DIVERSITY OF FREE-LIVING CILIATES IN THE SANDY SEDIMENT OF A SPANISH STREAM IN WINTER

This study had two objectives: to determine the number of (phenotypic) ciliate species co-existing in 1 m2 of sandy river sediment at a maxi mum temperature of 4-degrees-C; and to determine the ecological mechan ism(s) facilitating their co-existence. The ciliate community was dive rse (65 species 18 of which are new], belonging to 50 genera, from 17 orders). The sediment supported a superficial mat of diatoms (> 30 spe cies). These served as food for at least 16 ciliate species. The size frequency distribution of ingested diatoms was almost identical to tha t for the diatoms in the sediment: thus the probability of a diatom be ing ingested appears to be a simple function of its relative abundance . Two factors were probably important for the co-existence of ciliate species: wide variation in cell size and shape enabled them to occupy most habitats; and they deployed a variety of feeding mechanisms to co nsume the variety of microbial food types. Taken as a whole, the cilia te community was capable of feeding on all microbes, including other p rotozoa, up to a size of about 80 pm. Considering the broad diversity of ciliate habitats available within 1 m2, the importance of physical transport processes in the river basin, and the known cosmopolitan dis tribution of many ciliate species, it is believed likely that the spec ies richness we recorded is representative of the expanse of sandy sed iment in this river, on this occasion.