||Range of conditions
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.
||Moderately exposed (MCR.ModT & CMX.ModMx), Sheltered,
very sheltered (SCR.Mod)
||Strong (MCR.ModT), Moderately strong (MCR.ModT &
CMX.ModMx), Weak (SCR.Mod), Very weak (SCR.Mod)
||Mixed substrata (SCR.Mod), Cobbles, pebbles and Modiolus
shells (MCR.ModT), Muddy gravel and sand, with shells and stones (CMX.ModMx)
||Infralittoral lower (MCR.ModT), Circalittoral (all)
||10-20 m (SCR.Mod), 20- >50 m (CMX.ModMx)
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).
||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.
||Modiolus modiolus has been found in a variety of
turbid and clear water conditions.
||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).