Biodiversity & Conservation

Modiolus modiolus communities (clumps or beds) provide hard substratum in usually sedimentary areas. They accumulate a sediment of silt, organic rich faeces and pseudofaeces, and shell debris, forming raised beds, bound together by a matrix of byssus threads and horse mussels. Therefore, they significantly modify the habitat providing substratum, refuge and ecological niches for a wide variety of organisms.

Horse mussel beds support a diverse assemblage of suspension feeders, including barnacles (e.g. Balanus crenatus) and tube worms (e.g. Pomatoceros triqueter), hydroids (e.g. Sertularia argentea), anthozoans such as Alcyonium digitatum, bryozoans such as Alcyonidium mytili and Electra pilosa, ascidians (e.g. Dendrodoa grossularia), and brittlestars such as Ophiothrix fragilis and Ophiopholis aculata (Comely, 1981; Connor et al., 1997a; Holt et al., 1998).

Where present, red seaweeds and coralline algae are primary producers and are grazed by urchins, gastropods and chitons, however, fouling by algae and epifauna may be detrimental to the horse mussel bed (Witman, 1985; Holt et al., 1998).

Sea urchins, e.g. Echinus esculentus graze algae and epifauna on the horse mussel bed. Fouling organisms reduce the fitness of the horse mussels by reducing tissue weight and gametic development (Suchanek, 1985). Excessive fouling, especially by large algae such as kelp, increases drag and may result in removal of mussels by tidal streams, currents or wave action (see also Mytilus edulis) (Suchanek, 1985). Witman (1984; cited in Suchanek, 1985) noted that during 11 months of monitoring in the New England subtidal, 84% of fouled horse mussels were dislodged while 0% of un-fouled specimens were dislodged. Experimental removal of the sea urchin Strongylocentrotus droebachiensis from New England horse mussels beds, resulted in a 30 fold increase in dislodgement of horse mussels in the cleared areas due to the growth of kelps (Witman, 1984 cited in Suchanek, 1985; Holt et al., 1998). Therefore, Suchanek (1985) suggested that a facultative mutualism (sea urchins are not exclusive to mussel beds) exists between the mussel beds and sea urchins, where horse mussels benefit from the grazing activity of sea urchins, while the sea urchins benefit from the refuge from predation provided by the bed.

Witman (1985) demonstrated that the horse mussel beds in New England, USA provided a refuge from crab, lobster or fish predation for sea urchins, bivalves and brittlestars. He also showed that horse mussel beds provided a refuge from intense epifaunal grazing pressure by sea urchins. Horse mussel beds in British waters are also probably used as a refuge by similar species.

Starfish, crabs, lobsters and fish are probably significant predators on horse mussel beds. Starfish, crabs, lobsters and fish are generalists taking epifauna and horse mussels as prey (Witman, 1985). Juvenile horse mussels are subject to intense predation pressure, probably from starfish (e.g. Asterias rubens), the whelk Buccinum undatum, and crabs (e.g. Cancer pagurus) until they reach over 45-60mm in shell length (Brown & Seed, 1977; Comely, 1981; Sebens, 1985). Intense predation pressure may account for the low levels of recruitment and the bimodal population structure seen in populations of Modiolus modiolus, i.e. a peak of large individuals and a variable peak of smaller horse mussels (Brown & Seed, 1977; Holt et al., 1998). Witman (1985) noted that in New England horse mussel beds, crabs and lobsters were active at night while fish accounted for 71% of the total prey taken during the day.

The organic rich sediment that accumulates within and under the horse mussels bed supports an infauna of deposit feeders including polychaetes and holothurians (Comely, 1981; Witman, 1985; Brown & Seed, 1977; Holt et al., 1998).

Holt et al. (1998) stated that dense horse mussel beds were thought to be very stable in the long term, since they were observed in the same areas over long time periods. However, they noted that long term changes in the Modiolus modiolus population structure, and their associated community had not been studied. Ojeda & Dearborn (1989) examined the community structure of rocky intertidal habitats in Maine, USA between August 1984 and October 1986, including Modiolus modiolus beds. They noted no significant change in community biomass or density during the study period. However, species number varied with season and was maximal in summer, intermediate in autumn and spring but minimal in winter. They suggested that seasonal changes were probably due to migration or changes in activity of some of the species. In addition, it is likely that some seasonal changes occur in the abundance or extent of algal species within this biotope. Holt et al. (1998) suggest that some variation in Modiolus modiolus population structure must occur given the variable and sporadic nature of recruitment in the species.

Holt et al. (1998) suggested that most Modiolus modiolus communities consist of:

• very dense aggregations of horse mussel shells (living and dead) forming a single or multi-layered framework;
• a rich community of free living and sessile epifauna and predators;
• a very rich and diverse community of species, with low abundance, which shelters between the shells and byssus threads of the horse mussels and thrives on the rich sediment, and
• an infauna living within the rich sediment deposits built up by the bed.

Examples include:

• epibionts such as barnacles (e.g. Balanus crenatus or Balanus balanus), tubeworms (e.g. Pomatoceros triqueter), and coralline algae living on shells of Modiolus modiolus (Comely, 1981; Holt et al., 1998)
• sessile epifauna include bryozoans, hydroids, anthozoans such as Alcyonium digitatum, serpulid worms, sponges, saddle oysters and red algae such as Phycodrys rubens (Holt et al., 1998);
• mobile epifauna include whelks, starfish, sea-urchins, top shells, nudibranchs, and many decapod crustaceans (Holt et al., 1998);
• the brittlestars Ophiothrix fragilis and Ophiopholis aculata are commonly found in crevices between the horse mussels (Comely, 1981; Holt et al., 1998);
• infauna may typically include the heart urchin Spatangus purpureus, and bivalves Glycymeris sp., Astarte sulcata and Venus, spp. (Holt et al., 1998) as well as polychaetes and holothurians (Witman, 1985). The 'deep Venus community' is particularly characteristic of the biotope CMX.ModMx (Connor et al., 1997a; Holt et al., 1998), and
• demersal fish such as dragonets, small-spotted catshark (dogfish), butterfish, and sea scorpions (Holt et al., 1998).

Suspension feeding by the horse mussels and the suspension feeding community they support undoubtedly represent a significant contribution to secondary production in the benthic ecosystem (see Wildish & Fader, 1998). Filter feeding by horse mussel beds may be of great importance in channelling primary phytoplankton productivity to the benthos, termed 'benthic-pelagic' coupling. Navarro & Thompson (1997) demonstrated that Modiolus modiolus beds in Newfoundland fed on small phytoplankton but concentrated large diatoms in their pseudofaeces, and may contribute up to 40.9 mg dry weight per individual per day (faeces and pseudofaeces) hence cycling nutrients to the benthic ecosystem. Wildish & Fader (1998) reported that in the well mixed waters of the Bay of Fundy, horse mussel beds were able to feed on phytoplankton down to about 100m in depth and made a significant contribution to secondary benthic productivity (Holt et al., 1998; Wildish & Fader, 1998; Navarro & Thompson, 1997). In less well mixed areas, primary production reaches the benthos as organic particulates, detritus and dissolved organic matter. In shallow subtidal waters, or where light penetration is adequate for macroalgal growth, red algae contribute to primary productivity, and are probably utilized by a number of grazing species (see ecological relationships). However, no information on productivity levels were found.

• Recruitment in Modiolus modiolus is sporadic and highly variable seasonally, annually or with location (geographic and depth) (Holt et al., 1998). Some areas may have received little or no recruitment for several years. Even in areas of regular recruitment, such as enclosed areas, recruitment is low in comparison with other mytilids such as Mytilus edulis. For instance, in Strangford Lough small horse mussels (<10mm) represented <10% of the population, with peaks of 20-30% in good years (Brown & Seed, 1978; Figure 3). In open areas with free water movement larvae are probably swept away from the adult population, and such populations are probably not self-recruiting but dependant on recruitment from other areas, which is in turn dependant on the local hydrographic regime. In addition, surviving recruits take several to many years to reach maturity (3-8 years) (Holt et al., 1998).
• Established horse mussel beds are probably important for the recruitment (settlement and survival to maturity) of juveniles. Settlement within the byssus or mussel matrix of adults greatly increases the survival of juveniles in the face of intense predation pressure (Jones et al., 2000; Holt et al., 1998).
• Dense growth of foliose seaweeds or branching bryozoans and hydroids may provide an important settling area for bivalve spat, e.g. Pecten maximus, Chlamys spp. and Aequipecten opercularis, adults beds of which are often abundant in the vicinity of horse mussels beds (Holt et al., 1998).
• 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, planktonic development suggests considerable dispersal potential. However, recruitment is sporadic and Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols studies 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. In Ophiothrix fragilis recruitment success is heavily dependent on environmental conditions including temperature and food availability. In years after mild winters Ophiothrix fragilis occurred in extremely high densities in the Oosterschelde estuary in Holland (Smaal, 1994). Populations seem to be stable in the long term although there may be strong variation from year to year.
• Hydroids have limited dispersal potential due to a short planktonic larval stage, so that colonization is probably local. Recruitment would be dependent on currents and the proximity of hydroid communities.
• Bryozoa also have a short lived larva which does not travel far (see Umbonula littoralis). Similarly, the ascidian 'tadpole' larva has a very short planktonic life, dispersal is relatively poor and recruitment and colonization would depend on the proximity of nearby adult colonies in most species. Recruitment from distant colonies may take a long time.
• Red algae produce millions of non-motile spores, that may settle close to the adult or travel great distances depending on currents and turbulent deposition (Norton, 1992). For example, red algae colonized blocks within 26 weeks in the shallow subtidal (0.8m) and 33 weeks at 4.4m (Kain, 1975). Delesseria sanguinea was noted within 41 weeks (8 months) at 4.4m in one group of blocks and within 56-59 days after block clearance in another group of blocks. Recolonization occurred during winter months following spore release and settlement, but not in subsequent samples (Kain, 1975). This suggests that recolonization of Delesseria sanguinea in new areas is directly dependent on spore availability.

Holt et al., (1998) point out that where impacts are severe enough to clear extensive areas of a horse mussel bed, recovery would be unlikely even in the medium term. They also noted that both the time required for small breaks in beds to close up due to growth of surrounding clumps, and the survival of clumps torn from the bed is not known. Witman (1984 cited in Suchanek, 1985) cleared 115cm² patches in a New England Modiolus modiolus bed. None of the patches were recolonized by the horse mussel after 2 years, 47% of the area being colonized by laminarian kelps instead (Witman pers. comm. cited in Suchanek, 1985). No details on longer term studies were found.
The horse mussel is long-lived and reproduction over an extended life span may compensate for poor annual recruitment. However, any factor that reduces recruitment is likely to adversely affect the population in the long-term. Any chronic environmental impact may not be detected for some time in a population of such a long -lived species.
Overall, therefore, while some populations are probably self-sustaining it is likely that a population that is reduced in extent or abundance will take many years to recover, and any population destroyed by an impact will require a very long time to re-establish and recover, especially since newly settled larvae and juveniles require the protection of adults to avoid intense predation pressure.

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