Environmental Requirements

Physical Attributes

Because of its widespread distribution, intertidal habit, its abundance and ecological importance in many places, its use as a bio-indicator, its commercial importance, and the relative ease with which it can be kept alive in the laboratory, Mytilus edulis has been extremely widely studied. For general information on its biology, ecology and physiology, the reader is referred to Bayne (1976) and Gosling (1992b).

Mussels belonging to the genus Mytilus are widely distributed throughout the cooler waters of the world. The Mytilus edulis species complex is circumpolar in boreal and temperate waters, in both the southern and northern hemispheres (Soot-Ryen, 1955) extending from the Arctic to the Mediterranean in the North east Atlantic. The most important limiting factor for distribution world-wide is thought to be temperature (Stubbings, 1954).

Damage by extreme low temperatures is minimised in Mytilus by the use of nucleating agents in the haemolymph (Aunaas et al., 1988). Even in more temperate sites M. edulis is subject to lethal freezing conditions periodically, but they can survive even when tissue temperatures fall below -10°C (Williams, 1970), with large adults surviving laboratory conditions of -16°C for 24 hours (Bourget, 1983). Crisp (1964) observed that M. edulis within the British Isles was relatively unaffected by the severe, unexpected freeze of December 1962 to March 1963. Beds in the Wash have survived being buried under ice for up to several weeks in 1963, 1979 and in the mid 1980’s (Dare, pers. comm.) on permanently submerged offshore structures. In Sweden, mussels actively ingested seston at -10° C suggesting that they can utilise spring phytoplankton blooms in boreal waters even at low temperatures (Loo, 1992).

Tolerance of high temperatures and desiccation can explain the upper limit of M. edulis on the high shore (Seed & Suchanek, 1992). British M. edulis have an upper sustained thermal tolerance limit of about 29°C (Almada-Villela et al., 1982; Read & Cumming, 1967).

Recruitment or movement to cracks is known to afford better thermal protection on the upper shore (Suchanek, 1985). It can therefore be speculated that dense reef structures might afford some protection from extremes of temperature to the lower animals. In general, however, given the wide temperature tolerance of Mytilus, reefs, which are generally found quite low on the shore, are unlikely to be very sensitive to changes in temperature (but see possible indirect effects of cold elsewhere).

Reef areas are normally found on the lower third of the intertidal, and in the shallow subtidal, but can occur down to 10 m in some places such as the Wash and on Caernarfon Bar (Dare, pers. comm.). Dense subtidal populations of Mytilus are frequently reported from dock pilings and offshore platforms where low predation may be an important factor.

Lower zonational limits for M. edulis are usually set by biological factors, normally predation by starfish, crabs and gastropods (see chapter IV). Lower zonational limits can also be controlled by physical factors. Sand burial has been shown to limit lower regions of M. edulis zonation patterns in New Hampshire, USA (Daly & Mathieson, 1977) and this is probably important in some British locations, particularly in the case of cobble or boulder scars in areas of shifting sands such as in Morecambe Bay and the Solway Firth. Upper limits of distribution are set by physical factors, but growth and therefore size of animals is also affected by reduced feeding time at higher levels. It has been estimated that growth would be zero at approximately 55% aerial exposure (Baird, 1966), although clearly this will vary somewhat with local conditions.

M. edulis is tolerant of a wide range of salinity compared to other biogenic reefs species and may penetrate quite high into estuaries. However, it may stop feeding during short-term exposure to low salinities (Almada-Villela, 1984; Bohle, 1972) and the most well developed reefs therefore usually occur low on the shore in the mid to lower reaches of estuaries.

Almada-Villela (1984) reported greatly reduced shell growth for a period of up to a month or so upon exposure to 16‰ compared to 26 or 32‰, while exposure to 22‰ caused only a small drop in growth rate. In the longer term (in the order of weeks) M. edulis adapts well to low salinities (Almada-Villela, 1984; Bohle, 1972), and hence can even grow as dwarf individuals in the inner Baltic where salinities can be as low as 4-5‰ (Kautsky, 1982). Almada-Villela (1984) found that the growth rate of individuals exposed to 13‰ had recovered from almost zero initially, to over 80% of the rate of control animals in 32‰ after one month. It is therefore no surprise that Mytilus can do well in brackish lagoons and docks if there is a suitable supply of food.

M. edulis is widely recognised as being tolerant of a wide variety of environmental variables including salinity and oxygen tension as well as temperature and desiccation (Seed & Suchanek, 1992). It is capable of responding to wide fluctuations in food quantity and quality, including variations in inorganic particle content of the water, with a range of morphological, behavioural and physiological responses (Hawkins & Bayne, 1992). Mytilus is not necessarily particularly tolerant of anthropogenic chemicals, however (chapter VI).

Excessive levels of silt and inorganic detritus are thought to be damaging to Mytilus once they accumulate too heavily within the reef matrix (Seed & Suchanek, 1992), although the degree to which this might be influenced directly by water quality rather than production of faeces and pseudofaeces is unclear. Mytilus is capable of re-surfacing through a shallow covering of sediment. Mussels can colonise unpromising habitats such as former docks (Hawkins et al., 1992; Russell et al., 1983). In these semi-enclosed habitats sheets of mussels up to 30 cm thick can form. Their filtering activity can improve water quality (Allen & Hawkins, 1993; Wilkinson et al., 1996).

In general the best examples of biogenic Mytilus reefs occur on mixed substrata of small boulders, cobble and pebble on sandy or muddy substrata. It has long been suggested that larval Mytilus will settle on most substrata provided they are firm and have a rough, discontinuous surface (Maas Geesteranus, 1942). However, settlement is in many cases a two stage process; initial settlement occurs primarily on filamentous substrata such as sublittoral hydroids and algae, with subsequent secondary dispersal and reattachment later in areas with adult beds (see chapter IV).

In wave exposed areas Mytilus requires a hard and stable substratum such as rocks, or large boulders on which to form beds.

In sheltered areas infaunal beds may occur on gravel or even quite sandy areas, as reported in Lough Foyle (Erwin et al., 1986), at the mouth of Cookmere Haven, Sussex (E I Rees, pers. obs.), and occasionally in the Wash (Dare, pers. comm.) although it is likely that some harder substratum embedded within the more sandy areas is required. Dense settlement also occurs on cockle shells in the Wash and Burry Inlet, and on Lanice in the Wash (Dare, pers. comm.). In many of these cases the byssus of the embedded mussels do seem to serve a noticeable stabilising function. The authors are not aware of instances where true biogenic reefs have developed on such substrata but the potential may be there given the propensity for Mytilus juveniles to settle in the spaces afforded within adult beds (King et al., 1990; McGrorty et al., 1990).

Mytilus edulis is found in a wide range of exposures, from all but the very most exposed shores to extremely sheltered habitats, but in order to develop large reef structures considerable water movement is required. This provides increased oxygen and food supplies in such areas, and may also help to prevent ‘mussel mud’ (silt, faeces and pseudofaeces) from building up too quickly. In general, water movement in the best Mytilus reef areas is provided mainly by tidal currents, but in some cases wave action may also contribute, as in Morecambe Bay, although these beds tend to be more transient. On some open coasts M. edulis may form dense beds on boulder scars, where wave action is the main source of water movement, as on the Cumbrian coast, for example. However, it is debatable whether many of these areas would qualify as biogenic reefs as they tend not to be raised or massive, and are formed on what is essentially an underlying hard (though sometimes unstable) substrate. In general, the more stable reefs occur in smaller, more sheltered estuaries.

Individual animals have a wide variation in shell strength (Meire & Ervynck, 1986) which is one of the factors which influences mortality due to oystercatcher and crab predation. Shell strength tends to increase with increasing age/size, height on the shore and wave exposure. Large reefs which develop on mixed substrata in fairly exposed sites seem to be much more susceptible to being removed by tidal or wave action than do extensive beds on rock, the latter usually being both much more firmly attached and often less thick. Thicker reefs are attached to the underlying substratum only by the bottom layer of mussels, and accumulations of mussel mud in the deeper layers can kill the bottom mussels, exacerbating the problem. Mussel beds composed of a single year class of mussels may form only a thin layer sitting upon accumulated mussel mud; they are poorly attached to the substratum and are therefore more likely to be detached from the substratum than are mussel beds composed of a large number of years classes (McKay & Fowler, 1997).

Although a bed as a whole may be a persistent feature, the formation of patches within it is a dynamic process (Svane & Ompi, 1993). Those on the outside of patches tend to be larger and there are complex density dependent influences on a small scale on recruitment, growth and mortality (see also ‘longevity and stability’, chapter IV).

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