The Life Cycle and Population Dynamics of Idotea Pelagica and I. Granulosa (Isopoda: Valvifera) In South-East Ireland

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Abstract
Populations of Idotea pelagica and Idotea granulosa were studied monthly at Carnsore Point, County Wexford, from July 1978 to October 1979. The density of I. pelagica in mussel beds varied between 26 and 180/100 cm, reaching a peak in June. Numbers of I. granulosa in samples of Gigartina stellata fluctuated from month to month and averaged 200/100 g dry weight of algae. Peak densities were recorded in September of both years. Both species exhibited continuous growth, and breeding occurred throughout the year but with a main breeding period starting in winter. I. pelagica grew more rapidly and had an extensive breeding period lasting from December to August with two generations, some members of both generations producing second broods. For I. granulosa, the main breeding period was January to April, after which most adults died. Some of their offspring reached maturity in late summer giving rise to a lesser breeding peak in late summer and autumn but most did not mature until winter. Summer breeders of both species were smaller than those breeding in winter. The breeding cycles are compared with those of other populations in Britain and the Baltic and the possible reasons for the differences discussed.

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