The activity of three intertidal sand beach isopods (flabellifera: cirolanidae)

Abstract

The activity rhythms of Eurydice longicornis (Stüder), Pontogeloides latipes (Barnard) and Excirolana natalensis (Vanhoffen), three intertidal cirolanid isopods co-existing on South African sandy beaches, were studied in the field and under constant darkness in infra-red actographs in the laboratory. Both adults and immatures of all three species, collected on a high energy, microtidal beach, exhibit endogenous circatidal rhythms, modulated to a differing extent by a circadian rhythm. This results in peak swimming activity during diurnal high tides in immatures and during nocturnal high tides in adults of the three species. A clear semilunar rhythm could be demonstrated only in E. natalensis. The two species that occur highest on the beach, E. natalensis and P. latipes, show the strongest circadian modulation of the circatidal rhythm, while E. longicornis, which occurs lowest on the shore shows the strongest circatidal rhythm. Activity rhythms of these species are contrasted with those of other cirolanids from low energy, mesotidal beaches.