Seasonal variations in size and morphology of Acanthocyclops robustus (Copepoda Cyclopoida)

Abstract

The variations in the body length, swimming legs and some setae of the freshwater cyclopid Acanthocyclops robustus were analysed during an annual cycle in the field, and in laboratory experiments at different culture temperatures. In Lake Creteil, a shallow temperate sand-pit lake, there is a seasonal morphological variation of the setae: the plumose forms were restricted to the warmer season and the spinose ones to colder periods. Contrary to other years, the specific 'spine formula' of the swimming legs (3-4-4-4) did not vary during the reference year. In laboratory experiments, both sex and post embryonic developmental temperatures had an effect on the length of adults. A significant parental effect on body length was detected. Pairs with anomalous spine formulae produced offspring either with anomalous spine formulae or typical 3-4-4-4 formulae. No significant effects of parental culture temperatures, offspring culture temperatures and sex on the total number of spines were found. Modification of plumose into spinose setae did not occur, whatever the offspring developmental temperatures. The results presented here suggest that the alterations of the appendages of the cyclopid A. robustus could be compared to the cyclomorphosis in other zooplanktonic groups, and contribute to our understanding of the role of phenotypic induction in aquatic biology.