Environmental Requirements

Physical Attributes

Modiolus is clearly a boreal species, and the fact that dense aggregations seem to reach their southerly limit around British shores suggests a possible susceptibility to a long-term rise in summer water temperatures. Little published information was found on which to make an informed judgement, although it is clear that its upper thermal limit is lower than that of Mytilus edulis (Bayne, 1976). Being subtidal it is obviously 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 in a British context.

Dense populations which would represent biogenic reefs seem to be mainly restricted to depths of between 5 and 50 m in British waters, although bioherms have been recorded in over 80 m in Nova Scotia (Wildish & Fader, in press). Individuals have been found at depths of up to 280 m (Schweinitz & Lutz, 1976). Coleman (1973) demonstrated that M. modiolus exposed to air has an erratic heart rate, suggesting lack of physiological adaptation to aerial exposure, and loses water rapidly due to an apparent inability to control its gape sufficiently well. Lack of mobility, thin shell and restricted tolerance to changes in temperature and salinity have also been suggested as reasons for its poor ability to colonise the intertidal (Davenport & Kjorsvik, 1982). It is thus not surprising that intertidal occurrences are limited generally to low shore and pools.

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

Larval Modiolus will settle on a variety of shell and stone substrata, including Modiolus shell, and Modiolus may occasionally replace Mytilus as the main fouler of the deeper parts of offshore structures such as oil rigs. However, survival of small Modiolus is often low due to high predation, and there is evidence that the best refuge for juveniles is in the byssus threads of established clumps or aggregations of larger Modiolus, which also act as a suitable settlement substratum (Roberts, 1975). In more infaunal Modiolus beds it can be speculated that the lack of accessible byssus may be an important factor in reducing recruitment rates; there is some evidence that recruitment in ‘infaunal’ Modiolus reefs to the north of the Isle of Man is very sporadic (Holt & Shalla, 1997). Although clearly requiring some hard substratum for initial formation, Modiolus beds and reefs are capable of forming on a variety of sedimentary bottoms ranging from essentially muddy substrata in some sealochs to quite coarse mixed sediments containing much stones and shell.

Water movement appears to be an important factor in the build up of many of the denser reef areas, the majority being found in areas of moderate to strong tidal currents.

M. modiolus has been found to be more or less similar in tolerance of oxygen deficiency and hydrogen sulphide to M. edulis (Theede et al., 1969), both species being much more tolerant than many other groups such as gastropods, echinoderms and crustaceans. 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). It is also found in a variety of turbid and clear water conditions.

Fragility of individual Modiolus is clearly not particularly high, and the importance of reaching a certain minimum size to avoid predation is mentioned elsewhere. The fragility of reefs is also probably not particularly high, even in those situations where the animals are truly epifaunal; clearly semi-infaunal and infaunal reef areas are less fragile. Clumps upon muddy substrata are presumably more fragile than larger aggregations. Nevertheless, very physical activities such as impacts by towed fishing gear are known to be damaging, not only by disruption and flattening of clumps and larger aggregations, with reduction in the value of the habitat, but also by damage, and presumably mortality, to individual Modiolus (see chapter V). It should be noted also that the shells of old individuals can be very brittle due to the activities of the boring sponge Clione celata (Comely, 1978).

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