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Abstract
Ostracods are drastically reduced crustaceans, with never more than eight appendages enclosed between two valves, leaving only a limited number of morphological characters for species delineation. Conservative morphology of characters used to define genera, along with high variability of characters used to define species are creating problems in applying a morphospecies concept. A high intraspecific variability in a Lake Biwa (Japan) endemic, Physocypria biwaensis (Okubo, 1990), has been observed previously but was never studied in detail. Two sympatric forms, differing in pigmentation and size, suggest a presence of reproductive isolation. The aim of this study is to employ molecular and morphometric tools to aid in species delineation within P. biwaensis complex and reconstruct their phylogenetic relationships. A fragment of the mtCOI gene was amplified from 30 specimens, and an additional 37 specimens were studied for morphological characters. Resulting phylogenies showed that each morphologically distinct form is associated with a distinct phylogenetic group based on mtDNA. The average pairwise distance is very low (5%), indicating a recent divergence time. I speculate that there is a possibility that one of them originated in the lake, while the other probably colonized it afterwards. This seems to be supported with an apparent niche partitioning at different depths. In spite of the fact that traditionally used sexual characters are highly variable in these two species, the morphometric analysis of shell and soft part related characters clearly delineates them and suggests that such characters may be useful for future detection of seemingly cryptic ostracod species.
Highlights
Development of molecular tools for species delineation has considerably increased the number of cryptic species worldwide
Comparison of the mean values of all morphometric data between the two forms indicates that all average values for the dark form are smaller than for the light form (Fig. 6) and the t-Test statistics (S1 Table) shows that these differences are highly significant
The smallest measured light individual was 0.55 mm long while the largest was 0.62 mm
Summary
Development of molecular tools for species delineation has considerably increased the number of cryptic species worldwide. It has been shown [1] that they are almost evenly distributed among major metazoan taxa and biogeographic regions, which has theoretical and practical consequences for many spheres of biology, but in particular for the global conservation efforts and taxonomic initiatives. In the past decade several ostracod species from different lineages. The funders had no role in study design and analysis, decision to publish, or preparation of the manuscript
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