Habitat heterogeneity, body size and phenotypic diversity in Idotea granulosa (Isopoda) on the north-east coast of England

Samples were collected from six pools in order to examine the distribution, body length, base colour and phenotypic diversity (chromatophore pattern) of Idotea granulosa populations on five different species of algae (Ceramium, Corallina, Chondrus, Fucus and Enteromorpha), on each of two shores. Holbeck (HOL) pools had significantly higher habitat heterogeneity in terms of algal species diversity than did those on the more exposed Filey Brigg (FB). Idotea densities were significantly higher and mean body size significantly smaller on Corallina than the other algal species. Isopods were significantly larger on Fucus. There was no significant difference in base colour frequencies between shores but there was a positive association between the brown base colour and Corallina and the green colour and Fucus. The commonest chromatophore pattern morph was the granulosa-maculata that occurred in 44% of the isopods sampled and five out of ten chromatophore patterns accounted for <10% of the population. The uniformis morph was rare on both shores. There was no significant difference in chromatophore pattern frequency between the sexes. The lineata-maculata morph and maculata morphs showed a positive association with Corallina and Fucus respectively. There was a significant difference in chromatophore pattern frequency between adults and juveniles. The uniformis and maculata patterns occurred in higher frequencies than expected in the adult population whereas the reverse was true of the granulosa-maculata and lineata-maculata morphs. A total of 85 different phenotypes were recorded from both shores, with 51 of these being exclusively found in HOL pools. Holbeck pools had a significantly higher isopod phenotypic diversity than did those from FB. There was no significant difference in phenotypic diversity between adult and juvenile Idotea at HOL, however juveniles had a significantly higher phenotypic diversity than did adults at FB. Overall, there was a significant positive correlation between rockpool algal diversity and isopod phenotypic diversity.

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

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.

Osmoregulation in Ligia Oceanica and Idotea Granulosa

1. The osmoregulatory response of Ligia oceanica and Idotea granulosa to the range of the experimental variables was similar. They were both hyperosmotic relative to the medium and the difference between internal and external concentration increased as the salinity of the medium decreased. 2. In 100% sea water the osmotic concentration of the blood of Ligia oceanica was markedly above that of the medium, whereas in Idotea granulosa the blood was only marginally hyperosmotic. 3. In Ligia oceanica the blood concentration changed little in 100 and 75% sea water, but dropped significantly between 75 and 50% sea water, whereas blood concentration in Idotea granulosa dropped significantly throughout the test range of salinities. 4. The more efficient osmoregulation of Ligia oceanica in 25% sea water is reflected in the mean freezing-point depression of the blood, Δi = 1.65, compared with Δi = 0.90 in Idotea granulosa. 5. In both species the osmotic concentration of the blood was influenced by season, by temperature and by a temperature-salinity interaction. 6. Neither size nor sex of the animal influenced osmotic concentration of the blood.