Seasonal variation in population structure and biochemical composition of Jassa falcata (Crustacea, Amphipoda) off the island of Helgoland (North Sea)

Seasonal variations of size-frequency distribution, sex ratio, and percentage of egg-carrying females and juveniles in a population of Jassa falcata Leach inhabiting jetties at Helgoland Harbour (North Sea) are described. Reproduction occurs all the year round, but two maxima and thus, two main generations per year can be observed: a weaker one in winter, and a more pronounced one in late spring/early summer. Maximum body length is about 12 mm in females, and about 13 mm in males. Reproduction of females occurs soonest at 5 to 6 mm and ceases the latest at about 10 mm body length. Biochemical composition and energy contents of J. falcata show only in part a seasonal cycle. There is an inverse relationship between the protein and lipid fractions, whereas the former is negatively, the latter positively correlated with the amount of suspended food in the water. Protein, carbohydrote, lipid and the weight-specific energy equivalent show a decreasing trend with increasing size of the amphipods, while chitin significantly reveals an opposite trend.