Diel variation in the egg ratio of Hexarthra bulgarica in the high mountain lake La Caldera (Spain)

Hexarthra bulgarica is the dominant rotifer species in the lake La Caldera (south of Spain) both in number of individuals and in biomass. This species shows a nocturnal vertical migration and a simultaneous diel horizontal movement. In the present study, a detailed 24-h sampling program was carried out at four depths in six stations located along the two main transects of lake La Caldera. The results show the existence of diel cycles in oviposition of amictic and mictic (male and resting) eggs of this rotifer. The egg ratio for amictic and mictic eggs increases during the night. The oviposition of amictic eggs was also higher in the pelagic zone of the lake. Oviposition of amictic and mictic eggs was not related with density of H. bulgarica nor with density of competitors (Mixodiaptomus laciniatus). Our results suggest that UV radiation or an endogenous circadian rhythm of activity could be responsible of the diel cycle in oviposition of H. bulgarica.