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The ostracod fauna of Baltic brackish-water rockpools is made up of two groups: permanent members of the pools, and occasional guests from the littoral zone. The former group consists of Heterocypris salinus, H. incongruen8 and Cypridopsis aculeata. These species are characterized by rapid development (which starts when the water temperature approaches ~15 ~ a short life span, and 2 or 3 separate generations during the summer and autumn. The number of generations is determined by water temperature. Hibernation always takes place as eggs. Hatching and development during the late spring or early summer has been found to be mostly simultaneous. The spawning of the 3 species always starts epidemically. Reproduction is entirely parthenogenetic in the investigated area. Under natural conditions, C. aculeata may be found with either H. 8alinus or H. incongruens, but these two latter species have never been recorded together. H. incongruens is less tolerant to high salinities than the other 2 species and, even after successive adaptation, it does not resist salinities higher than 16 %~ H. salinus has been found in 35.2 %o S in the field, and has been kept in 30 %o S in the laboratory after successive adaptation. The optimum salinity-temperature range for this species is 5 to 10Y~o S and 15 ~ when both survival and development are considered. Corresponding figures for C. aculeata are 0.5 to 20 go S and 15 ~ although this species, like H. salinus, survives longest at 5 ~ The very rapid development, parthenogenetic reproduction and short life span of these species must be considered as favourable adaptations to the variable and unstable environment of the roekpool ecosystems.

Stomach contents of 112 bycaught harbor porpoises (Phocoena phocoena) collected between 1989 and 1996 in the Kattegat and Skagerrak seas were analyzed to describe diet composition and estimate prey size, to examine sample size requirements, and to compare juvenile and adult diets. Although porpoises preyed on a variety of species, only a few contributed substantially to the diet. Atlantic herring (Clupea harengus) was the dominating prey species for both juveniles and adults. Our results, in combination with those from previous studies, suggest that where herring is a dominant food source, porpoises prey primarily on size classes containing mature or maturing individuals. Further, we also show that Atlantic hagfish (Myxine glutinosa) may be an important food resource, at least for adult porpoises. Examination of sample size requirement showed that, depending on the taxonomic level used to describe the diet, a minimum of 35–71 stomachs are needed to be confident that all common prey species will be found.

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