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\ud river sediment at a maximum temperature of 4 "C; and to determine the ecological mechanism(s) facilitating their\ud co-existence. The ciliate community was diverse (65 species [8 of which are new], belonging to 50 genera, from 17\ud orders). The sediment supported a superficial mat of diatoms (> 30 species). These served as food for at least 16\ud ciliate species. The size frequency distribution of ingested diatoms was almost identical to that for the diatoms in the sediment: thus the probability of a diatom being 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 consume\ud the variety of microbial food types. Taken as a whole, the ciliate community was capable of feeding on all microbes,\ud including other protozoa, 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\ud distribution of many ciliate species, it is believed likely that the species richness we recorded is representative of\ud the expanse of sandy sediment in this river, on this occasion.