Biodiversity & Conservation

The major ecological relationships are between space occupiers and their predators. Space for colonization may be freed by physical factors such as storms or by biological factors such as predation. Ecological relationships important for the function of the community are given below. The following information has been summarised from studies by Suchanek (1985), Tsuchiya & Nishihira (1985 & 1986), Seed & Suchanek (1992) and Holt et al. (1998) to which the reader should refer for further details. Please note that recent evidence suggests that the Mytilus edulis communities studied by Suchanek 1985 and Tsuchiya & Nishihira (1985 & 1986) were probably Mytilus trossulus and Mytilus galloprovincialis respectively (Seed, 1992), although their community ecology is probably similar.

Competition Mytilus edulis is a active suspension feeder on phytoplankton, organic particulates and dissolved organic matter, which probably significantly reduces the suspended particulate food (seston) available to other suspension feeders in the biotope.

Other suspension feeders include the surrounding barnacles, tube worms (e.g. Pomatoceros spp.), hydroids (e.g. Obelia geniculata), bryozoans (e.g. Electra pilosa) and interstitial bivalves such as Lasaea adansoni.

Mytilus edulis competes for space with other species such as barnacles and fucoids.

Where present the biogenic mud under the bed support deposit feeders or detritivores such as polychaetes (e.g. Cirratulus cirratus and terebellids).

Predation and herbivory The macroalgae (e.g. Fucus vesiculosus, Mastocarpus stellatus, and Ceramium spp.) provide primary production to the community and the surrounding ecosystem directly to grazers, or indirectly in the form of abraded algal particulates and detritus, algal spores, algal exudates and dissolved organic matter. The macroalgae support mesoherbivores such as gammarid amphipods and isopods feeding on algal epiphytes and macroalgae (Brawley, 1992b; Tsuchiya & Nishihira, 1985, 1986).

Epifloral/faunal grazers, such as chitons, limpets, littorinids (e.g. Littorina littorea, Littorina saxatilis, and Littorina obtusata) feed within and around the mussel bed, grazing on benthic microalgae and macroalgae (sporeling and adult plants), and bulldozing newly settled invertebrate larvae (Hawkins & Hartnoll, 1983).

Grazers have been shown to reduce excessive fouling by epifauna and large macroalgae, and encourage recovery from disturbance in intertidal Mytilus californianus populations (Suchanek, 1985; Seed & Suchanek, 1992). This biotope is characterized by the presence of macroalgae and gastropod and mesoherbivore grazing probably prevents the algae and epifauna smothering the mussel bed, although the patches of mussels provide a refuge for the macroalgae from the intense grazing by limpets on the surrounding substratum.

Predation is the single most important source of mortality in Mytilus edulis populations (Seed & Suchanek, 1992; Holt et al., 1998). Many predators target specific sizes of mussels and, therefore influence population size structure. For example, Carcinus maenas was unable to consume mussels of ca. 70mm in length and mussels >45mm long were probably safe from attack (Davies et al., 1980; Holt et al., 1998).

The lower limit of intertidal mussel populations may be limited by predation by starfish (e.g. Asterias rubens), Carcinus maenas and the dog whelk Nucella lapillus, although dog whelk predation is of more importance in sheltered sites (Holt et al., 1998).

Flatfish such as Platichthys flesus (plaice), Pleuronectes platessa (flounder) and Limanda limanda (dab), where present, feed on mussels.

Birds are important predators of mussels, and oystercatchers, herring gulls, eider ducks and knot have been reported to be major sources of Mytilus edulis mortality. For example, in the Ythan estuary bird predation accounted for 72% of mussel production, with oystercatchers and herring gulls being each responsible for 15% and mussels are regarded as a staple food of oystercatchers (Dare, 1976; Holt et al., 1998). Although, probably of greatest importance in sedimentary habitats, bird predation, probably significantly affects the population dynamics of intertidal mussel beds.

Scavengers probably feed on dead mussels within the matrix, e.g. flatworms, small crabs and polychaetes (Kautsky, 1981; Tsuchiya & Nishihira, 1985,1986), while other polychaetes (e.g. scale worms), small crabs and nemerteans and predatory within the matrix.

Little season change in the dominant species is likely. Mytilus edulis spawns in spring and summer and in some areas again in last August and September, with settlement occurring 1-4 weeks later. However, while recruitment can be annual, it is often sporadic and unpredictable (see below). The species richness of the macro-invertebrate fauna associated with mussel patches was shown to fluctuate seasonally, probably reflecting random fluctuations in settlement and mortality typical of marine species with planktonic larvae (see Seed, 1996 for discussion). Ephemeral green algae may show a peak in abundance during the spring.

Winter storms can result in gaps forming in the mussel bed, especially where the mussels are fouling by macroalgae or epifauna, due to wave action and drag, or direct impact by wave driven debris, e.g. logs (Seed & Suchanek, 1992). Winter storms will also reduce or damage fucoids and macroalgal cover (e.g. Mastocarpus stellatus). Crab and fish tend to move to deeper water in the winter months, so that predation is probably reduced. Macroalgae probably exhibit minimal cover in winter, growing back in spring and reaching maximum cover in summer. Seed (1996) reported that the invertebrate communities within mussel patches exhibit significant temporal and small-scale spatial variations in diversity and abundance, that probably reflect the stochastic nature of larval recruitment and settlement.

Holt et al. (1998) suggested that moderately exposed mussel beds on rock surfaces could be relatively stable and long-lived. In the intertidal, low shore beds probably consist of young individual mussels, due to the intense predation due to starfish and crabs, with few surviving to their second or third year. However, predation pressure decreases with increasing height up the shore, so that mid-shore or high shore populations may consist of a twenty or more year classes, although their growth rates and hence size were much reduced (Seed & Suchanek, 1992). Seed & Suchanek (1992) suggested that although mussel assemblages found in the upper intertidal or most sheltered sites, experience the least change per unit time, and may be considered more 'stable' (Lewis, 1977), if disturbed, these assemblages would recover much slower than lower intertidal and more exposed sites.

The biotope consists of bedrock supporting large patches or a band of large, dense Mytilus edulis, which themselves support fucoids and a few red algae (Connor et al., 1997b). The large patches of Mytilus edulis and barnacle covered substratum denote areas of different habitat complexity and species richness. Patches (or 'islands') of mussels support a diverse community (see Suchanek,1985; Tsuchiya & Nishihira, 1985, 1986) whereas the interstices of barnacles provide shelter for small species (Barnes, 2000). The habitat complexity and species diversity of the shore depending on the relative abundance of mussel and barnacles, the presence of macroalgae and crevices.

Rock surface between mussel patches

• The rock surface may provide a complex of upward facing, overlapping and fissured habitats.
• The upward facing surfaces are likely to be colonized by limpets, barnacles and fucoids. Barnacles form a tightly packed covering over the substratum excluding other species. Dead barnacles leave gaps in the covering that can be exploited by small invertebrates. Small interstitial species occupy relatively stable microclimates in-between barnacles or in dead barnacles shells, including the small littorinids Littorina neglecta and Littorina saxatilis, the bivalve Lasaea adansoni, intertidal mites, amphipods and isopods (see Barnes, 2000 for review).
• Red foliose algae and encrusting coralline algae may be present, especially in the shelter of fucoids.
• Overhanging surfaces and fissures provide local shelter and may be colonized by sponges, hydroids and barnacles.
• The periphery of the mussel patches and beds attract feeding dog whelks Nucella lapillus.
• Wave sheltered large crevices and gullies provide refuges for dog whelks and littorinids, while crevices provide refuges for predatory nemerteans and polychaetes (e.g. Eulalia viridis).

Patches or bands of mussels
Holt et al. (1998) noted that a raised bed was not present in this biotope and most associated organisms were capable of growing on the substratum in the absence of Mytilus edulis. The mussels bed can be divided into three distinct habitat components: the interstices within the mussel matrix; the biodeposits beneath the bed; and the substratum afforded by the mussel shells themselves (Suchanek, 1985; Seed & Suchanek, 1992). Although, the beds in this biotope are probably mostly composed of only a single layer of mussels, Tsuchiya & Nishihira (1985,1986) demonstrated that old large mussels in Japan and mussel patches accumulated biogenic sediments and shell fragments. Please note that recent evidence suggests that the Mytilus edulis communities studied by Suchanek 1985 and Tsuchiya & Nishihira (1985 & 1986) were probably Mytilus trossulus and Mytilus galloprovincialis respectively (Seed, 1992), although their community structure is probably similar.

• The interstices between the mussels provide refuge from predation, and provide a humid environment protected from wave action, desiccation, and extremes of temperature. In the intertidal, Mytilus sp. beds the species richness and diversity increases with the age and size of the bed (Suchanek, 1985; Tsuchiya & Nishihira, 1985,1986; Seed & Suchanek, 1992). The mussel matrix may support sea cucumbers, anemones, boring clionid sponges, ascidians, crabs, nemerteans, errant polychaetes and flatworms (Suchanek, 1985; Tsuchiya & Nishihira, 1985,1986).
• Mussel faeces and pseudo-faeces, together with silt, build up organic biodeposits under the beds, which support infauna such as sediment dwelling sipunculids, polychaetes and ophiuroids (Suchanek, 1978; Seed & Suchanek, 1992, Tsuchiya & Nishihira, 1985,1986). Flushing by wave action may prevent the build up of the thick layer of biodeposits found in Mytilus reefs.
• Mytilus edulis can use its prehensile foot to clean fouling organisms from its shell (Theisen, 1972). Therefore, the epizoan flora and fauna is probably less developed or diverse than found in beds of other mussel species but may include barnacles (e.g. Elminius modestus) and tubeworms (e.g. Pomatoceros species)
• Mobile epifauna include isopods, chitons (e.g. Lepidochitona cinerea) and gastropods such as littorinids (e.g. Littorina littorea) and top shells (e.g. Gibbula umbilicalis), which obtain refuge from predators, especially birds, within the mussel matrix, emerging at high tide to forage (Suchanek, 1985; Seed & Suchanek, 1992).
• Intense growth may occasionally give rise to hummocks in which the bed is raised off the surface and the available space colonized by small crabs and dog whelks (Seed & Suchanek, 1992; Davenport et al., 1998).
• The mussels provide a substratum for the attachment of macroalgae such as Fucus vesiculosus and to a lesser extent other fucoids, foliose and filamentous algae e.g. Ceramium species, Mastocarpus stellatus and Palmaria palmata.

Fucoids and other macroalgae

Where macroalgae are present the community also supports small crustaceans such as gammarid amphipods (e.g. Hyale prevosti) and isopods (e.g. Idotea granulosa) and gastropods (e.g. Littorina obtusata and Littorina saxatilis) (Tsuchiya & Nishihira, 1985,1986; Seed & Suchanek, 1992; JNCC, 1999). Ephemeral algae such as Ulva spp. and Ulva lactuca may also grow on the mussels themselves.

Rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al., 1998). Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.

Macroalgae such as Fucus vesiculosus are primary producers of organic carbon, which is utilized directly by grazing invertebrates. Raffaelli & Hawkins (1999) reported an estimate of the productivity of intertidal fucoids as 160 gC/m²/year in moderately wave exposed habitats. Only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1999). Macroalgae exude considerable amounts of dissolved organic carbon, which is taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea and may enter the food chain of local, subtidal ecosystems, or be exported further offshore.

Mytilus spp. communities are highly productive secondary producers (Seed & Suchanek, 1992; Holt et al., 1998). Low shore mussels were reported to grow 3.5-4cm in 30 weeks and up to 6-8cm in length in 2 years under favourable conditions, although high shore mussels may only reach 2-3cm in length after 15-20 years (Seed, 1976). Seed & Suchanek (1992) suggested that in populations of older mussels, productivity may be in the region of 2000-14,500 kJ/m²/yr. In Killary Harbour, western Ireland, the shore population of mussels contributed significantly to the larval population of the inlet. Kautsky (1981) reported that the release of mussel eggs and larvae from subtidal beds in the Baltic Sea contributed an annual input of 600 tons of organic carbon/yr. to the pelagic system. The eggs and larvae were probably an important food source for herring larvae and other zooplankton. Dense beds of bivalve suspension feeders increase turnover of nutrients and organic carbon in estuarine (and presumably coastal) environments by effectively transferring pelagic phytoplanktonic primary production to secondary production (pelagic-benthic coupling) (Dame, 1996). The Mytilus edulis beds probably also provide secondary productivity in the form of tissue, faeces and pseudofaeces (Seed & Suchanek, 1992; Holt et al., 1998)

• Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Jørgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark but Lutz & Kennish (1992) suggested that larval mortality was approximately 99%. Larval mortality is probably due to adverse environmental conditions, especially temperature, inadequate food supply (fluctuations in phytoplankton populations), inhalation by suspension feeding adult mytilids, difficulty in finding suitable substrata and predation (Lutz & Kennish, 1992). Widdows (1991) suggested that any environmental factor that increased development time, or the time between fertilization and settlement would increase larval mortality.
• Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in-between individual mussels of established populations. Persistent mussels beds can be maintained by relatively low levels of recruitment e.g. McGrorty et al., (1990) reported that adult populations were largely unaffected by large variations in spat fall between 1976-1983 in the Exe estuary.
• The Mytilus edulis bed may act as a refuge for larvae or juveniles, however, the intense suspension feeding activity of the mussels is likely to consume large numbers of pelagic larvae. Commito (1987) suggested that species that reproduce with cocoons, brood their young or disperse as juveniles will be favoured (see gastropods below).
• While Asterias rubens, for example, is widespread, and fecund, with a pelagic larvae capable of widespread dispersal, recruitment in starfish is sporadic, unpredictable and poorly understood (Seed, 1993).
• Barnacles such as Semibalanus balanoides have a planktonic nauplii larvae, which spends ca 2 months in the plankton, with high dispersal potential. Peak settlement in Semibalanus balanoides occurs in April-May in the west and May-June in the east and north of the British Isles, However, settlement intensity is variable, subsequent recruitment is inhibited by the sweeping action of macroalgal canopies (e.g. fucoids) or the bulldozing of limpets and other gastropods (see MarLIN review for details).
• The propagules of most macroalgae tend to settle near the parent plant (Schiel & Foster, 1986; Norton, 1992; Holt et al., 1997). For example, the propagules of fucales are large and sink readily and red algal spores and gametes and immotile. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition (zygotes or spores being thrown against the substratum). For example, spores of Ulva spp. have been reported to travel 35km, Phycodrys rubens 5km and Sargassum muticum up to 1km, although most Sargassum muticum spores settle within 2m. The reach of the furthest propagule and useful dispersal range are not the same thing and recruitment usually occurs on a local scale, typically within 10m of the parent plant (Norton, 1992). Vadas et al. (1992) noted that post-settlement mortality of algal propagules and early germlings was high, primarily due to grazing, canopy and turf effects, water movement and desiccation (in the intertidal) and concluded that algal recruitment was highly variable and sporadic. However, macroalgae are highly fecund and widespread in the coastal zone so that recruitment may be still be rapid, especially in the rapid growing ephemeral species such as Ulva spp. and Ulva lactuca, which reproduce throughout the year with a peak in summer. Similarly, Ceramium species produce reproductive propagules throughout the year, while Mastocarpus stellatusproduce propagules form February to December, and exhibit distinct reproductive papillae in summer (Dixon & Irvine, 1977; Burrows, 1991; Maggs & Hommersand, 1993).
• Gastropods exhibit a variety of reproductive life cycles. The common limpet Patella vulgata, the top shell Gibbula umbilicalis, and Littorina littorea have pelagic larvae with a high dispersal potential, although recruitment and settlement is probably variable. However, Littorina obtusata lays its eggs on the fronds of fucoids form which hatch crawl-away miniature adults. Similarly, the dog whelk Nucella lapillus lays egg capsules on hard substrata in damp places on the shore, from which crawl-aways emerge. Therefore, their dispersal potential is limited but probably designed to colonize an abundant food source. In addition, most gastropods are relatively mobile, so that a large proportion of recruitment of available niches within a mussel bed would involve migration. Nucella lapillus is an exception, as they generally do not move far, averaging 100mm /tidal cycle, or between 30cm or 10m per year when in the vicinity of an abundant food source (see MarLIN reviews for details; Fish & Fish, 1996).
• Many species of mobile epifauna, such as polychaetes that may be associated with patches of mussels or rock crevices, have long lived pelagic larvae and/or are highly motile as adults. Gammarid amphipods brood their embryos and offspring but are highly mobile as adults and probably capable of colonizing new habitats from the surrounding area (e.g. see Hyale prevosti review).

Development of the community from bare or denuded rock is likely to follow a similar succession to that occurring after an oil spill. The loss of grazing species results in an initial proliferation of ephemeral green then fucoid algae, which then attracts mobile grazers, and encourages settlement of other grazers. Limpet grazing reduces the abundance of fucoids allowing barnacles to colonize the shore. Recovery of rocky shore populations was intensively studied after the Torrey Canyon oil spill in March 1967. Areas affected by oil alone recovered rapidly, within 3 years. But other sites suffered substantial damage due to the spilled oil and the application of aromatic hydrocarbon based dispersants. In the latter sites, populations of fucoids were abnormal for the first 11 years, and limpet Patella vulgata populations were abnormal for at least 10-13 years. Recovery rates were dependant on local variation in recruitment and mortality so that sites varied in recovery rates, for example maximum cover of fucoids occurred within 1-3 years, barnacle abundance increased in 1-7 years, limpet number were still reduced after 6-8 years and species richness was regained in 2 to >10 years. Overall, recovery took 5-8 years on many shores but was estimated to take about 15 years on the worst affected shores (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999).

This biotope is characterized by the presence of dense Mytilus edulis. Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992). Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). The presence of macroalgae in disturbance gaps in Mytilus califorianus populations, where grazers were excluded, inhibited recovery by the mussels. In New England, U.S.A, prior barnacle cover was found to enhance recovery by Mytilus edulis (Seed & Suchanek, 1992). While good annual recruitment is possible, recovery of the mussel population may take up to 5 years. However, recovery of the mussel population may be delayed by 1-7 years for the initial macroalgal cover to reduce and barnacle cover to increase. Therefore, the biotope may take between 5 -10 years to recover depending on local conditions.

Once, the patches of mussels have returned colonization of the associated community is dependant on the development of a mussel matrix, younger beds exhibiting lower species richness and species diversity than older beds, and hence growth rates and local environmental conditions. Tsuchiya & Nishihira (1986) examined young and older patches of Mytilus (probably Mytilus galloprovincialis) in Japan. They noted that as the patches of mussels grew older, individuals increased in size, and other layers were added, increasing the space within the matrix for colonization, which also accumulated biogenic sediment. Increased space and organic sediment was then colonized by infauna and epiphytes and as the patches and mussels became older, eventually epizoic species colonized the mussel shells. Macroalgae could colonize at any time in the succession. Tsuchiya & Nishihira (1986) did not suggest a timescale.

None entered