Modiolusmodiolus(L.) from the Scottish West Coast. I. Biology

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Abstract
The population structure, seasonal biology and flesh content of Modiolus from three different subtidal habitats were studied. In an area of strong currents recruitment is normal, and the population is relatively young. In an adjacent, quiet water habitat recruitment is negligible and the population consists of ageing animals which suffer extensive physical damage from the boring sponge Cliona celata Grant. In the population from deeper water (13 m) there has been no recruitment for a numb~r of years, and the population is dominated by old animals which grow to a large size and remain in apparently good condition, but suffer from possible senile changes. There is also a notable incidence of tumours and pearl formation not found elsewhere. Flesh content is related primarily to the gonad size. There is a well defined gonadial cycle in the populations from the fast current and deep water habitats. The third population was consistently poor. The evidence indicates that there is a spring and late summer spawning, but gametes may be released throughout the year. The observations suggest that the current-swept population is viable, but the adjacent population is possibly at risk as recruitment is so low and the ravages of Cliona so considerable. The deep water population is certainly dying out, and can only be saved by a successful spat-fall within the next few years.