Life cycle, food, and growth of the amphipodAmpelisca macrocephalaliljeborg from the Øresund

Abstract

The lifespan of Ampelisca macrocephala was found to be one and a half year for males and two years for females. A few females become three years old and breed a second time. It is estimated that the length of the animal increases by about 10 % at each molting and that approximately 18 molts take place before the egg-carrying stage is reached. Growth is fast in spring, slower in summer and autumn when the ovaries and testes develop in animals about to breed. and stagnating in winter when the females carry embryos. The latter stay in the marsupium for five months, during which time their length increases from about 0.8 to about 1.3 mm. The average egg number per female was found to be about 60. The feeding behaviour is described, and it was found that the animal will accept zooplankton as food in the laboratory. In the field. however, the intestinal contents of collected animals indicate that they feed on detritus. The periods of growth and reproduction seem to be closely correlated with the periods of primary and secondary production in the Øresund, whereas temperature was found unimportant as a governing factor. The Ampelisca population studied was heavily infested with the turbellarian endoparasite Kronborgia amphipodicola Christensen & Kanneworff, and studies on the influence of this parasite revealed that it affects growth and causes sterility in all infested host animals. Thus, due to the latter fact. the breeding population of Ampelisca was found to be reduced by about 50 %.