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

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.

The growth, reproduction and body color pattern of Cleantiella isopus (Isopoda: Valvifera) in Hakodate Bay, Japan

“…In C. isopus, guarding males were always larger than females, and there was no correlation in body length between guarding males and guarded females (Fig. 6) indicating non sizeassortative mating; a similar trend was also reported in I. pelagica (Healy & O'Neill, 1984).…”

“…In contrast, the lowest water temperature (3°C) matched the lowest water temperature of the Baltic Sea (Salemaa, 1979). Healy & O'Neill (1984) have shown that low temperature is critical for isopods and their activity drops when the water temperature falls below 6°C. Therefore the defined breeding periods of isopods living in the Baltic Sea and Hakodate Bay are likely caused by low temperature which suppresses growth and breeding.…”

“…Idotea species (Sheader, 1977;Salemaa, 1979;Healy & O'Neill, 1984). Healy & O'Neill (1984) reported that males died soon after copulation, which might contribute to the sex ratio shift in the population.…”

“…Healy & O'Neill (1984) reported that males died soon after copulation, which might contribute to the sex ratio shift in the population. Variation of male's size distribution was also observed in other Idotea species (Sheader, 1977;Healy & O'Neill, 1984), but the cause and significance were unknown. Precopulatory mate guarding is widely known within Valvifera (Sheader, 1977;Salemaa, 1979;Jormalainen et al, 1992).…”

“…The breeding period and its length differ among species or conspecific populations that occupy different habitats (Naylor, 1955b;Sheader, 1977;Salemaa, 1979;Healy & O'Neill, 1984). Although about 20 species have been reported in Japan, as far as we know, there is only one study about the feeding habit of Idotea ochotensis (Suzuki et al, 2002).…”