Diel cycles of reproduction and vertical migration in the marine cladocerans Pseudevadne tergestina and Penilia avirostris

Abstract

The marine cladocerans Pseudevadne tergestina and Penilia avirostris use different strategies to avoid visual predators. Pseudevadne tergestina exhibited a pronounced diel cycle in reproduction, but did not perform diel vertical migration. Parthenogenetic females with fully developed embryos were absent during the day in both fishless enclosures and in the natural environment. This observation suggests that the diel cycle was due to nocturnal release of neonates and not the result of selective predation by fish. The need to use the compound eye to locate food would force P. tergestina to remain in the illuminated surface water during the day and release their offspring in darkness thus decreasing their visibility to fish. The diel cycle in reproduction in P. avirostris was not as pronounced as that in P. tergestina, but females with mature embryos were still more common at night than during the day. As a grazer of phytoplankton, P. avirostris performed diel vertical migration and retreated to dark, deeper waters during the day, avoiding visual predators.