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 10oC,
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.
Keystone (structuring) species
Importance of habitat for other species
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.
Time for community to reach maturity
No information available.