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image of Crustaceana

This study reports on cyclomorphosis of Daphnia laevis in a seasonal floodplain in the Okavango Delta, Botswana. Data were sampled during the flooding season 1998, with laboratory experiments supplementing the field observations. As newborn fish fry entered the floodplain, the relative helmet size and tail spine length in D. laevis gradually increased, whereas total body length decreased. No relationship between temperature and cyclomorphosis was detected. Gut content analysis of 170 fish fry (predominantly Cichlidae) showed extensive planktivory, but D. laevis was negatively selected among the zooplankton. In general, the species of Cladocera were concentrated in the littoral, which was dominated by dense mats of Algae and macrophytes. Distribution of D. laevis, however, showed an opposite pattern, with highest densities (max. 114 ind. l-1) in the open pool area, where refugia from predation were scarce. This strongly suggests that the extensive cyclomorphosis observed is essential for the co-existence of fish fry and D. laevis. To our knowledge, this is the first study of cyclomorphosis in D. laevis.

Dies ist ein Bericht über Cyclomorphose bei Daphnia laevis in einer saisonal überfluteten Fläche im Okavango Delta, Botswana. Freilanddaten wurden in der Überflutungsperiode des Jahres 1998 gesammelt und durch Laborexperimente ergänzt. Nach dem Erscheinen frisch geschlüpfter Jungfische nahm die Länge von Helm und Dorsocaudalstachel bei D. laevis allmählich zu, während sich die Körperlänge verringerte. Ein Zusammenhang zwischen Cyclomorphose und Temperatur war nicht erkennbar. Mageninhaltsanalysen von 170 Jungfischen (vorwiegend Cichliden) ergaben, daß Zooplankton eine wesentliche Nahrungskomponente ist, wobei D. laevis allerdings kaum betroffen war. Im Allgemeinen waren Cladoceren im Litoral konzentriert, wo dichte Algenmatten und Makrophythen dominierten. Im Gegensatz dazu zeigte D. laevis höchste Dichten (max. 114 Ind. l-1) im zentralen, offenen Bereich der Überflutungsfläche, wo Refugien vor Räubern Mangelware sind. Cyclomorphose wird als unabdingbare Voraussetzung für das gemeinsame Vorkommen von Jungfischen und D. laevis erachtet. Unseres Wissens ist dies die erste Untersuchung von Cyclomorphose bei D. laevis.


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