Diel vertical migration and endogenous swimming rhythm in Asterope mariae (Baird) and Philomedes interpuncta (Baird) (Crustacea Ostracoda Cypridinidae)

Among the benthic ostracods Asterope mariae and Philomedes interpuncta, the adult males emerge at dusk from the benthos and swim straight up to the su rface, where they soon concentrate in the neustonic realm. The migration st arts when the irradiance is almost 1 mu W cm(-2) and the maximal density oc curs at the surface when the light has reached 0.005 mu W cm(-2), before it is completely dark. The migration ends <1 h later and very few animals are observed in the water column later in the night. Upward migration does not result from a positive phototaxis to dim light, but rather from a strong n egative geotaxis and an increase in kinesis or swimming activity which occu r at around 1 mu W cm(-2). This activity is induced both by the end of the diurnal period of photoinhibition and by an endogenous circadian rhythm. Th is rhythm could be clearly observed in both species from the recordings mad e during up to 2 weeks in constant darkness. The length of the period was r oughly 24 h (24 h 3 min in Asterope, 23 h 10 min in Philomedes). The timing of the activity phase in total darkness varied depending on the post-captu re history of the experimental animals, especially on the time of onset of the experiment. The adaptive advantage of this rapid diel vertical migratio n may a question of feeding, but this may also increase the nocturnal dispe rsal of the reproductive and thus promote the reproduction of the species.