Biodiversity & Conservation

The ecology of circalittoral Mytilus edulis beds have been poorly studied and little information was found. Mussel beds colonizing artificial substrata such as jetty piles and the legs of oil production platforms, together with data on mussel beds in general has been used to derive the following information. Mytilus edulis is a active suspension feeder on organic particulates and dissolved organic matter.

Mytilus edulis probably competes for space with other species such as Sabellaria spinulosa and Tubularia indivisa and other mussel species (e.g. Musculus discors).

Epifloral/faunal grazers, such as limpets, chitons and sea urchins (e.g. Echinus esculentus), 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).

Fish, starfish, crabs and lobsters are potential predators on subtidal mussels beds (Kautsky, 1981; Paine, 1976; Seed, 1993; Seed & Suchanek, 1992). The common starfish Asterias rubens and the plaice Pleuronectes platessa were observed feeding on Mytilus edulis in the biotope off Flamborough Head (Brazier et al., 1998).

Kautsky (1981) examined subtidal mussel beds in the Baltic Sea and reported that mussels were a major food source for the flounder (Platichthys flesus) but probably of only minor importance for eelpout (Zoarces viviparus) and cod (Gadus morhua). The lower limit of Mytilus edulis beds is usually set by the intensity of predation. The formation of a bed at depth suggests either a scarcity of predators or the rapid growth of the individual mussels during a lull in predator numbers to a size above the handling size of most predators. 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).

Starfish would be expected to be significant predators in the subtidal, however, the population dynamics of starfish populations are poorly understood (Seed, 1993). 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).

Scavengers probably feed on dead mussels within the matrix, e.g. flatworms and polychaetes (Kautsky, 1981; Tsuchiya & Nishihira, 1985,1986). However, Kautsky (1981) demonstrated little scavenger activity in the subtidal Mytilus edulis beds in the Baltic Sea.

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.

Little information concerning the population dynamics of subtidal Mytilus edulis populations was found. Kautsky (1981) reported that no major fluctuations in distribution and abundance of Mytilus edulis was noted in the Baltic Sea over a ten year period, although a large proportion of biomass fluctuated with the build up and subsequent release of gametes. However, his studied population was not significantly affected by predation. It is likely that subtidal populations are periodically removed or significantly reduced by sporadic and unpredictable swarms or starfish.

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 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). However, Connor et al., (1997a) reported that the species richness of the MCR.MytHAs biotope was not particularly high. Similarly, Holt et al. (1998) noted that a raised beds was not present and most associated organisms were capable of growing on the substratum in the absence of Mytilus edulis.
• Anemones such as Urticina felina and Sagartia elegans, and branching bryozoans such as Flustra foliacea are probably attached directly to the substratum surface, and penetrate the mussel matrix.
• Mussel faeces and pseudo-faeces, together with silt, build up organic biodeposits under the beds. The biodeposits attract infauna such as sediment dwelling sipunculids, polychaetes and ophiuroids (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. Balanus crenatus and erect branching bryozoans, in particular, may be epizootic in MCR.MytHaS.

Mytilus spp. communities are highly productive secondary producers (Seed & Suchanek, 1992; Holt et al., 1998). Kautsky (1981) estimated that the subtidal Mytilus edulis beds in the Baltic Sea contained a biomass of 10,200 tonnes dry weight in July-August, of which 1500 was meat. Kautsky (1981) also estimated that the mussel beds contributed up to 600 tons of organic carbon to the pelagic ecosystem, as eggs and larvae. However, the Baltic Sea subtidal mussel beds were subject to low levels of predation due to the reduced salinities and therefore more productive than might be expected of mussel beds in other subtidal locations.
No information concerning the productivity of circalittoral Mytilus edulis beds in the UK (MCR.MytHAs) was found. However, it is likely that they represent an important food resource for a number of predatory species, especially starfish, decapod crustaceans and fish (see ecosystem relationships).

• 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 1 month later. JØrgensen (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, and 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. 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 spat fall between 1976-1983 in the Exe estuary.
• 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).
• Anthozoans, such as Alcyonium digitatum and Urticina felina are long lived with potentially dispersive pelagic larvae and are relatively widespread. They are not restricted to this biotope and would probably be able to recruit rapidly (refer to the Key Information reviews).
• Balanus crenatus is an early colonizer of available space, with a dispersive, pelagic nauplius larvae and likely to recruit into the population rapidly.
• Flustra foliacea, and other bryozoans have a short-lived, pelagic larvae, with probably poor dispersive abilities. However, bryozoans are widespread, and not restricted to this biotope and are likely to recruit from neighbouring populations fairly rapidly. Recruitment is likely to be aided by the strong tidal streams inhabited by this biotope.
• Ascidians such as Molgula manhattensis have external fertilisation but short lived larvae, so that dispersal is probably limited. Where neighbouring populations are present recruitment may be rapid but recruitment from distant populations may take a long time.
• 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.

Holt et al. (1998) suggested that the associated species in this biotope (MCR.MytHAs) could colonize the rock surface in the absence of Mytilus edulis. Therefore, the occurrence of this biotope requires 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). Therefore, while good annual recruitment is 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. However, no information on recovery in subtidal Mytilus spp. populations was found. The associated community is likely to colonize the substratum or mussel matrix rapidly.

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