Genetic Identities of Cryptic Species in the Strombidium stylifer/apolatum/oculatum Cluster,Including a Description of Strombidium rassoulzadegani n. sp.

ABSTRACT. To assess diversity among cryptic species of the ciliate genus Strombidium, we characterized the small subunit ribosomal DNA gene (SSU‐rDNA) from several lineages that had been identified previously as distinct based on the internal transcribed spacer regions of the rDNA locus. We sequenced SSU‐rDNA from four members of a cryptic species cluster of ciliates from tidepools in the North Atlantic Ocean. One of the sequences was very similar (>99% similarity) to that of Strombidium apolatum. The other three sequences differed from each other and from the closest named species on GenBank by 4–10%. We were able to cultivate only one of these three species. Here we name it Strombidium rassoulzadegani n. sp. and describe its morphology, behavior, and feeding. The history of observations of tidepool Strombidiidae is discussed along with hypotheses about how they may partition the tidepool niche for coexistence. Given the apparent high degree of cryptic diversity of ribotypes in the Strombidiidae, we recommend that no new species descriptions be made without accompanying genetic information.