Habitat requirements

Habitat factor Range of conditions
Salinity Full

Dense populations of very young Modiolus modiolus do occasionally occur subtidally in estuaries, although the species is more poorly adapted to fluctuating salinity than other mussel species (Bayne 1976). Dense populations of adults are not found in low salinity conditions. Pierce (1970) established tolerance limits of 27 – 41 ‰ for Modiolus modiolus based on ventilation behaviour and byssus formation.

Wave exposure Moderately exposed (MCR.ModT & CMX.ModMx), Sheltered, very sheltered (SCR.Mod)
Tidal streams Strong (MCR.ModT), Moderately strong (MCR.ModT & CMX.ModMx), Weak (SCR.Mod), Very weak (SCR.Mod)
Substratum Mixed substrata (SCR.Mod), Cobbles, pebbles and Modiolus shells (MCR.ModT), Muddy gravel and sand, with shells and stones (CMX.ModMx)
Zone Infralittoral – lower (MCR.ModT), Circalittoral (all)
Depth range 10-20 m (SCR.Mod), 20- >50 m (CMX.ModMx)

Coleman (1973) demonstrated that Modiolus modiolus exposed to air has an erratic heart rate, suggesting a lack of physiological adaptation to ariel exposure, and that it loses water rapidly due to an apparent inability to control its gape effectively. Dense populations therefore seem restricted to around 5 – 50 m in British waters, although bioherms have been recorded in over 80m in Nova Scotia (Wildish & Fader, in press). Lack of mobility, thin shell and restricted tolerance to changes in temperature and salinity has been suggested as reasons for the poor ability of Modiolus to colonise the intertidal (Davenport & Kjorsvik 1982).

Temperature Modiolus modiolus is clearly a northern species, and the fact that dense aggregations seem to reach their southerly limit around British coasts suggests a possible susceptibility to a long-term rise in summer water temperatures. There is little published information on the temperature tolerance of Modiolus, although it is clear that it has a lower upper thermal limit than Mytilus edulis (Bayne 1976). Being subtidal it is protected from major short-term fluctuations. It has been suggested that an inability to tolerate temperature changes is one of the factors which prevents Modiolus from colonising the intertidal to any extent (Davenport & Kjorsvik 1982). Low winter water temperatures would not pose any threat to Modiolus modiolus populations around Britain.
Water quality Modiolus modiolus has been found in a variety of turbid and clear water conditions.
Nutrients Work in Newfoundland has demonstrated that Modiolus modiolus is capable of tolerating intermittent availability of food supplies, reducing feeding activity during periods of low phytoplankton (autumn and winter) and increasing clearance rate during spring and early summer (Navarro & Thompson 1996).

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