Ecology ofIdoteaspp. (Isopoda) in the northern Baltic

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Abstract
The population dynamics and bteeding biology of Idotea baltica, I. chelipes and I. granulosa wete studied in the littoral belts of two northern Baltic rocky shore habitats. Population sizes are largest in the aurumn, after occupation of the Fucus belt by a new Idotea generation from the belt of filamentous algae. Two periods of dectease in the population density, one in late autumn and another after the breeding period in the following summer were observed. Adult males are eliminated from populations before females. Most individuals in each species breed simultaneously in eatly summer. Brood numbers are related to female size, but great marsupial mortality occurs during incubation. Idotea juveniles grow exp.nentially in late summer, but during winter the growth is delayed. Another period of intensive growth occurs in spring before breeding. I. baltica is the dominant species in both habitats examined. I. chelipes and I. granulosa were found not to coexist. The roles of different factors affecting the distribution and geographical variation in Idotea are discussed.