In-situ studies on the life cycle of Diastylis rathkei (Cumacea: Crustacea)

Abstract

Aspects of the life cycle of the cumacean Diastylis rathkei Kröyer, the most important food item of demersal fishes in the Western Baltic Sea, were investigated from the Underwater Laboratory (UWL) “Helgoland” in Lübeck Bay. In August the population density was 10.1±3.6 individuals 100 cm^-2 in fine-medium sand (Md=292 μm), and 3.6±2.4 individuals in coarser medium sand (Md=470 μm). In both substrata, abundance values decreased until October, at each station to a different extent. Particular attention was paid to vertical migrations of D. rathkei. Plankton catches and numerous direct observations provided evidence that the pelagic phase, observed at night, is always combined with ecdysis. During short periods of high swarming and moulting activities, the population structure changed considerably: juveniles of the 3rd stage dominated in early August; thereafter they were replaced by “prematures”, the last juvenile stage. The first mature individuals were observed in October. Some data are given on growth rate.