Day and night densities, reproductive biology and life cycle of Amphilocus neapolitanus (Della Valle, 1853), associated with Sargassum cymosum C. Agardh, 1820, were studied during 13 months. The density of juveniles, males and females was higher in night samples than in day samples. Sex ratio was strongly favorable toward the females. Ovigerous females were present in similar densities in both the periods while the non-ovigerous females were predominant at night. The egg number was not correlated with the size of the females. Breeding occurred throughout the year.
Population structure; reproductive biology; life cycle; Amphipoda; Sargassum
