Geographical variations in the reproduction of the horse mussel,Modiolus modiolus(Mollusca: Bivalvia)

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Abstract
The reproductive cycles of four widely separatedModiolus modioluspopulations from W. Sweden, W. and N. Norway, and Northern Ireland were examined in relation to their contrasting temperature regimes. Reproductive condition was analysed on the basis of the degree of development and numerical density of the gametes, and on size-frequency distributions of oocytes.All the Scandinavian populations had well defined spawnings, those of W. Sweden and W. Norway occurring in June and followed by a period of redevelopment and further gamete release. The N. Norway population spawned in July and subsequent redevelopment was not accompanied by further spawnings. Both Norwegian populations overwintered in a relatively ripe condition, but that from W. Sweden spent the winter in a low state of reproductive development. The N. Ireland population contrasted with the others in being continuously ripe and having no overt spawning, but rather a slow, constant release of gametes throughout the year.These differences are assessed in terms of the effects of the local temperature regimes on the interacting processes of gametogenesis and spawning. It is concluded that seasonal variability in temperature plays a major role in determining the extent and duration of gametogenesis, whilst spawning is more dependent upon the attainment of a limited range of minimum temperatures. The northern limit ofM. modiolusis therefore more likely to be determined by failure of conditions suitable for spawning and larval development than by a failure of gametogenesis.

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