In ecological studies, especially in those dealing with energy circulation in nature, determinations of the energy content of organisms are inevitable. Energy determinations are, however, laborious and time-consuming. Average conversion factors based on different species form various areas and seasons may often be a shortcut for overcoming this problem. To establish general energy conversion factors for aquatic invertebrate groups, we used 376 values of J. mg-' DW and 255 values of J. mg-' AFDW, representing 308 and 229 species, respectively. The dry-weight-to-energy factors were highly variable both within and between taxonomic groups, e.g.: Porifera, 6.1 J. mg-' DW; insect larvae, 22.4 J * mg-' DW (median values). The energy-conversion factors related to AFDW showed a much smaller dispersion with a minimum median value of 19.7 J * mg-' AFDW (Ascidiacea) and a maximum of 23.8 J. mg-' AFDW (insect larvae). Within taxonomic groups, the 95% confidence intervals (AFDW) were only a few percent of the median values. The use of energy-conversion factors based on AFDW is preferable due to their lower dispersion. For aquatic macrobenthic invertebrates, a general conversion factor of 23 J. mg-' AFDW can be used.

In the Baltic Sea, the presence of dense populations of the deposit—feeding amphipods Pontoporeia affinis and P. femorata has been hypothesized to inhibit recruitment of the bivalve Macoma balthica. Experimental tests with P. affinis and newly settled M. balthica in small, flow—through aquaria corroborated this hypothesis. Survival of small M. balthica spat decreased with increasing density of Pontoporeia affinis. The increase in mortality was strong enough potentially to explain the field observation that M. balthica is generally absent or rare in areas with dense populations of Pontoporeia spp. Three hypotheses as to the mechanism for the decreased M. balthica survival were tested. The results wee inconsistent with hypotheses of suffocation after burial or starvation as the mechanism of death, but corroborated the hypothesis of direct physical injury to M. balthica spat following contact with P. affinis adults. In the presence of P. affinis, spat survival was lower in shallow than in deep sediment; encounters between the two species should be more frequent in shallow sediment. Adult P. affinis were found to crush newly settled M. balthica spat, presumably ingesting the remains. M. balthica spat are not, however, common enough to be an important food source for P. affinis. It is suggested that while interference with larval recruitment by adults is likely to be an important structuring factor in marine soft bottom communities, it is unlikely to depress community biomass below carrying capacity, as suggested by Peterson (1979). Virtually all communities contain species with brood protection, the young of which are less susceptible to interference by adults. Where such interference is strong, species with brood protection are likely to become dominant, as are Pontoporeia spp. over large areas in the Baltic Sea.

Field studies in the Baltic Sea indicate that the growth rate of the deposit-feeding benthic amphipod Monoporeia affinis (Lindström), which is found in densities of up to ≅25 000 ind. m -2

The food composition of 132 specimens of Halicryptus spinulosus, collected in the Asko-Landsort area (northern Baltic proper) was investigated. More than 40 % of the specimens had empty guts. Detritus was the most frequent constituent of contents of the intestines, but is assumed to play a minor role as an energy source. Other food items recorded were polychaetes, oligochaetes, crustaceans, chironomids, other H. spinu/osus specimens and undetermined eggs. The food composition of smaller and bigger specimens differed significantly. It is concluded that this species is mainly carnivorous.

No abstract