Population density, size, age, reproduction and microdistribution in the Jaera albifrons group (Isopoda).

Specimens of species of the Jaera albifrons group (Isopoda) were collected from 25 stones each month over a period of one year from two different shores. The density was found to be 4-20 per m² undersurface in winter and 2 500-11 400 in summer. The animals usually die within one year. Females always out-number males. Reproduction took place between April and September-November. A strong correlation between size and number of eggs was found. The egg number/brood varied from a mean of 8 up to a mean of 60. A theoretical potential productivity of a hibernating female was estimated to be 9200 descendants in the autumn. Up to 60 % of the females in a sample were carrying dead eggs and young. At one of the localities two species occurred. No evidence of differences in microdistribution was found between these. On exposed sites, however, spatial segregation between the sexes and between different size classes was found. [ABSTRACT FROM AUTHOR]