Species Boundaries in Tintinnid Ciliates: A Case Study – Morphometric Variability, Molecular Characterization, and Temporal Distribution of H elicostomella species ( C iliophora, T intinnina)

Tintinnids are a large group of planktonic ciliates with diverse morphologies. The range of variability in lorica shapes and sizes can be very high even within a single species depending on life stages and environmental conditions, which makes the delimitation of different species based on morphological criteria alone very difficult. Accordingly, comparisons of morphological and molecular variability in tintinnids are necessary to provide a pragmatic approach for establishing species boundaries within this diverse and poorly understood group. We investigated the temporal distribution of species of the hyaline tintinnid H elicostomella ( C iliophora, T intinnina), which were collected daily from S eptember 2008 to A ugust 2009 in J angmok B ay of G eoje I sland on the south coast of K orea. Two forms – a long form and a short form, were discovered. The long form was found in cold waters in F ebruary and M arch whereas the short form occurred in warm waters from J une to O ctober. Thus, these two forms were seasonally isolated. However, all the morphological characteristics for these two forms overlap to some degree. A comparison of the small subunit ( SSU ) rDNA , ITS 1‐5.8 S ‐ ITS 2, and ITS 2 sequences from two forms revealed 0.5%, 3.8%, and 5.6% divergences, respectively. Morever, one compensatory base change ( CBC ) and three hemi‐ CBC s were identified from ITS 2 secondary structures of these two forms. All these data suggest that these two forms represent two distinct species despite their highly similar lorica morphology. The phylogenetic position of the genus H elicostomella was also examined using SSU rDNA sequences.