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

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 Cr£teil. a shallow temperate sand-pit lake, there is a seasonal morphological variation of the setae: the plumose forms were restricted to the wanner 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. Introduction The freshwater cyclopoid copepod Acanthocyclops robustus (G.O.Sars, 1863) is a common lacustrine species. Gross morphological variation is common, resulting in taxonomic confusion with Acanthocyclops vernalis (Fischer, 1853), another highly variable species. The systematic status of these two cyclopids was strongly debated until their definite distinction on the basis of chromosome number (RUsch, 1960; Einsle, 1977), the shape of the genital segment (Petkovski, 1975; Kiefer, 1976) and ecological preferences (Fryer, 1985). An extensive morphological analysis of three North American species of Acanthocyclops (A.brevispinosus Herrick, 1884, A.vernalis and A.robustus) was recently carried out by Dodson (1994). The size and morphology of A.robustus are highly variable. Like many other zooplanktonic organisms, this species displays seasonal variation in length (Einsle, 1977; Vijverberg, 1977; Reed, 1986). This phenomenon was first pointed out by Lilljeborg (1901) and has been noted for copepods in different genera, e.g. Cyclops strenuus strenuus (Elgmork, 1959), Acanthocyclops carolinianus (Yeatman, 1959), Diacyclops thomasi (Reed and Aronson, 1989), Metacydops minutus (Maier, 1992), Mixodiaptomus kupelwieseri (Champeau, 1965), Eudiaptomus gracilis and Eudiaptomus graciloides (Hofmann, 1979), and Eudiaptomus vulgaris (Tonolli, 1971). Moreover, Smyly (1955) has shown that the length of the furcal rami of Eucyclops spp. changes seasonally in ponds near Windermere (UK). As a rule, the specific 'spine formula' of the swimming legs is a reliable taxonomic criterion for freshwater cyclopidae (Kiefer, 1929; Gurney, 1933). However, this is not the case for A.robustus, nor for A.vernalis, nor the North American species A.carolinianus (Yeatman, 1944) (Yeatman, 1959). In addition, the morphology of the swimming © Oxford University Press 907