Species Boundaries in Tintinnid Ciliates: A Case Study – Morphometric Variability, Molecular Characterization, and Temporal Distribution of Helicostomella species (Ciliophora, Tintinnina)

Abstract

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 Helicostomella (Ciliophora, Tintinnina), which were collected daily from September 2008 to August 2009 in Jangmok Bay of Geoje Island on the south coast of Korea. Two forms - a long form and a short form, were discovered. The long form was found in cold waters in February and March whereas the short form occurred in warm waters from June to October. 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, ITS1-5.8S-ITS2, and ITS2 sequences from two forms revealed 0.5%, 3.8%, and 5.6% divergences, respectively. Morever, one compensatory base change (CBC) and three hemi-CBCs were identified from ITS2 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 Helicostomella was also examined using SSU rDNA sequences.

Multigene-Based Analyses of the Phylogenetic Evolution of Oligotrich Ciliates, with Consideration of the Internal Transcribed Spacer 2 Secondary Structure of Three Systematically Ambiguous Genera

“…Previous studies have reported the monophyly of the family Strombidiidae (9,11,31). However, recent studies based on SSU rRNA gene sequence data revealed that the tail-less genus Laboea clusters with the Tontoniidae rather the Strombidiidae (8,32), which renders the family Strombidiidae paraphyletic.…”

Declines in both redundant and trace species characterize the latitudinal diversity gradient in tintinnid ciliates

“…At present it is not clear which phenomenon is more common. Among tintinnids and foraminfera cryptic species are generally segregated either temporally or spatially (for example, de Vargas et al, 1999;Xu et al, 2012;Santoferrara et al, 2015), whereas in polymorphic species the different morphotypes occur together (for example, Dolan et al, , 2014Dolan, 2015).…”

“…However whether or not, for example, an East Sea/Sea of Japan population of a 'species' is genetically distinct from that in the High Arctic would not change species inventories or distributions for the geographically distinct assemblages. Given that cryptic forms of tintinnids appear to be segregated either temporally, that is, seasonally (Xu et al, 2012) or spatially, that is, distinct water masses (Santoferrara et al, 2015), they are unlikely to represent hidden diversity within an assemblage. Hence, in comparing assemblages from different systems the true problematic phenomenon is polymorphism that may inflate apparent species richness of an assemblage.…”

Southern Ocean Biogeography of Tintinnid Ciliates of the Marine Plankton

“…However, there is a recent report of two forms of Helicostomella , genetically distinct, but largely overlapping in morphological characteristics (Xu et al. ).…”