Life cycles, size and reproduction of the two coexisting calanoid copepodsArctodiaptomus alpinus (IMHOF, 1885) and Mixodiaptomus laciniatus (LILLJEBORG, 1889) in a small high-altitude lake

This paper reports for the first time the sympatric occurrence of the two c alanoid copepods Mixodiaptomus laciniatus and Arctodiaptomus alpinus. In th e small high-altitude lake Seehornsee (Northern Calcareous Alps of Austria) , both species were found to be monocyclic. Arctodiaptomus alpinus hatched from resting eggs in May and June, developed fast and produced resting eggs in August. In M. laciniatus, one year old adults produced subitaneous eggs in spring and early summer. Early development of M. laciniatus was as fast as in A. alpinus but retarded from copepodid stage III onwards. As a resul t of their different reproductive strategies (subitaneous vs. resting eggs) and their different modes of development, both species grew up at the same time but were clearly separated in sexual activity. Body length measuremen ts of all developmental instars revealed that both species were nearly equa l in size until CII, but towards maturity A, alpinus grew larger than M. la ciniatus. Arctodiaptomus alpinus hatched slightly before M. laciniatus, whi ch resulted in an average body size difference of at least 25% between the populations during summer. Less than 2 percent of the M. laciniatus populat ion reached maturity during summer (parallel to A. alpinus), grew as large as A. alpinus and produced exclusively resting eggs like A. alpinus. This s ubpopulation, which was not separated from its competitor in any dimension, is contradictory to established theories of niche separation, but might re present a long-term strategy to survive exceptional winter conditions.