Diel cycle in the percentage abundance of parthenogenetic females with embryos of different developmental stages in four species of marine cladocerans

Abstract

The diel cycle in the percentage abundance of parthenogenetic females carrying embryos of different developmental stages in the marine cladocerans Penilia avirostris, Pseudevadne tergestina, Pleopis polyphymoides and Pleopis schmackeri was studied at the edge of an artificial rocky shore area in a semi-enclosed bay in Hong Kong. Females carried embryos during both day and night, but females with fully-developed embryos were found predominantly at night. The diel cycle in the abundance of females with mature embryos was most pronounced in P. tergestina, and less prominent, but still clearly noticeable in P. avirostris, P. schmackeri and P. polyphymoides. The absence or scarcity of females with mature embryos during daytime could be caused by both selective predation by visual predators and nocturnal maturation and release of embryos. Juveniles of Acanthopagrus schlegeli (black seabream) were the most abundant planktivorous fish in the study area in spring. Stomach content analyses revealed that these daytime predators fed intensively on marine cladocerans and exhibited a strong selection for females with mature embryos. On the other hand, the gradual decline in the percentage of females with advanced embryos during the latter part of the night, when feeding by visual predators presumably had not yet begun, suggests that there was a tendency for nocturnal release of neonates in marine cladocerans.