Abstract
Biomodal (flat/globular or slender/tall) shell/body shapes are associated with dichotomous (simultaneous reciprocal or non-reciprocal) modes of copulation behaviour in the fully-shelled stylommatophoran snails. In flat-shelled groups that copulate simultaneously reciprocally, no study has found an example of enantiomorphism that persists within a population. However, the original description of a flat camaenid snail, Trichocathaicaamphidroma, noted that it is dextral- or sinistral-coiled. By examination of shell surface morphology, we found that shell specimens classified as those of this species include shells of three different morphological species. Namely, T.amphidroma, Trichocathaicavestita (Pilsbry, 1934), comb. n., and Trichocathaicamacrosquamata Páll-Gergely, sp. n. In each of the three species, both sinistral and dextral shells have been collected from presumably one area. Ethanol-fixed soft bodies of single dextral and sinistral individuals of T.vestita, which were available for the first time for interchiral comparison of genital morphology in the present genus, differed from each other in the pattern of penial microsculpture. They might represent enantiomorphs that have recently diverged in allopatry instead of enantiomorphism within a population or species. However, their shell and genital differences were not discrete enough to divide them taxonomically into two morphologically distinct species. Our results demonstrate the importance of evaluating individual variation relative to differences between incipient species in penial morphology, especially between conchologically indistinguishable enantiomorphs in the flat groups. We revise the taxonomy of the genus Trichocathaica including the above-mentioned new species, and Trichocathaicaputeolata Páll-Gergely, sp. n.
Highlights
Left-right reversal of development seldom evolves in the Bilateria (Utsuno et al 2011)
We found that all 72 specimens labelled as Trichocathaica amphidroma were collected from the areas of Luding and Wasigou in west Sichuan, China
The present study discovered that both dextral and sinistral individuals are frequently found in the flat-shelled stylommatophoran snail genus Trichocathaica
Summary
Left-right reversal of development seldom evolves in the Bilateria (Utsuno et al 2011). The chiral phenotype, dextral (clockwise-coiled) or sinistral (counter clockwise-coiled), exhibits maternal inheritance (Okumura et al 2008; Asami et al 2008; Utsuno and Asami 2010; Utsuno et al 2010; Davison et al 2016) In stylommatophorans, their bimodal shell shapes are associated with the dichotomous mating modes. Groups with the flat/globular shaped shell copulate simultaneously reciprocally, whereas those with the tall shape copulate non-reciprocally, exceptional cases are present (Asami 1993; Davison and Mordan 2007) In both of the modes, copulation between the reversed mutant and wild type (interchiral copulation) is less successful than intrachiral copulation with the same morph, which results in positive frequency-dependent selection (Johnson 1982). Frequency-dependent selection against the less common chiral morph is playing a key role for suppression of chiral speciation at least in stylommatophorans
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