Habitat requirements
Habitat factor Range of conditions
Salinity Full, variable. Mytilus edulis is tolerant of a wide range of salinity compared to other biogenic reef species and may penetrate quite far up estuaries. However, it may stop feeding during short-term exposure to low salinities (Almada-Villela 1984; Bohle 1972) and the most well-developed beds 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).
Wave exposure Sheltered, Very sheltered, Extremely sheltered
Substratum Mixed boulders, cobbles and pebbles on muddy sediment. In sheltered areas infaunal beds may occur on gravel or even quite sandy areas, 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 Loughor Estuary where the byssus of the embedded mussels seem to serve a stabilising function. It has long been suggested that larval Mytilus will settle on most substrata provided they are firm and have a rough, discontinuous surface (Mass Geesteranus 1942). Settlement is in any case 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.
Zone Eulittoral-mid, Eulittoral-lower
Height Reef areas are normally found on the lower third of the intertidal, and in shallow subtidal, but can occur down to 10 m in some places such as the Wash and on Caernarfon Bar. Lower zonational limits for M. edulis are usually set by biological factors, normally predation by starfish, crabs and gastropods, and by physical factors. Sand burial has been shown to limit lower regions of M. edulis zonation patterns in New Hampshire, USA (Daly & Mathieson 1977). This is probably important in some British locations, particularly in the case of cobble and boulder scars in areas of shifting sands such as 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.
Temperature Mytilus edulis is widely distributed throughout the cooler waters of the world. The most 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, Denstad & Zachariassen 1988). Even in more temperate sites M. edulis is periodically subject to potentially lethal freezing conditions periodically, but they can survive even when tissue temperatures fall below –10oC (Williams 1970). 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 29oC (Almada-Villela, Davenport & Gruffydd 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.
Water quality Mytilus 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). 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.

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