Reproduction, development and growth

Longevity and stability

Small scale movements

Feeding

Parasites and diseases

A discussion of the reproductive biology of Mytilus edulis is found in Seed & Suchanek (1992), including an account of larval ecology (Lutz & Kennish, 1992).

Many populations of M. edulis exhibit some spawning all year round, with major peaks of spawning in spring and often a number of further ‘opportunist’ spawnings later in the summer, with more restricted spawning periods where adult feeding conditions are poor. Spawning on the west coast of Britain occurs a few weeks earlier than on the colder east coast (Seed & Suchanek, 1992). Reproductive output (see below) is size and site related, and is also influenced by temperature, food supply and tidal exposure; 10-fold variations in reproductive effort have been recorded from six contrasted sites on the English and Welsh coasts (Bayne et al., 1983). Fecundity also varies from year to year, probably reflecting adjustments to energy allocations according to variation in food supply (Thompson, 1979, working in the North west Atlantic).

Larval growth to metamorphosis during spring and early summer, at around 10° C, normally takes about 2-4 weeks (Lane et al., 1985; Seed, 1976; Seed & Suchanek, 1992; Widdows, 1991). Under optimum conditions it may take 20 days or less (Bayne, 1965; Seed & Suchanek, 1992; Sprung, 1984a; Sprung, 1984b). Temperature is probably the main influence on the duration of metamorphic delay, with longer delays at lower temperatures (Strathman, 1987), but salinity, food supply and non-availability of suitable substratum may also delay settlement, and larval life may last for up to three months (Widdows, 1991) or even over six months in some cases (Lane et al., 1985). Once the larvae reach the pediveliger stage and they are competent to settle metamorphosis can be delayed for up to 7 weeks. Upon settling the young mussels are known as spat.

M. edulis has a two-stage extended dispersal strategy. A primary settlement of post-larvae usually settles onto 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. In some cases beds of filamentous algae such as Polysiphonia, Corallina and Mastocarpus seem to provide a pool of young mussels which might account for some of the sporadic recruitment seen in some places. However, it has become apparent recently that sometimes only a single settlement may occur, directly onto adult beds (e.g. King et al., 1990), and further study is needed to fully understand settlement and recruitment processes (see Seed & Suchanek, 1992 and Lutz & Kennish, 1992 for further information and references).

Spatfall and recruitment in some beds of mussels is very variable year on year. An exceptional (at least for the area concerned) spatfall on parts of the Cumbrian coast was reported to consist of densities of up to 175,500 m^-2 (Perkins, 1987; Perkins, 1988). Unlike some other invertebrates, high densities of the adults do not inhibit the settlement of spat (Commito, 1987). Persistent stable beds can be maintained by relatively modest spat recruitment into the crevices and shelter of the byssal threads of the adults. McGrorty et al. (1990) demonstrated that in stable beds in the Exe Estuary, recruitment to adult populations was relatively unaffected by very large variations in spatfall - during the period 1976 - 1983 spat recruitment varied by a factor of 17 (min - max), but adult numbers varied by only 1.5 suggesting strong damping over the first year (McGrorty et al., 1990). First winter mortality averaged 68%. The adults suffered large losses (mean 39%) after spawning and 24% winter mortality mainly due to bird predation.

There have been several co-operative attempts to compare recruitment at different locations along European coasts (see Dijkema, 1992), though there are staff resource implications in doing this. Recruitment is favoured by cold preceding winters caused by decreases in predator populations and delays in the arrival of newly settled crabs and shrimps on the flats which allows the spat to reach a larger size before the onset of predation.

Growth and production rates within Mytilus biogenic reefs can be extremely high. In a population in Morecambe Bay, which is characterised by high rates of mortality, the production by two year classes was calculated as 2.5 to 3 times their maximum standing crop; most of this was by first-year mussels and production had virtually ceased after sixteen months, with few mussels surviving beyond the third year (Dare, 1976). Low shore mussels in favourable areas can grow to 3.5 - 4 cm in 30 weeks (Orton, 1914) and to 60-80 mm in length within 2 years (Seed, 1976). Such rapid production and turnover seems to be a characteristic of many of the biogenic Mytilus communities in estuarine and other enclosed areas. Over a one year period Craeymeersch et al. (1986) estimated production on an intertidal bed in the Eastern Scheldt to have been 156g AFDW / m² and the P:B ratio to have been 0.5. By means of cage exclosure experiments Egerrup & Laursen (1992) estimated that annual predation on a mussel bed in the Danish Wadden Sea was 116 g AFDW / m² from a mean annual biomass of 740 g AFDW / m². This was equivalent to 17% of the biomass and 81% of the secondary production. The most important predators were eider and oystercatcher. Crab predation was thought to have been insignificant in this experiment on an established bed. Self-thinning as mussels grow is also reported as there are limits to multilayered packing (Hughes & Griffiths, 1988), although in flat bed situations, the fate of displaced mussels is not clear. It is possible that they can wash to locations where they can embyss again.

The above figures contrast with rates on higher rocky shore areas where M. edulis might only reach 20-30 mm after 15-20 years (Seed, 1976).

In contrast to Modiolus, Mytilus can reproduce in its first year of life (Seed & Brown, 1977).

Mussels are capable of living to very old ages in certain conditions and Mytilus edulis has been reported as reaching 18-24 years in the Danish Wadden Sea (Thiesen, 1968). However, it seems likely that the majority of animals in biogenic reef areas are very young, since these reefs have a tendency to grow rapidly and be detached by water movement when they become well developed. There are areas in Morecambe Bay, and probably elsewhere, which regularly (though not invariably) receive heavy juvenile recruitment but which rarely develop into true beds, losses probably being attributed to a mixture of predation and loss to wave action later in the year. Many biogenic reefs probably consist largely of mussels up to two or three years of age (Dare, 1976; Seed, 1976) (see also chapter I). In some cases heavy predation by starfish and birds is probably also partly responsible for this (see later). However, more stable reefs do occur in smaller, sheltered estuaries (see the example of the Exe Estuary in the discussion on recruitment, section a).

Over time, beds in particular places may for natural reasons vary in the positions they occupy on the continuum between thin, patchy beds and well developed reefs. Particularly at times when the biomass of mussels is high, mounds may form. When stocks are low for whatever reason the mounds may barely be detectable. Because mussel mud is highly cohesive, once it has consolidated, the deposits may last for years after the mussels have largely gone.

Four surveys of the extent of intertidal mussel beds in the German part of the Wadden Sea since 1949 showed that the distribution of the beds remained rather constant, although the abundance of the mussels varied considerably due to irregular mass spatfalls, ice drift, storm surges and parasitism (Obert & Michaelis, 1991). During the 1980s the mussel populations declined due to the co-occurrence of increasing eider predation, intensification of the mussel fishery and a series of ice winters. Niels & Thiel (1993) using aerial surveys make a clear distinction between persistent beds in relatively sheltered situations and beds in exposed situations in the Schleswig-Holstein part of the Wadden Sea. Using records from 1937, 1968 and 1978 to compare with those from 1989-1991 they indicated that there were great similarities in the extent of the persistent beds over the long-term. In the Dutch parts of the Wadden Sea the distribution of the beds also remained relatively constant over the 1949 - 1988 period, though figures given by Dankers & Koelemaij (1989) indicate the overall biomass of mussels varying by a factor of over 30.

In the Danish part of the Wadden Sea Jensen (1992) showed that there no obvious differences between macrobenthos populations present in the 1930s and in the 1980s. There was no difference at all in cockle growth rates between the 1930s and the 1980s in situations not directly influenced by terrestrial run-off, but some observations suggested that mussels had extended their range along the low water line.

It is widely recognised that Mytilus, although apparently sedentary, can actually move appreciable distances to readjust positions within clumps or to resurface when covered by sand. However, when Raffaelli et al. (1990) created large numbers of small bare patches (5-140 cm²) in dense mussel beds in the Ythan estuary, to simulate patches made by feeding eider, they observed that mussels rarely encroached back onto them over 30-50 day periods.

There has been a great deal of work on the feeding ecology of Mytilus. For an introduction to this subject and other aspects of Mytilus physiology the reader is referred to Hawkins & Bayne (1992). M. edulis is a filter feeder capable of removing particles down to 2-3 µm with 80-100% efficiency (MØhlenberg & Riisgåd, 1977), and which shows a great range of adaptations to changing conditions, including the ability to regulate filtration rates, ingestion rates and the production of pseudofaeces dependent upon the quantity and quality of plankton and other particulate matter in the water (with a near immediate response), physiological adaptations to changing nutrient conditions (usually taking days or weeks), and morphological changes to filtration apparatus (months). Organic and bacterial food sources adhering to particles and resuspended benthic algae and other organic matter may also be important as food sources.

There is a wide range of known diseases, and more especially parasites, of Mytilus which probably represents the amount of work carried out on Mytilus rather than any special susceptibilities. These have been reviewed by Bower (1992). Those which are, or have been, considered to be most important are considered below.

It is thought that trematodes of the families Bucephalidae and Fellodostomidae, which use mussels as primary hosts, are among the more serious parasites. Some of these are known to cause a wide range of problems including parasitic castration and death, (Cousteau et al., 1990; Feng, 1988), and on occasion mass mortalities (Munford et al., 1981) in Mytilus edulis and other mussels. Mussels higher on the shore contain have a higher incidence of parasitism by a trematode for which the oystercatcher is the next host, but the true importance of this parasite is unclear. The supposition that mussels suffer less in general from parasites (and diseases) than oysters is probably an artefact of the shorter history of intensive cultivation (Bower, 1992).

Another important parasite is the polychaete Polydora ciliata, which burrows into the shell, weakening it and rendering the mussel more susceptible to predation by birds and shore crabs. This may cause particular problems for mussels greater than 6 cm in length (Kent, 1989), which when healthy are normally relatively free of predation, and can cause substantial mortalities (Lauckner, 1983).

The parasitic copepod Mytilicola intestinalis is widely prevalent in M. edulis, and mass mortalities of cultivated mussels have often been attributed to it, but a ten year study carried out in Cornwall suggested that M. intestinalis may be a commensal rather than a harmful parasite (Davey, 1989), and the true position is unclear (Bower, 1992).

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