Mytilus edulis beds on variable salinity infralittoral mixed sediment
Ecological and functional relationships
Mytilus edulis is a active suspension feeder on organic particulates and dissolved organic matter.
The production of faeces and pseudofaeces enriches the underlying sediment providing a rich food source for infauna detritivores, deposit feeders, meiofauna and bacteria.
Dense beds of suspension feeding bivalves are important in nutrient cycling in estuarine and coastal ecosystems, transferring phytoplankton primary production and nutrients to benthic secondary production (pelagic-benthic coupling) (Dame, 1996).
Other suspension feeders include epifaunal barnacles and tube worms e.g. Pomatoceros triqueter.
Epifloral/faunal grazers, such as limpets and chitons may use the mussel bed as a refuge. Their grazing reduces epiflora/faunal fouling of Mytilus edulis shells, hence reducing the potential for dislodgement of the mussels due to strong water flow or storm surges (Suchanek, 1985).
The organic rich 'mussel mud' provides a food source for deposit feeding polychaetes (e.g. Scoloplos armiger and Capitella capitata and oligochaetes (e.g. Tubificoides spp.) and surface deposit feeders (e.g. Polydora spp. and Macoma baltica
Scavengers probably feed on dead mussels and other organic material within the mussel matrix, e.g. flatworms, polychaetes and amphipods (Kautsky, 1981; Tsuchiya & Nishihira, 1985,1986).
The interstices within the mussel matrix and mussel mud support epifaunal and infaunal predators such as scale worms (e.g. Harmothoe spp.), nereids (e.g. Nephtys spp.) and other polychaetes and nemerteans.
Fish, starfish, crabs and lobsters are potential predators on subtidal mussels beds (Kautsky, 1981; Paine, 1976; Seed, 1993; Seed & Suchanek, 1992).
Mussels were a major food source for the flounder (Platichthys flesus) in Morecambe Bay and subtidal mussel beds in the Baltic Sea (Dare, 1976; Kautsky. 1981) but probably of only minor importance for eelpout (Zoarces viviparus) and cod (Gadus morhua in the Baltic Sea (Kautsky, 1981).
The lower limit of Mytilus edulis beds is usually set by the intensity of predation, e.g. from Asterias rubens and Nucella lapillus in eastern England (Seed, 1969) or Liocarcinus spp., Carcinus maenas , Nucella lapillus and Marthasterias glacialis in Ireland (Kitching & Ebling, 1967; Holt et al., 1998). However, predation risk is size dependant, i.e. 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).
Periodic, and sporadic swarms of starfish have been observed to decimate mussel populations, and subtidal settlements in the Wash were destroyed by Asterias rubens annually (Dare, 1976, 1982; Seed, 1969; Holt et al., 1998).
Birds are major predators in intertidal beds but this biotope is probably only vulnerable during extreme low tides to most predatory wildfowl, however , eider ducks are capable divers. Eider duck consume large numbers of mussels, primarily over winter. Raffaelli et al. (1990) recorded the removal of 4500 mussels /m² (within the preferred size of 10-25mm) within 60 days by a flock of 500 eider. Eider remove mussels in clumps, which they shake to remove the target mussel. This results in additional mortality for those mussels removed from suitable substratum in the clump and leaves bare patches in the mussel beds, which may increase the risk of the loss of further mussels by water movement. Eider may, therefore, significantly affect the mussel bed (Seed & Suchanek, 1992; Holt et al., 1998).
Otters may prey on mussel beds.
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.
Seasonal and longer term change
Mussels are capable of living to up to 18-24 years of age, however, the majority of mussels in biogenic reefs are probably young consisting of 2 -3 year old individuals due to predation and the dislodgement of clumps of mussels by wave action and storms (Holt et al., 1998). As mussel beds grow in size, individual mussels become more attached to other mussels than to the underlying substratum, so that large beds may be 'rolled up' and removed by wave action. Therefore, mussel beds may vary in size and extent, and show a continuum between thin patchy beds and well developed reefs (Holt et al., 1998). However, more stable reefs occur in sheltered environments. For example, in the German Wadden Sea, the distribution of mussels has been relatively constant since 1949 but the abundance of mussels varied due to irregular recruitment, storm surges, ice drift, and parasitism. In the Dutch Wadden Sea the distribution of mussel beds was relatively constant from 1949-1988 although the biomass varied 30 fold (Holt et al., 1998).
Habitat structure and complexity
Sub-tidal Mytilus edulis
beds have been little studied but probably have features in common with intertidal beds or subtidal beds of other mussel species (e.g. Modiolus modiolus
). Mussels beds 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).
- The gaps between interconnected mussels form numerous interstices for a variety of organisms. In 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). However, the species richness of the IMX.MytV biotope is not particularly high (Connor et al., 1997a). Holt et al. (1998) noted that this biotope may form raised beds (biogenic reefs) and stabilize the substratum, perhaps resulting in a higher species diversity than in the sediments alone.
- Mussel faeces and pseudo-faeces, together with silt, build up organic biodeposits under the beds. The biodeposits attract infauna such as sediment dwelling sipunculids, oligochaetes, and polychaetes (Suchanek, 1979; Seed & Suchanek, 1992). However, in areas of strong tidal streams, flushing may prevent the build up of a thick layer of biodeposits.
- Epizoans may use the mussels shells themselves as substrata. However, 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. Barnacles and tubeworms may be epizoic, however this biotope does not support a diverse epifauna.
Mytilus spp. communities are highly productive secondary producers (Seed & Suchanek, 1992; Holt et al., 1998). For example, in Morecambe Bay, Dare (1976) estimated that production by two year old classes was 2.5-3 times their maximum standing, even though mussels in this area suffer high rates of mortality. In favourable areas low shore mussel can grow 3.5 -4cm in 30 weeks and 6-8 cm in length in 2 years (Orton, 1914; Seed, 1976). Rapid production and turnover are characteristic of estuarine or sheltered communities (Holt et al., 1998). Production of an intertidal bed in the Eastern Scheldt was estimated to be 156 g ash free dry weight (AFDW) / m² in one year (Craeymeersch et al., 1986). Similarly, Egerrup & Layrsen (1992; cited in Holt et al., 1998) estimated that annual predation on a Danish Wadden Sea mussel bed accounted for 17% of the biomass and 81% of the secondary production from a mussel biomass of 740 g AFDW /m². Dame (1996) suggested that dense beds of suspension feeding bivalves are important in nutrient cycling in estuarine and coastal ecosystems, transferring phytoplankton primary production and nutrients to benthic secondary production (pelagic-benthic coupling) and improving the productivity of the entire system.
- Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Gametogenesis and spawning varies with geographic location, e.g. southern populations often spawn before more northern populations (Seed & Suchanek, 1992). Spawning is protracted in many populations, with a peak of spawning in spring and summer and settlement approximately one month later. Jörgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark. 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 fertilisation 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. Occasional recruitment to circalittoral populations may occur as individuals dislodged from the intertidal. Competition with surrounding adults may suppress growth of the young mussels settling within the mussel bed, due to competition for food and space, until larger mussels are lost (Seed & Suchanek, 1992). However, young mussels tend to divert resources to rapid growth rather than reproduction. 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 spatfall 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 (e.g. occasionally in Urticina felina) or disperse as juveniles will be favoured.
- Recruitment in echinoderms is highly variable, for example, Asterias rubens is widespread, fecund, and with a pelagic larvae capable of widespread dispersal, however, recruitment in starfish is sporadic, unpredictable and poorly understood (Seed, 1993).
- Nucella lapillus mates in gregarious aggregations and lays capsules, cemented to the substratum, in which the larvae develop until released as miniature adult crawl-aways. There is no pelagic phase, and although passive mucous rafting may occur occasionally, dispersal is limited to about 10-30cm. However, adults are relatively long-lived (about 6 years) and a female can produce up to 1030 hatchlings per year (see review).
- Most species of polychaete encountered within the biotope are widespread and have a dispersive pelagic larvae (Fish & Fish, 1996), and can potentially disperse and recruit over a wide range, depending on the hydrographic regime. The larvae of Scoloplos armiger are benthic (Fish & Fish, 1996), however, passive transport of juveniles has been shown to be important for the recruitment of species in sedimentary habitats (Olafsson et al., 1994), and other polychaetes with purely benthic stages are capable of colonizing new habitats rapidly, e.g. Arenicola marina.
Time for community to reach maturity
The occurrence of this biotope requires the presence of dense Mytilus edulis beds. 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). Therefore, while good annual recruitment and rapid growth are possible, recovery of the mussel population may take up to 5 years. In certain circumstances and under some environmental conditions recovery may take significantly longer. The associated community is likely to colonize the substratum or mussel matrix rapidly.
This review can be cited as follows:
Mytilus edulis beds on variable salinity infralittoral mixed sediment.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=36&code=1997>