Indosphenia cochinensis (Preston, 1916)

Oliver, P. Graham, Hallan, Anders, Jayachandran, P. R., Joseph, Philomina, V. F. Sanu, & Nandan, S. Bijoy, 2018, Taxonomy of myid bivalves from fragmented brackish-water habitats in India, with a description of a new genus Indosphenia (Myidae, Myoidea, Myidae), ZooKeys 799, pp. 21-46 : 40-42

Indosphenia cochinensis (Preston, 1916)

Cuspidaria cochinensis Preston, 1916: 39, figs 17, 17a.

Cuneocorbula cochinensis (Preston, 1916): Oliver et al. 2017: 1224-5, fig. 2.

N.B.

Cuspidaria cochinensis (Fig. 12 a–c) was transferred from the Cuspidariidae into the corbulid genus Cuneocorbula by Oliver et al. (2016). Besides its extremely corbulid-like shell, the hinge is reminiscent of that of corbulids in which there is a relatively prominent tooth in the right valve. However, following this review, this tooth is no longer regarded as equivalent to the large anterior tooth possessed by Potamocorbula and Lentidium . The tooth in C. cochinensis (Fig. 7o) is a thickening of the hinge margin and thus structurally identical to the pseudo tooth seen in Mya and Sphenia . The chondrophore of C. cochinensis (Fig. 7n) has a posterior flange and this too is identical to the form seen in Sphenia . Consequently, we transfer Cuneocorbula cochinensis to Indosphenia in the Myidae .

Discussion.

The form of the hinge in Indosphenia is more similar to that of juvenile Mya and Sphenia supporting the placement of Indosphenia in the Myidae rather than the Corbulidae . The molecular phylogenetic analysis conducted herein, using COI places Indosphenia kayalum in the Myidae , although with no significant bootstrap support. However, in combination with the morphological data, we consider this molecular result as adding further confidence to this family placement.

The variability in shell form across the species herein assigned to Indosphenia can be seen both visually in the shells figured and in the graphical display of morphometric parameters (Fig. 13). The morphometric parameters show considerable variation within, and overlap between, most populations. Of the samples measured, that of I. sowerbyi from the Cooum River appears to be the most distinctive, even from the Adyar population of the same species only six kilometres distant. Using only dried shells from historical collections we cannot employ molecular techniques to distinguish any effects of geographical distance from habitat. In the Cochin Backwater, there are two quite distinct ecological forms that are separated ecologically. Indosphenia cochinensis is infaunal, living in sand and found close to the entrance of the Kodungallur-Azhikode Estuary, where average salinity is 15.6 ‰. This contrasts with I. kayalum that lives byssally attached among algae and below in muddy sediment in an isolated embayment of Lake Vembanad, with a very low salinity of 5 ‰. These locations are approximately 40 km apart, but they are poorly connected, having separate channels into the Indian Ocean.

The environment may be more important than geographical separation in directing the form of the shell in Indosphenia , so although we have aggregated all of the Port Canning forms under I. abbreviata , it is possible that the two primary forms represent different species. The length/height and length/tumidity ratios (Fig. 13a, c) are significantly different but we have very poor locality and habitat details, only the general location of Port Canning, to make any inferences. Within the estuary of the Matla River there is likely to be a gradient of salinity and a variety of habi tats. Taxonomic problems with brackish water faunas were raised by Muus (1967) and genetic divergence was reviewed by Cognetti and Maltagliati (2000) but studies do not reveal consistent results. The European brackish water cockle Cerastoderma glaucum ( Bruguière, 1798) is morphologically variable, having received 44 species, subspecies, or varietal names ( MolluscaBase 2018a). The variation was considered to be ecophenotypic, but a recent molecular investigation has shown that many popula tions are sufficiently genetically distinct to warrant some nomenclatural recognition ( Tarnowska et al. 2010). A similar result was found for the bivalve Mytilaster minimus (Poli, 1795) ( Camilli et al. 2001).

González-Wangüemert and Vergara-Chen (2014) studying Pomatoschistus marmoratus (Risso, 1810) found significant diversity within Mar Menor lagoon, but Cavraro et al. (2017) did not find genetic diversity within the Venice lagoon for Aphanius fasciatus (Valenciennes, 1821). Both studies found significant diversity between lagoon and marine populations of the same species. We cannot be certain if there is genetic diversity within the Port Canning samples, but it cannot be excluded.

The morphological complexity of Indosphenia suggests that the brackish waters of India may well represent fragmented habitats and as such would make excellent sites for the study of genetic isolation and divergence.