Indosphenia abbreviata (Preston, 1907)

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 : 30-32

Indosphenia abbreviata (Preston, 1907)

Corbula abbreviata Preston, 1907: 215, fig. 1; Ramakrishna and Dey 2010: 269.

Corbula alcocki Preston, 1907: 215, fig. 2.

Corbula calcarea Preston, 1907: 216, fig. 3; Ramakrishna and Dey 2010, 269.

Corbula gracilis Preston, 1907: 216, fig. 4; Ramakrishna and Dey 2010: 270.

Corbula pfefferi Preston, 1907: 216, fig. 5.

Potamocorbula abbreviata (Preston): Huber 2010: 771; Subba Rao 2017: 431; MolluscaBase 2018b.

Type material examined.

Corbula alcocki , Syntype, 1 shell, Port Canning, purchased from Preston, HMUK1909.8.18.33.

Corbula gracilis , Syntype, 1 shell, Port Canning, purchased from Preston, HMUK1909.8.18.32.

The type material of C. abbreviata , C. calcarea , and C. pfefferi was not available for study.

Other material examined.

Sphenia sp. 20 shells and single valves, "Port Canning" [can be interpreted as the Matla River at Port Canning], SE of Kolkata, 22°18.7'N, 88°40.6'E, ex Godwin-Austin Coll, NHMUK 20170342.

Sphenia sp. 5 shells and 1 single valve, Pt. Canning, Matla River, ex Blandford Coll, HMUK 20170343.

Corbula intumescens manuscript name of Stoliczka. 4 shells. Canning, ex. Dr F. Day, HMUK91.9.19.20-3.

Corbula tumescens manuscript name of Day 4 shells. Port Canning, NHMUK 88.12.4.703-6.

Type localities.

All species described by Preston (1907) are localised as "Port Canning, Lower Bengal; in brackish pools". Port Canning lies on the Matla River approximately 80 km upstream from the mouth of the estuary. Both the manuscript names of Stoliczka have the same locality of "Port Canning".

Description.

Shell (Fig. 5 a–d) small, reaching 10 mm in length, thin, fragile, inequivalve - left valve slightly smaller; slightly inequilateral, beaks slightly to the front of centre (PL/AL = 1.3); moderately inflated (L/T = 2.1). Beaks prosogyrate, directed anteriorly. Outline subovate-sub-rostrate, L/H = 1.6, anterior end broadly rounded, ventral margin curved, posterior end much narrower than anterior end, sub-rostrate. Posterior area demarcated by sharp carina running from beak to ventral margin of rostrate posterior end. Postero-dorsal margin concave. Posterior end rounded, but often eroded and obscured by periostracum extending beyond shell. Sculpture of weak commarginal lines and raised threads forming weak projections on posterior carina.

Prodissoconch (Fig. 9b) consisting of small P1 (65 µm) and much larger P2 (182 µm); P1 with punctate micro sculpture; P2 with commarginal ridges crossed by sparse radial threads. Hinge myid, with chondrophore in left valve. Chondrophore (Fig. 7a) with proportionately narrow ligament insertion plate and long posterior flange; ridge between ligament area and flange weak. Right valve with very weak pseudo cardinal tooth (Fig. 7g) commonly rising dorsally to obscure beak.

Intraspecific variation.

Some shells in the unidentified samples from Port Canning are distorted with the rostrate posterior end upturned (Fig. 5e), while others are truncate posteriorly (Fig. 5f). While the overall outline is different, the hinge remains identical in structure (Fig. 7c). The syntype of Corbula alcocki (Fig. 5g, h) represents the posteriorly truncated form, whereas the syntype of C. gracilis (Fig. 5i, j) is closer to the typical shape. The samples labelled as Sphenia tumescens and the sample collected by Blandford, contain larger shells (up to 12 mm in length) (Fig. 6f). Of these, many are more elongate and narrower (Fig. 6 a–d, g) and with a larger length to height ratio of 1.9, and where the posterior dorsal margin is less concave. Of the species described by Preston (1907), that of calcarea may be representative, but it is less than half the size of the shells described here. In one shell, from the tumescens sample, radial striations are present (Fig. 6e), most visible on the interior ( Preston’s pfefferi form). The prodissoconch (Fig. 9a) is identical to that of the typical form in having a radial sculpture.

Remarks.

The shells from Port Canning are highly variable, with many showing distortions of some degree, although the overall form is broadly rounded with a deep anterior end and a sub-rostrate posterior end. Some samples contain a higher proportion of shells that are less deep and more elongate, and these have the manuscript names of tumescens Stolizcka and intumescens Day.

While the extremes of this form look very different, there are intergrades, and in all of these the prodissoconch has the same pattern of radial striations.

While Port Canning is given as the locality, the precise habitat is not provided for these samples, although one does mention the Matla River. Preston (1907) stated that his species came from brackish pools, so it may be that this species inhabits a range of habitats within the Matla River estuary. Port Canning is some 80 km from the open sea of the Bay of Bengal and is undoubtedly subject to much variation in salinity, certainly between isolated pools and the main river channel. Without supporting molecular data, we regard these forms to be part of a single species. The earliest available name for this taxon is abbreviata Preston, 1907, which is based on individuals that in the context of the current sample set may not be typical, in having the posterior end shortened. However, we consider that Huber (2010) was the first reviser of this group of species and we therefore retain the earliest name and the one given priority by him. This is also the name adopted by Subba Rao (2017).