COMPARISON OF LIFE HISTORY PARAMETERS IN COEXISTING SPECIES OF THE GENUS JASSA (AMPHIPODA, ISCHYROCERIDAE)

Species of the genus Jassa dominate marine fouling communities where they can reach remarkably high densities. Several species have often been found co-occurring in the same habitats, such as the three congeners J. falcata (Montagu, 1808), J. marmorata Holmes, 1903 and J. herdmani (Walker, 1893) in the hard bottom communities of the island of Helgoland (German Bight, North Sea). In ecological and physiological studies, these species have frequently not been distinguished. To get an idea of the extent of ecological differentiation among the species, some life history parameters were analyzed and compared. In a laboratory experiment, survival rates and moulting frequencies were studied in adults under two different temperature regimes. Field samples were analyzed with respect to brood size and body size. The results revealed clear differences among the three studied species with respect to life cycle and reproduction as well as adaptations to temperature: survival rates of adults, and how these are affected by temperature; intermoult periods of adult females and their dependence on age; adult body size; ova per brood; brood size/body size relationship. The total number of moults in females was not affected by temperature, which indicates an endogenously fixed maximal number of moults. The findings highlight the importance of a strict discrimination of Jassa spp. to properly understand patterns of distribution.