Benthic ciliate identification and enumeration: An improved methodology and its application

Abstract

The study of the ecological role played by benthic ciliates is hampered by the lack of a methodology to accurately enumerate benthic ciliates with good taxonomic resolution. As a result, a technique was developed that combined 2 previously published methods developed to identify and count pelagic ciliates and benthic flagellates, respectively. The new method utilizes centrifugation in a non-linear density gradient to separate ciliates from sediment, and the QPS (quantitative protargol stain) silver-staining technique to stain the cilia and nuclei of ciliates which, after centrifugation, are concentrated on cellulose nitrate filters. The wide applicability of the method was shown by utilizing it to count and identify ciliates in cores taken from intertidal sediment and sampled on a 2 mm depth interval. The intertidal cores had a total of 41 species or morphotypes present, but no more than 21 species or morphotypes in any single 2 mm layer. Total ciliate abundance was as high as 2500 cells ml-1, with the upper layers having higher abundance than deeper layers. There was no obvious pattern with depth either for the number of species found in any one layer, or for ciliate diversity. The method was further utilized in an experiment that explored the interactions between benthic ciliates and the ostracod Cyrideis trosa. Ostracods reduced the final abundance of ciliates, but this effect was confined to the upper 5 mm of sediment. Two-thirds of the ostracods were found in the 5-10 mm layer at the end of the experiment, but while there was an effect on ciliate diversity in this layer, there was no effect on total ciliate abundance. We conclude that the method is suitable for studying both the natural distribution and diversity of benthic ciliates and their response to experimental manipulations.