The life history of Cyclocypris ovum (Ostracoda) in a permanent karstic lake

Abstract

We studied the life history of Cyclocypris ovum, a small candonid ostracod, in a Spanish mountain karstic lake (Lake La Cruz). Samples were collected from the littoral area of the lake between June 1996 and December 1998 and the relative abundances of all developmental instars were analysed. Hatching took place between November and May and juvenile development lasted around 6 months; adult life-span was 6-8 months. In summer, a sharp decline in adult population densities was observed, and juveniles completely disappeared until the next hatching period. This population decline is probably a result of mortality, and the commencement of dormancy in immature adults, which may move towards the bottom to enter dormancy buried in the sediments. These processes may be triggered by the combined effects of summer whiting and an increase in water temperature above 20°C. Densities of mature adults progressively increase in autumn, suggesting that the end of dormancy is stimulated by decreasing temperatures and turbulence. In this way, females lay eggs that will hatch mainly during the overturn period. In permanent waters, dormancy of an immature adult stage which optimises reproduction and growth under favourable conditions, could be more adaptive than resting egg production (as commonly occurs in ostracods inhabiting temporary waters). Further research is needed to know to what extent temperature variability and calcium carbonate precipitation affect the life history of this small, nektonic ostracod, which is common in northern Europe but rather rare at low altitudes in the Iberian Peninsula.