain crabs (e.g. Pisidia longicornis), and sea squirts (e.g. Aplidium spp., Clavelina lepadiformis 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 carnivores of zooplankton and other small animals include, hydroids (e.g. Tubularia indivisa and Nemertesia antennina), soft corals (e.g. Alcyonium digitatum), while larger prey are taken by anemones and cup corals (e.g. Caryophyllia smithii and Actinothoe sphyrodeta) (Hartnoll, 1998).
  • Sea urchins (e.g. Echinus esculentus) are generalist grazers, removing 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 epifaunal communities and succession (Sebens, 1985; 1986; Hartnoll, 1998) and are no doubt important in this biotope (see temporal change below).
  • Other grazers include topshells (e.g. Gibbula cineraria), small crustacea (e.g. amphipods) and Calliostoma zizyphinum, which grazes hydroids.
  • Specialist predators of hydroids and bryozoans include the nudibranchs (e.g. Doto spp., and Onchidoris spp.) and pycnogonids, (e.g. Achelia echinata), while the nudibranch Tritonia hombergi preys on Alcyonium digitatum, and some polychaetes also take hydroids. Nudibranch life cycles may be closely linked to the life cycles of their prey, and local nudibranchs populations grow rapidly and may denude the hydroid colonies, creating gaps in the epifaunal turf (see Chester et al., 2000).
  • Starfish (e.g. Asterias rubens and Crossaster papposus), crabs and lobster are generalist predators feeding on most epifauna, including ascidians.
  • Scavengers include polychaetes, small crustacea such as amphipods, starfish, 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 gunnerellus feeding mainly on small crustacea, 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. Filter feeders reduce the concentration of suspended particulates and deplete food to other colonies/individuals downstream (intra and inter specific competition). Sebens (1985, 1986) demonstrated an 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. The epifauna of vertical rock walls became dominated by large massive species, depending on the degree of predation, especially by sea urchins. However, encrusting bryozoans and encrusting corallines may survive overgrowth (Gordon, 1972; Sebens, 1985; Todd & Turner, 1988).

Seasonal and longer term change

Seasonal change
Many of the species within the community demonstrate seasonal changes in abundance and reproduction. 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 hydroid Tubularia indivisa is annual, dying back in winter (Fish & Fish, 1996), while the uprights of 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 fronds of macroalgae (when present), the uprights of hydroids, 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. 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).
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 the bryozoan turf, leaving bare rock or encrusting corallines behind (Keith Hiscock pers comm.). The biotope probably represents an intermediate community, in which ascidians and soft corals have not become dominant probably due to predation pressure or competition with erect bryozoans. The similar biotope IR.AlcByH is characterized by a greater abundance of the soft coral Alcyonium digitatum, possibly representing another stage in the epifaunal succession suggested by 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; 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 of 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).

Habitat structure and complexity

This biotope may occur as part of other faunal biotopes of steep slopes and vertical surfaces. The biotope may occur on steep slopes, vertical surfaces or under overhangs. The species composition probably varies with depth from the lower infralittoral to deep circalittoral. For example, macroalgae probably out compete the faunal turf species on the tops of bedrock ridges but decline on vertical surfaces and with depth. Similarly, Bugula turbinata tends to dominate in shallower examples, while deeper records have a mixture of Bugula species dominated by Bugula plumosa. The different abundance of different Bugula species is presumably caused by their different preferences for the effects of oscillatory rather than unidirectional water flow (caused by wave action, the effect of which decreases with depth), due to slight differences in colony structure.
  • The bedrock is covered by a layer of encrusting corallines, encrusting bryozoans, and sometimes barnacles, especially Balanus crenatus and Verruca stroemia.
  • Encrusting epifauna are overgrown by dominant erect bryozoans and hydroids (e.g. Bugula species, Scrupocellaria reptans, Tubularia indivisa and Nemertesia antennina) interspersed with encrusting sponges (e.g. Pachymatisma johnstonia, Haliclona viscosa, Stelligera rigida), ascidians (e.g. Dendrodoa grossularia), Alcyonium digitatum and Urticina felina. The dominance by erect bryozoans and hydroids and ascidians forms a faunal turf over the substratum.
  • The faunal 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.
  • The erect bryozoans and hydroids support a variety of epizoics that use them as substratum and in some cases affect their growth rates. For example, Bugula flabellata is almost invariably attached to other bryozoans such as Flustra foliacea, while Crisia eburnea grows on macroalgae, hydroids and other bryozoans. Similarly, Alcyonidium parasiticum is epizoic on hydroid stems or the bryozoan Cellaria spp. and the sponge Esperiopsis fucorum may grow on the stem of Tubularia species or on the test of ascidians.
  • Mobile species include decapods crustaceans such as shrimp, crabs and lobsters, sea urchins, starfish and fish. Gobies, shannies and butterfish probably utilize available rock ledges and crevices, while large species such as flounder and cod probably feed over a wide area.
  • 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, while species richness decreases with proximity to the sediment/ rock interface, which favours species such as the sponges Polymastia spp. or the anemone Urticina felina (Stebbing, 1971b, Eggleston, 1972b; Sebens, 1985, 1986; Connor et al., 1997a; Hartnoll, 1998; Brazier et al., 1999).
  • Periodic disturbance of the community due to physical disturbance by storms, extreme scour, or fluctuations in predation, especially by sea urchins, may encourage species richness by preventing dominance by a few species (Osman, 1977; Sebens, 1985, 1986; Hartnoll, 1998).

Productivity

The presence of macroalgae in the shallower examples of this biotope 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. Although, the biotope occurs within moderately strong to strong water flow that could remove a large proportion of the reproductive output, 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 Aglaophenia plumosa and Sertularia argentea 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). Tubularia indivisa releases a slow crawling actinula larvae with potentially very limited dispersive range (Fish & Fish, 1996). However, 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, Securiflustra securifrons, 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). Actinothoe sphyrodeta is capable of reproducing asexually by fission (Manuel, 1988). Juvenile anthozoans are susceptible to predation by sea urchins or overgrowth by ascidians (Sebens, 1985; 1986). However, recruitment of rare and scarce species, where they occur, such as the sunset cup coralLeptopsammia pruvoti, the Weymouth cup coral Hoplangia durotrix, and the soft coral Alcyonium hibernicum, is likely to take a very long time, e.g. up to 25-30 years or not occur at all in Leptopsammia pruvoti (see MarLIN review).
  • Ascidians such as Molgula manhattensis 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 anthozoa 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 recolonized 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.

Jensen et al. (1994) reported the colonization of an artificial reef in Poole Bay, England. They noted that erect bryozoans, including Bugula plumosa, began to appear within 6 months, reaching a peak in the following summer, 12 months after the reef was constructed. Similarly, ascidians colonized within a few months e.g. Aplidium spp. Sponges were slow to establish with only a few species present within 6-12 months but beginning to increase in number after 2 years, while anemones were very slow to colonize with only isolated specimens present after 2 years (Jensen et al., 1994.). In addition Hatcher (1998) reported a diverse mobile epifauna after a years deployment of her settlement panels.

Hydroids are often initial colonizing organisms in settlement experiments and fouling communities (Standing, 1976; Brault & Bourget, 1985; Sebens, 1986; Jensen et al., 1994; Hatcher, 1998). In settlement experiments, the hydroids Cordylophora caspia, Obelia dichotoma and Obelia longissima colonized artificial substrata within ca 1-3 months of deployment (Standing, 1976; Brault & Bourget, 1985: Sandrock et al., 1991). Similarly, Hatcher (1998) reported that Tubularia larynx colonized settlement panels within only 68 days (ca 2 months). Once colonized the hydroids ability to grow rapidly and reproduce asexually is likely to allow them to occupy space and sexually reproduce quickly.

Overall, bryozoans, hydroids, and ascidians are opportunistic, grow and colonize space rapidly and will probably develop 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.

Additional information

None entered.

Preferences & Distribution

Recorded distribution in Britain and IrelandRecorded from only a few sites in the British Isles, including Orkney, Portland Harbour, Fowey estuary, the Lleyn Peninsula, Bardsey and west Anglesey. However, the biotope has been recorded as parts of other habitats and is probably, therefore, under recorded.

Habitat preferences

Depth Range
Water clarity preferences
Limiting Nutrients Data deficient
Salinity
Physiographic
Biological Zone
Substratum
Tidal
Wave
Other preferences Steep, vertical or overhanging hard substrata.

Additional Information

The abundance of bryozoans is positively correlated with supply of stable hard substrata and hence with current strength (Eggleston, 1972b; Ryland, 1976). The community stability and diversity also requires stable substrata (Osman, 1977; Dyrynda, 1994). 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 and disperse larvae or medusae. Animal communities tend to dominate steep, vertical or overhanging surfaces, while macroalgae tend to dominate horizontal or gently sloping surfaces (< 60°) (Hartnoll, 1983). Steep or vertical surfaces reduce incident light, reduce siltation and may prevent settlement by algal spores, which tend to roll down hill (see Norton, 1992, Hartnoll, 1983). Species composition varies with depth, with decreasing numbers of foliose or filamentous algae due to light attenuation, and increasing numbers of species adapted to unidirectional flow (e.g. planar species of hydroids) than oscillatory flow as the effects of wave action attenuate (Riedl, 1971; Hiscock, 1983).

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

Additional information

Circalittoral faunal turf biotopes support a large number of sessile, interstitial, and mobile cryptofauna (Hartnoll, 1998). The MNCR identified ca 370 conspicuous species within this biotope, although a smaller proportion of species is likely to occur in any one example of the biotope.

Some examples of this biotope may include the nationally rare sunset cup coral Leptopsammia pruvoti and Weymouth cup coral Hoplangia durotrix, and the nationally scarce soft coral Alcyonium hibernicum (Keith Hiscock, pers comm.).

Sensitivity reviewHow is sensitivity assessed?

Explanation

This biotope is dominated by species of the erect bryozoan Bugula, which if lost would result in loss of the biotope as described. The faunal turf of bryozoans and hydroids such as Tubularia indivisa and Nemertesia antennina provide substrata for many species or epizooics, refuges for mobile species, and are active suspension feeders, important in the structure and succession of the biotope. The sensitivity of hydroids has been represent by Nemertesia ramosa with reference to reviews of hydroid biology . Similarly, ascidians and sponges are important competitors for space within the community. Their sensitivity has been represented by Clavelina lepadiformis, Morchellium argus and Halichondria panicea respectively. Echinoderms are mobile generalist grazer and predators that affect community succession, recruitment and diversity and have been represented by Echinus esculentus and Asterias rubens.

Species indicative of sensitivity

Community ImportanceSpecies nameCommon Name
Important functionalAsterias rubensCommon starfish
Important characterizingBugula flabellataAn erect bryozoan
Important characterizingBugula plumosaAn erect bryozoan
Important characterizingBugula turbinataAn erect bryozoan
Important structuralClavelina lepadiformisLight bulb sea squirt
Important functionalEchinus esculentusEdible sea urchin
Important structuralHalichondria paniceaBreadcrumb sponge
Important structuralMorchellium argusA sea squirt
Important structuralNemertesia ramosaA hydroid

Physical Pressures

 IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
High High Moderate Major decline Moderate

Removal of the substratum will result in removal of all the sessile species, together with most of the slow mobile species (crustaceans, sea urchins and starfish) and an intolerance of high has been recorded.
Recoverability will depend on recruitment from neighbouring communities and subsequent recovery of the original abundance of species, which may take many years, especially in slow growing sponges and Anthozoa. Therefore, a recoverability of high has been recorded (see additional information below).

High High Moderate Decline Moderate

Although, overhangs and vertical surfaces are unlikely to suffer from smothering, this biotope occurs on steep slopes and the sides of boulders, which may collect sediment. Smothering by 5cm of sediment will prevent feeding and reduce growth and reproduction, interfere with respiration and potentially cause localised anoxia, and interfere with larval settlement. Tall erect species, e.g. Nemertesia antennina or large Alcyonium digitatum may escape the smothering due to their size, while some hydroids may survive as dormant stages. However, the dominant Bugula species, encrusting sponge species and ascidians are likely to be damaged or killed by smothering. Therefore, an intolerance of high has been recorded. A recoverability of high has been suggested (see additional information below).

Low Very high Very Low Minor decline Low

Suspension feeding organisms may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. Bryozoan turfs form preferentially on steep surfaces and under overhangs and 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 intolerant.

This biotope occurs in moderately wave exposed sites with moderately strong to 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, which may be important for species with several generations per year (e.g. Bugula spp.). The biotope also occurs extensively in areas subject to high levels of suspended sediment, e.g. North Devon (Keith Hiscock pers comm.). However, in areas of weak water flow and increased depth (reduced effects of wave action), an increase in suspended sediment increase siltation to the detriment of several members of the community, especially the bryozoans. Therefore an intolerance of low has been recorded for most examples of the biotope. Recovery is likely to be rapid (see additional information below).

Low High Moderate Not relevant

A decrease in suspended sediment may decrease food availability for the duration of the benchmark (one month) but otherwise not adversely affect the biotope. Therefore, an intolerance of low has been recorded.

Not relevant Not relevant Not relevant Not relevant Not relevant

Bryozoans, sponges, soft corals, and hydroids are probably highly intolerant of desiccation. However, this biotope is circalittoral, occurring below 5-10m depth and possibly to great depths and unlikely to be exposed to the air and desiccation.

Not relevant Not relevant Not relevant Not relevant Not relevant

An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).

Not sensitive* Not relevant

An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).

Intermediate High Low Decline Low

The abundance of bryozoans is positively correlated with supply of stable hard substrata and hence with current strength (Eggleston, 1972b; Ryland, 1976). The community stability and diversity also requires stable substrata (Osman, 1977; Dyrynda, 1994). 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, e.g. Okamura (1984) reported that an increase in water flow from slow flow (1-2cm/s) to fast flow (10-12cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera.
This biotope occurs in moderately strong to weak tidal streams and moderate wave exposure. The oscillatory water movement caused by wave action is probably of greater importance in sites with only weak tidal streams. However, an increase in water flow from moderately strong to very 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, Actinothoe sphyrodeta, 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 and Ascidia mentula may be excluded (see Hiscock, 1983). A proportion of the Bugula colonies may also be damaged or removed by very strong currents. In addition, very strong water flow may interfere with larval settlement and recruitment. Therefore, an intolerance of intermediate has been recorded. Loss of several intolerant species is likely to reduce species richness. Recovery is likely to be rapid (see additional information below).

High High Intermediate Decline Low

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. A decrease in water flow rates in the proximity of sediment is likely to result in greater siltation (see above). Most hydroids utilize a narrow range of water flow rates for effective feeding, and feeding efficiency decreasing a high water flow rates (Gili & Hughes, 1995). Similarly, water flow rates affect filter feeding efficiency in bryozoans, the exact preferred ranges depending on species, e.g. Okamura (1984) reported that an increase in water flow from slow flow (1-2cm/s) to fast flow (10-12cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera.
A decrease in water flow, e.g. from moderately strong to very weak or negligible will probably result in impaired growth and reproduction of suspension feeders due to a reduction in food availability, an increased risk of siltation (see above) and encourage colonization by other species of hydroids, ascidians, sponges and anemones, resulting in significant changes in the community and possibly the loss of the dominant hydroid/ bryozoans turf. For example, species of Bugula, Tubularia indivisa, Actinothoe sphyrodeta may be lost, while the remaining species may increase in abundance. Therefore, an intolerance of high has been recorded as the biotope is likely to change. Recoverability is likely to be rapid as many of the characteristic species would remain in reduced abundance and could re-colonize rapidly (see additional information below).

Low Very high Very Low Minor decline 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. For example, Bugula turbinata is a predominantly southern species in British water (Lewis, 1964; Hayward & Ryland, 1998) but has been recorded as far north as Shetland. A long term increase in temperature may increase its abundance in northern British waters and allow the species to extend its range. Similarly, the sponges Clathrina coriacea occurs from the Arctic to South Africa, Pachymatisma johnstonia occurs south to Spain, while Haliclona oculata is widespread. Asterias rubens and Echinus esculentus are probably intolerance of short term increases in temperature at the benchmark level.

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). Therefore, an intolerance of low has been recorded to represent the effects of temperature on growth and reproduction in many species.

Low Very high Moderate Minor decline Low

Temperature is a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). 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. However, Bugula turbinata and Bugula plumosa are predominantly southern species extending in range to the Mediterranean (Lewis, 1964; Hayward & Ryland, 1998). A long term decrease in temperature may reduce their extent in British waters, probably by interfering with growth and reproduction. They will probably be replaced by more northern species (perhaps Bugula flabellata or Bugula purpurotincta) and therefore not change the biotope. Therefore, an intolerance of low has been recorded to represent the effects of temperature on growth and reproduction in many species and changes in species composition.

Low Very high Very Low Minor decline Low

An increase in turbidity is likely to result in a decrease in phytoplankton and macroalgal primary production, which may reduce food available to the suspension feeders within the community. As a result, growth rates and reproduction may be decreased, and some species may not be able to keep up with predation (e.g. see Gaulin et al., 1986). Similarly, rapid growing bryozoans with multiple generation per year (e.g. Bugula species and Bicellaria ciliata) probably have relatively high food demands and their growth and reproduction may be impaired, as would their ability to cope with predation. Therefore, increased turbidity may result in loss of condition and reduced growth rates but no mortality, so an intolerance of low has been recorded. Recovery is likely to be rapid (see additional information below).

Intermediate High Low Minor decline Low

An decrease in turbidity may increase phytoplankton and hence zooplankton productivity and potentially increase food availability. Increased light penetration may allow macroalgae to colonize deeper water. Macroalgae effectively compete for space and grow over and may smother fauna. Hydroid and bryozoan communities in the infralittoral tend to occupy steep or vertical surfaces, while macroalgae dominate horizontal, flat surfaces (Hartnoll, 1983, 1998). Therefore, decreased turbidity may allow macroalgae to colonize the more shallow examples of this biotope, resulting in loss of a proportion of the biotope, although some members of the community are likely to survive even in the presence of macroalgae. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

Intermediate High Low Minor decline Low

This biotope occurs in moderately wave exposed habitats. The oscillatory flow generated by wave action is potentially more damaging than unidirectional flow but is attenuated with depth (Hiscock, 1983). Many of the 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 Esperiopsis fucorum, and the hydroids Tubularia indivisa Sertularia argentea. 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 is likely to decrease sea urchin and starfish predation, allowing larger, massive species (e.g. sponges, Alcyonium digitatum, anemones and ascidians) 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.
Therefore, 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, and hence fewer bryozoans. Therefore, a proportion of the biotope is likely to be lost or changed and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

High High Intermediate Decline Low

The moderately strong currents and tidal streams in this biotope are probably more important for water movement than wave induced oscillatory flow. Where the tidal streams are weak, the biotope is likely to be more intolerant. A decrease in wave action from e.g. moderately exposed to very sheltered may allow more delicate species, such as Nemertesia ramosa, ascidians and sponges to increase in abundance. However, species adapted to strong water movement may be reduced in abundance or lost e.g. Tubularia indivisa, Flustra foliacea, Pachymatisma johnstonia and Actinothoe sphyrodeta. However, species more tolerant of sheltered conditions may increase in abundance e.g. Caryophyllia smithii (see Hiscock, 1983). Reduced wave action may also result in an increase in sea urchin predation and hence increased patchiness and species richness (Sebens, 1985; Hartnoll, 1998).
Overall, the dominant bryozoans in this biotope will probably not be adversely affected by a decrease in wave exposure in moderately strong currents but examples of the biotope in weak currents are likely to be more intolerant due to the overall decrease in water movement (see water flow). In addition, the decreased wave action is likely to result in an increase in starfish and sea urchin predation. Therefore, an intolerance of high has been recorded. Recovery of the full community may take many years (see additional information below).

Tolerant Not relevant Not relevant Not relevant Not relevant

Hydroids, bryozoans, sponges and ascidians are unlikely to be sensitive to noise or vibration at the benchmark level. Mobile fish 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 Not relevant Low

Hydroid and bryozoan polyps or barnacle cirri may retract when shaded by potential predators, however the community is unlikely to be affected by visual presence. Mobile fish species may be temporarily scared away from the areas but few if any adverse effects on the biotope are likely to result.

Intermediate High Low Decline Moderate

Erect epifaunal species are particularly vulnerable to physical disturbance. Hydroids and bryozoans are likely to be removed or damaged by bottom trawling or dredging (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Hydroid and bryozoan matrices were reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998 and references therein). The removal of rocks or boulders to which species are attached by the passage of mobile fishing gears (Bullimore, 1985; Jennings & Kaiser, 1998) results in substratum loss (see above). Magorrian & Service (1998) reported that queen scallop trawling removed emergent epifauna from horse mussel beds in Strangford Lough. They suggested that the emergent epifauna such as Alcyonium digitatum were more sensitive than the horse mussels themselves and reflected early signs of damage. However, Alcyonium digitatum is more abundant on high fishing effort grounds suggests that this seemingly fragile species is more resistant to abrasive disturbance than might be assumed (Bradshaw et al., 2000), presumably owing to good recovery due to its ability to replace senescent cells, regenerate of damaged tissue and early larval colonization of available substrata. Species with fragile tests that occur in the biotope such as Echinus esculentus and the brittlestar Ophiocomina nigra and edible crabs Cancer pagurus were reported to suffer badly from the impact of a passing scallop dredge (Bradshaw et al., 2000). Scavengers such as Asterias rubens and Buccinum undatum were reported to be fairly robust to encounters with trawls (Kaiser & Spencer, 1995) may benefit in the short term, feeding on species damaged or killed by passing dredges. However, Veale et al. (2000) did not detect any net benefit at the population level.

Overall, physical disturbance by mobile fishing gear is likely to remove a proportion of all groups within the community and attract scavengers to the community in the short term. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high due to repair and regrowth of hydroids and bryozoans and recruitment within the community from surviving colonies and individuals (see additional information below). Severe physical disturbance will be similar in effect to substratum loss (see above).

High High Moderate Major decline Low

Most sessile species, such as bryozoans (e.g. Bugula species), sponges (e.g. Halichondria panicea), ascidians (e.g. Clavelina lepadiformis) and hydroids (e.g. Nemertesia species) can not reattach to the substratum if removed, and may be damaged or destroyed in the process. Hydroids and sponges may be able to grow and reattach from fragments, aiding recovery. Mobile species, such as amphipods, gastropods, small crustaceans, crabs and fish are likely to survive displacement. Anemones (e.g. Actinothoe sphyrodeta) are strongly but not permanently attached and will probably reattach to suitable substrata. However, the dominant bryozoans and hydroids are likely to be lost and an intolerance of high has been recorded. Recovery of the full community is likely to take many years and a recoverability of high has been recorded (see additional information below).

Chemical Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
High High Moderate Major decline Low

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. Moran & Grant (1993) reported that settlement of marine fouling species, including Bugula neritina was significantly reduced in Port Kembla Harbour, Australia, exposed to high levels of cyanide, ammonia and phenolics.

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, 1968). Possible sub-lethal effects of exposure to synthetic chemicals, may result in a change in morphology, growth rate or disruption of reproductive cycle. Smith (1968) also noted that large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus.

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 suggests that epifaunal species (including, bryozoan, hydroids and ascidians) may be at least inhibited by the presence of TBT.

Therefore, hydroids crustaceans, gastropods, and ascidians are probably intolerant of TBT contamination while bryozoans are probably intolerant of chemical pollution and 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
Low Very high Very Low Minor decline 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). However, Bugula neritina was reported to survive but not grow exposed to ionic Cu concentrations of 0.2-0.3 ppm (larvae died above 0.3ppm) but die where the surface leaching rate of Cu exceeded 10µg Cu/cm²/day (Ryland, 1967; Soule & Soule, 1979). Ryland (1967) also noted that Bugula neritina was less intolerant of Hg than Cu. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton and their tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999). Waters containing 25 µg / l Cu caused developmental disturbances in Echinus esculentus (Kinne, 1984) and heavy metals caused reproductive anomalies in the starfish Asterias rubens (Besten, et al., 1989, 1991). Sea urchin larvae have been used in toxicity testing and as a sensitive assay for water quality (reviewed by Dinnel et al. 1988), so that echinoderms are probably intolerant of a heavy metal contamination. 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).
Overall, the dominant bryozoans may be tolerant and hydroids manifest only sublethal effects. The sea urchin Echinus esculentus is probably highly intolerant of heavy metal contamination. Heavy metals contamination may, therefore, reduce reproduction and recruitment in starfish and sea urchins, potentially reducing predation pressure in the biotope. Therefore, an intolerance of low has been recorded to represent the sublethal effects on dominant bryozoans and hydroids. Loss of predatory sea urchins, may result in an increased dominance by some species and a slight decrease in species richness.

Hydrocarbon contamination
High High Moderate Major 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. For example:

  • 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).
  • Laboratory studies of the effects of oil and dispersants on several red algal species concluded that they were all sensitive to oil/dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages (O'Brien & Dixon, 1976; Grandy, 1984, cited in 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).
  • Amphipods, especially ampeliscid amphipods, are regarded as especially sensitive to oil (Suchanek, 1993).
  • Smith (1968) reported dead colonies of Alcyonium digitatum at depth in the locality of Sennen Cove (Pedu-men-du, Cornwall) resulting from the combination of wave exposure and heavy spraying of dispersants sprayed 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).

If the physiology within different animals groups can be assumed to be similar, then bryozoans, amphipods, 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. Therefore, the dominant bryozoans and several members of the community may be lost or damaged as a result of acute hydrocarbon contamination, and an intolerant of high 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

Insufficient
information found.

Changes in nutrient levels
Intermediate High Low 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 turf especially in its shallower examples, although increased turbidity due to phytoplankton abundance may offset the effect of nutrient enrichment (Hartnoll, 1998). Moderate nutrient enrichment may increase the food available to the community in the form of phytoplankton, zooplankton or organic particulates. However, eutrophication may result in deoxygenation (see below) or algal blooms. While 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).

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, 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).

This biotope occurs in areas subject to moderately strong tidal streams, so that prolonged deoxygenation is unlikely to occur. However, the potential toxic effects of the algal blooms and the siltation caused by death of an algal bloom may result in loss of several members of the community, especially ascidians, and 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

This biotope occurs in full salinity and is unlikely to encounter variation in salinity.

High High Intermediate Not relevant

Although circalittoral, shallow examples of the biotope may be affected by variable or reduced salinity, resulting, for example, from hyposaline effluents. Several of the species identified as indicative of intolerance may be of 'intermediate' or 'high' intolerance to a reduction in salinity. Ryland (1970) stated that, with a few exceptions, the Gymnolaemata were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna.

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.

Echinoderms are generally unable to tolerate low salinity (i.e. they are stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). At low salinity e.g. sea urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. Although, local adaptation to reduced salinity may occur (see Stickle & Diehl, 1987), the inability of echinoderms to osmoregulate makes them intolerant of short term acute or chronic long term reductions in salinity, e.g., a sudden inflow of river water into an inshore coastal area caused mass mortality of the Asterias vulgaris at Prince Edward Island, Canada (Smith, 1940, in Lawrence, 1995).

 

Overall, the majority of the species in the community are likely to be intolerant of a reduction in salinity, resulting in loss off many species and an impoverished community. The biotope would probably be replaced by reduced salinity epifaunal communities (e.g. the estuarine biotope A3.363) and therefore lost. Hence, an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).

Intermediate High Low Major decline Very low

Little information on the effects of oxygenation on bryozoans was found. 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, and suggested that the amphipods Gammarus tigrinus and Ampelisca agassizi were intolerant of hypoxia. Echinoderms are probably intolerant of hypoxia (see reviews). 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 water movement (see water flow). Although evidence is limited, the dependence of this community (especially bryozoans and hydroids) on water movement suggests a high dependence on a turnover of nutrients and oxygen. 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, and an intolerance of intermediate has been recorded, albeit with very low confidence. Recoverability is likely to be rapid (see additional information below).

Biological Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
Intermediate High Low Minor decline Low

Epizooics were shown to reduce growth rates in Flustra foliacea (Stebbing, 1971a) and may have similar effects on other bryozoans. 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. However, sea urchin populations have 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 bryozoans faunal in circalittoral faunal turfs (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 massive growth forms, e.g. ascidians, sponges and anemones, resulting in a loss of at least a proportion of the biotope as described. Therefore, an intolerance of intermediate has been recorded, albeit at very low confidence. Once predation and grazing pressure return to prior levels recovery will probably be rapid.

No information Not relevant No information Insufficient
information
Not relevant

No information found.

Low Very high Very Low No change Low

Although, not subject to extraction at present, many bryozoans have been recently found to contain pharmacologically active substances (Hayward & Ryland, 1998). Therefore, species of Bugula and other bryozoans may be subject to harvesting in the future. Few species present within this biotope are known to be subject to extraction or harvesting. However, the use of mobile fishing gear in the vicinity of the biotope, such as scallop dredges and beam trawls result in physical disturbance to the sediment surface, and an increase in suspended sediment (Hartnoll, 1998). Furthermore, potting and fixed netting (their placement and collection) for crabs, crayfish and lobster would probably result in abrasion and physical disturbance although this has been dealt with in 'Physical Disturbance' above. Echinus esculentus has been collected by diving in the past (Nichols, 1984). The loss of functionally important predators such as sea urchins, and to a lesser extent crabs and lobster may affect community structure (see microbial pathogens, Sebens, 1985; Hartnoll, 1998). However, other important functional species such as Asterias rubens are likely to remain and a low intolerance has been suggested.

Intermediate High Low Minor decline Low

Additional information

Recoverability
The majority of the species within this biotope have short lived pelagic larvae, with limited powers of dispersal, resulting in good local recruitment but poor long distance dispersal (see recruitment). Exceptions include, mobile crustaceans and echinoderms with long-lived planktonic larvae, and Nemertesia antennina and Alcyonium digitatum which can probably disperse up to 50 m or over 100 km respectively (Hughes, 1977; Hartnoll, 1998).

Bugula and other bryozoan and hydroid species exhibit multiple generations per year (see recruitment), that involve good local recruitment, rapid growth and reproduction. Bryozoans and hydroids are often opportunistic, fouling species, that colonize and occur space rapidly. For example, hydroids would probably colonize with 1-3 months and return to their original cover rapidly, while Bugula species have been reported to colonize new habitats within 6 -12 months (see recruitment). However, Bugula has been noted to be absent form available habitat even when large populations are nearby (Castric-Frey, 1974; Keough & Chernoff, 1987), suggesting that recruitment may be more sporadic.

Where the population is reduced in extent or abundance but individuals remain, local recruitment, augmented by dormant resistant stages and asexual reproduction, is likely to result in rapid recovery of the dominant bryozoan species, hydroids, probably within 12 months. Colonial ascidians would probably recover their original cover with two years, while sponges and anemones may take longer to recover but would probably regain original cover within less than five years (see 'time for community to reach maturity').

Where the community was destroyed and recovery is dependant on recruitment from other areas, bryozoans, hydroids and ascidians would probably recruit rapidly from other neighbouring areas (see Jensen et al., 1994; Hatcher, 1998). However, sponges and especially Anthozoa may take many years to recruit and develop. In addition, recruitment of rare and scarce species, where they occur, such as Leptopsammia pruvoti, Hoplangia durotrix, and Alcyonium hibernicum, is likely to take a very long time, e.g. up to 25-30 years or not occur at all in Leptopsammia pruvoti (see MarLIN review).

Therefore, studies of settlement and the effects of disturbance suggest rapid colonization, so that the biotope would be restored and recognisable within 5 years. But the species richness and a full community may take 5-10 years to recover, depending on local conditions. Communities isolated by distance from reproductive populations by geography or hydrography may take longer to develop. The recovery of any rare and scarce species with infrequent recruitment, e.g. Leptopsammia pruvoti and Hoplangia durotrix is likely to be very low (Keith Hiscock pers comm.).

Importance review

Policy/Legislation

- no data -

Exploitation

Few species present in this biotope are subject to exploitation. Many bryozoans have been recently found to contain pharmacologically active substances (Hayward & Ryland, 1998). Therefore, Bugula species and other bryozoans may be subject to harvesting in the future.

Crabs, crawfish and lobster may be subject to harvesting by pots, creels, or fixed bottom-set nets in circalittoral faunal biotopes (Hartnoll, 1998).

Additional information

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Citation

This review can be cited as:

Tyler-Walters, H., 2002. Bugula spp. and other bryozoans on vertical moderately 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/105

Last Updated: 20/05/2002