Reef Habitats

Predators

Competitors

Wider effects on the Environment

The associated biota of Mytilus reefs has been little studied, but does not appear generally to be particularly rich or diverse in comparison with stable subtidal biogenic reefs such as those of Modiolus or Serpula vermicularis, S. alveolata in the Severn Estuary, and possibly the more stable examples of S. spinulosa reefs. Nevertheless, these often represent the only hard substrate communities in the area, so that they may be regarded as important in terms of increased habitat heterogeneity. A variety of small infaunal invertebrates is 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, may be frequent. Briggs (1982) found at least 34 species associated with the matrix of partly-infaunal mussel beds in Lough Foyle, of which almost half were crustaceans. Dense beds in intertidal mussel communities in the southern North Sea had only 12 associated species (Asmus, 1987), and this might be more typical of the rapidly growing communities present in the larger Mytilus reefs, especially in estuarine areas, although production rates of associated species might still be high.

In other areas, more stable and long-lived Mytilus beds such as those found on moderately exposed rock surfaces probably build up much more diverse associated communities, with typically 50-100 species (see Seed, 1996 for a review) although many of these would probably not be regarded as biogenic reefs.

A consequence of enhanced biodeposition of labile organic matter is that mussel mounds have more deposit feeding oligochaetes than the surrounding flats (Dittmann, 1990).

Newcombe (1935) pointed out that sediments below dense mussel beds can become more anoxic, leading to the loss of some infaunal bivalve species such as Mya arenaria.

It has also been suggested that the high rate of suspension feeding in the mussel mounds favours species that reproduce with cocoons, brood the young, or which disperse as juveniles rather than as planktonic larvae (Commito, 1987).

On permanently submerged offshore structures, fewer predators and the opportunity for continuous feeding are thought to be the factors which accounts for the success of M. edulis in creating very dense aggregations of large mussels. It is possible that relative lack of predators in some enclosed, especially more estuarine, areas might thus be partly responsible for the build up of reefs in these areas.

A number of invertebrate predators can be very important in regulating Mytilus populations, particularly crabs and starfish. On parts of the east coast of England the lower limits of M. edulis beds are controlled by predation by Asterias rubens and Nucella lapillus, although the latter may be more important in exposed than in sheltered areas (Seed, 1969); gastropods, unlike starfish, were not regarded as important predators of Mytilus in Morecambe Bay (Dare, 1976) nor in the Wash (Dare, pers. comm.) or the Danish Wadden Sea (Thiesen, 1968). In Ireland Liocarcinus, Carcinus, Nucella and Marthasterias are thought the most likely candidates to control the lower limits of mussel beds (Kitching & Ebling, 1967). Crab-proof cages on the shore in the Menai Strait resulted in good survival of M. edulis whereas in unprotected control areas there was very rapid loss of all live mussels due to predation by Carcinus maenas (Davies et al., 1980). Work in Nova Scotia has demonstrated that there is an upper size limit of around 70 mm in length for predation by C. maenas on M. edulis, although in the UK most mussels longer than 45 mm are probably safe (Davies et al., 1980) In Washington State Suchanek (1981) observed ‘herds’ of roaming starfish Pisaster ochraceus eliminating large beds of M. edulis in days. Similar rapid destruction of Mytilus beds by Asterias has been observed on rocky shores in Europe (Seed, 1969) and on boulder scars around MLWST in Morecambe Bay. In the latter case, Asterias has been found at densities up to 450 m^-2, and the swarm, which covered up to 2.25 ha at one time, may have cleared up to 4,000 tonnes of first year mussels between June and September (Dare, 1976; Dare, 1982). In the Wash, Asterias destroys most sublittoral settlements each year, and also attacks cultivated plots at or below MLWST. These predatory species are sensitive to desiccation so that mussels have a spatial refuge on the shore above MLWNT.

Invertebrate predators of reef-forming bivalves including Mytilus are reviewed in Seed (1993).

Other important predators include flatfish; in Morecambe Bay, flounders were found to contain the remains of up to 570 (average 150) small mussels (up to 15 mm in length) per fish, and plaice and dabs were similarly important (Dare, 1976). Flounders have also been found to be important predators of mussels in Liverpool Docks (Hawkins, pers. comm.)

Bird predation on mussels can also be important, and may significantly affect the development of reefs. It has been studied extensively (see reviews in Seed & Suchanek, 1992, and 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). Eider are also sometimes regarded as a nuisance on submerged mussel farms. More than 60% of the adult eider diet may consist of mussels (Seed & Suchanek, 1992). Knot, turnstones, sandpipers, herring gulls and even crows are also known to feed on intertidal mussels, while scoters also dive for small mussels. In Morecambe Bay, Dare (1976) identified oystercatchers, herring gulls, eider ducks and knot as major sources of Mytilus edulis mortality. There were differences in the size range of Mytilus taken, with knot being important when the mussels were only a few mm long, herring gulls at around 3-20 mm length, and oystercatchers and eiders at larger sizes. For eiders, Raffaelli et al. (1990) reported a rather smaller preferred size of 10-25 mm in the Ythan estuary, where over a 60 day period a flock of 500 removed approximately 36% of the larger mussels (6-30 mm). They also reported that eider feed by removing large clumps, leaving bare patches on the rock. Many of the Mytilus which are shaken from these clumps but not eaten may die later. Taking this into account, they calculated that a flock of around 500 eiders probably removed around 4,000 m² of mussels (around 25% of the mussel bed under study), during November of the study year. Eiders are known to be present all year around, but there are reduced numbers during winter and a rapid rise during spring, often to over 4,000, when females especially feed voraciously, in preparation for a long fast over their incubation period (Baird & Milne, 1981; Raffaelli et al., 1990). Baird & Milne (1981) reported that in the Ythan estuary, bird predation accounted for 72% of the annual M. edulis production (other predators being negligible in these terms) with flocks of over 4000 eider being responsible for 42% and oystercatchers and herring gulls 15% each. It was concluded by Raffaelli et al. (1990) that direct and indirect mortality caused a significant impact upon the population dynamics of M. edulis, although there was surprisingly little evidence of significant impact upon the associated invertebrate community. The likelihood that areas of mussel beds from which clumps had been removed could be more susceptible to subsequent removal by water movement was not mentioned, perhaps because the Ythan is a sheltered estuary, but this would appear to be a possibility; the disruption of the integrity of mussel clumps on mussel ropes is recognised as an important side effect of eider predation. Hosomi (1984) noted that the tendency of M. galloprovincialis to recruit strongly around edges as well as upon individuals of established beds might serve to reduce the removal of mussels by wave action.

Mytilus is often a staple food of oystercatchers in the winter. In the east Scheldt, Holland, 40% of the annual mussel production is consumed by oystercatchers (Meire & Ervynck, 1986), and mussel production is probably the major limiting factor for density of overwintering flocks (Craeymeersch et al., 1986). In Conway, North Wales, oystercatchers were found to consume up to 574 mussels (average length 25.7 mm) or 186 mussels (average 37.5 mm) each in a low tide (Drinnan, 1958). Considerable selection by oystercatchers has been shown against mussels which are barnacle encrusted, thick shelled or otherwise difficult to open (Leopold et al., 1989; Meire & Ervynck, 1986).

It is clear that bird predation may significantly affect the quality of biogenic reefs.

Crisp (1964) noted that, although M. edulis was relatively unaffected by unusually low temperatures, there was an increased mortality due to predation on mussels by birds in parts of South Wales. It was thought that the cold may have rendered some species, including mussels, too torpid to resist attack by birds. In the same paper it was observed that there were heavy mortalities of cockles Cardium edule in the Solway Firth caused by oystercatchers and gulls "for whom they offered during one period the only accessible food", and heavy predation on moribund cockles by oystercatchers in Morecambe Bay was noted. The possibility of increased foraging in intertidal areas by birds during extremely cold weather must therefore also be considered.

In addition to the species already mentioned, a wide variety of other organisms have been found to be important predators on mytilids in some circumstances, including limpets, predatory gastropods, Cancer, lobsters, urchins, fish, otters, seals and even walrus and turtles (see relevant references in Seed & Suchanek, 1992), and pink shrimp Pandalus montagui take spat in the Wash (Dare, pers. comm.) but none of these are likely to be of major importance in the main areas of biogenic M. edulis reefs.

Possible competitive interactions between Mytilus and Sabellaria alveolata have been discussed in the relevant section above on S. alveolata. These interactions tend to occur on more exposed boulder scars where mussel have less tendency to form well developed reefs, and so are probably more important with regard to the Sabellaria reefs than Mytilus reefs.

It has often been established that the efficient removal of particles in Mytilus beds can deplete the seston available in the benthic boundary layer downstream of them. In some bays the mussel beds are of a scale such that by their filter feeding they play a particularly important role in energy flow over much wider areas than the actual beds. At one level they compete for phytoplankton with beds of cockles that are often on flats inshore. "Rain shadow" effects have been reported. Mussel beds result in significant depletion of phytoplankton at the bottom of the water column (Frechette & Grant, 1991) and function as systems, not just as populations of mussels (Dame & Dankers, 1988). Using a canalised flume system installed on a natural bed in the Wadden Sea, Asmus & Asmus (1991) measured a 37% uptake of phytoplankton passing through the 20 m flume in summer. An equivalent high nutrient release from the bed was also measured. Using a benthic seston sampler Muschenheim & Newell (1992) estimated that on an ebb tide a mussel bed was capable of processing the water over the bed to a depth of 7 cm but allowing for significant refiltration the effective feeding zone was of the order of 3.5 cm. They fed preferentially on high concentrations of resuspended benthic diatoms.

Mytilus reefs have a strong stabilising effect on sediment, for periods varying from a few months to many years, and it has been suggested that in their absence large scale changes to whole estuary complexes may occur (McKay, pers. comm.).

Mussel beds are extremely important in the generation of organically enriched biodeposits that provide nutrition for wide ranges of deposit feeding invertebrates not just in them but over wide areas of tidal flats around them. In the Northern Baltic, Kautsky & Evans (1987) estimated in situ biodeposit production per gram of mussel (dry weight including shell) as 1.76 g or 0.33 g AFDW. During the growing season (April - September) the biodeposit had a higher organic content and a higher nitrogen content than naturally sedimenting material. With the ecologically similar M. chilensis the mean annual deposition rate was measured at 234g/m²/day of which 21% was organic matter (Jaramillo et al., 1992). Modification of the benthos and the redox discontinuity under mussels cultivated in suspension has been shown in Gallicia and in Sweden (Mattsson & Linden, 1983). Parallels may be drawn with the generalised patterns of macrobenthic succession accompanying organic over-enrichment as postulated by Pearson & Rosenberg (1978).

75% of the >2mm carbonate fraction of sediments in the Wadden Sea consists of fragmented shells. Eiders crush Mytilus internally when feeding and Cadee (1994) suggests that a third to a half of the shell fragments could have been produced in this way.

Mytilus in reefs and beds are clearly of great importance as food for bird populations, particularly eiders and oystercatchers for which Mytilus is a stable food for at least part of the year. The interrelationships between birds and mussels are discussed in the sections on predators and competitors above, and reported detrimental effects of low mussel stocks on oystercatchers and eiders are mentioned in chapter VI.

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