The associated biota of Mytilus beds has been little studied, but does not appear generally to be particularly rich or diverse in comparison with stable subtidal biogenic reefs. Nevertheless, the Mytilus beds often represent the only hard substrate communities in an area (e.g. an estuary), so they may be regarded as important in terms of increased habitat heterogeneity. A variety of small infaunal invertebrates are found within the accumulations of mussel mud, with some larger mobile animals such as Littorina littorea, Gammarus spp., polychaetes and small Carcinus maenas in between the mussels and dead shells. These are hunted by foraging birds such as turnstones, curlews, redshank and gulls. The shells themselves may support encrusting fauna such as barnacles, and algae, particularly Fucus vesiculosus and sometimes green algae such as Enteromorpha spp., may be frequent. It has been suggested that the high rate of suspension feeding in the mussel mounds favours species that reproduce with cocoons, brood their young or which disperse as juveniles rather than as planktonic larvae (Commito 1987).

Larval growth to metamorphosis during spring and early summer, at around 10^oC, normally takes about 2-4 weeks (Lane, Beaumont & Hunter 1985; Seed 1976; Seed & Suchanek 1992; Widdows 1991). Mytilus edulis has a two-stage extended dispersal strategy. A primary settlement of post-larvae usually occurs on sublittoral filamentous substrata such as hydroids and algae. Then, after growing to around 1-2 mm in length, the spat detach and move to the adult beds, aided by the secretion of long byssus threads which help the young mussels to drift in the water until a secondary settlement site is found. Spatfall and recruitment in some beds of mussels is very variable year-to-year, and unlike some other invertebrates, high densities of the adults do not inhibit the settlement of spat (Commito 1987).

No information available.

A number of invertebrate predators, particularly crabs and starfish, can be important in regulating Mytilus populations. Other important predators include flatfish; in Morecambe Bay, flounders were found to contain the remains of up to 570 (average 150) small mussels per fish, and plaice and dabs were similarly important (Dare 1976). Bird predation on mussels may significantly affect the development of reefs (see reviews in Seed & Suchanek 1992; Meire 1993). Oystercatchers and eider ducks are very widely reported as feeding extensively on Mytilus, and may be responsible for heavy mortalities in wave-protected bays and estuaries (Seed & Suchanek 1992). In addition to the species already mentioned, a wide variety of other organisms have been found to be important predators on Mytilus and include limpets, predatory gastropods, crabs, lobsters, urchins, fish, otters and seals.

Surveys of intertidal mussel beds in the German part of the Wadden Sea showed that the distribution of the beds remained rather constant, although the abundance of mussels varied considerably due to irregular spatfalls, ice drift, storm surges and parasitism (Obert & Michaelis 1991). During the 1980s the mussel populations declined due to increasing eider predation. In the Danish part of the Wadden Sea Jensen (1992) showed that there were no obvious differences between macrobenthos populations present in the 1930s and in the 1980s.

No information available.

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