Indosphenia Oliver, Hallan & Jayachandran

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 : 28-30

Indosphenia Oliver, Hallan & Jayachandran gen. n.

Type species.

(here designated) Indosphenia kayalum sp. n.

Nominal species included.

Sphenia sowerbyi EA Smith, 1893; Corbula alcocki Preston, 1907; Corbula gracilis Preston, 1907; Cuspidaria annandalei Preston, 1915; Cuspidaria cochinensis Preston, 1916.

By inference from descriptions Corbula abbreviata Preston, 1907; Corbula calcaria Preston, 1907; Corbula pfefferi Preston, 1907; Corbula chilkaensis Preston, 1911.

Description.

Slightly inequivalve, left valve smaller than right valve, almost equilateral to posteriorly extended, rather inflated. Outline subovate, anterior end broadly rounded; posterior narrowed, sub-rostrate. Sculpture of commarginal lines and very thin, weak lamellae; rostrum with a defined keel, at least in early growth stages. Pallial sinus very shallow, adductor muscle scars subequal, posterior scar subcircular, anterior scar elongate. Right valve with sub-umbonal, depressed resilifer accommodating chondrophore from left valve. Anteriorly, small projecting pseudo-tooth appears as extension of anterior margin (Fig. 3d). Left valve with projecting laminar chondrophore (Fig. 3d) plus shallow triangular depression anteriorly. Ligament attachment in narrow deep groove and extending over anterior part of chondrophore; ligament separated from posterior flange by weak ridge. Chondrophore extending posteriorly as narrow flange with median flexure in some individuals, its posterior end rounded and its outer face slightly sinuous in juveniles; in adults, posterior flange projects beyond ligamental portion and is rounded.

In larger specimens, anterior tooth on right valve can be eroded and scarcely visible, chondrophore can project further and flange can be reduced. Mantle edge fused except for pedal gape and short paired fused siphons. Mantle patterned with darkly pigmented radiating blotches. Gills with both demibranchs. Labial palps small. Byssus of very fine threads, but not observed in all species.

Etymology.

Indosphenia - combining the taxon provenance (India) with the related genus Sphenia . Gender feminine.

Remarks.

The molecular and morphological data strongly suggest that Indosphenia is a member of the Myidae . Huber (2010) postulated that all Corbula species described from Port Canning by Preston (1907) should be considered as a single species in the genus Potamocorbula . This conclusion was apparently reached on the basis of Preston’s descriptions rather than examination of actual specimens. Given that Preston did not describe the hinge and seemingly confused the anterior and posterior ends, it is not surprising that their true relationship went unrecognised until now.

The structure of the hinge of Indosphenia is generally identical to that of Sphenia sensu stricto as represented by its type species S. binghami , and its Indian Ocean counterpart S. perversa . However, this structure is also very similar to that seen in the juveniles of both Mya arenaria and Mya truncata . Consequently, hinge structure would not appear to be a useful character at the generic level. Instead, it suggests that Sphenia represents a neotenous retention of the juvenile byssate characters of Mya .

Compared to the deep burrowing habits of Mya , Sphenia exhibits a byssate nestling habit, which is manifested in their differing characters; typical Sphenia (Fig. 4) is irregular, heteromyarian and strongly inequilateral with the anterior end reduced. The posterior end is subtruncate and the siphons are large with well-developed musculature (Fig. 11e) and a corresponding deep and broad pallial sinus inside the shell valves. The following species share the nestling habit with corresponding similarities in shell form and anatomy: S. binghami , (see Yonge 1951), S. perversa (see this paper), S. antillensis Dall & Simpson, 1901 (see Narchi and Domaneschi 1993), S. fragilis (H & A Adams, 1854), S. hatcheri Pilsbry, 1899 (see Pastorino and Bagur 2011), S. coreanica Habe 1951 (see Zhang et al. 2012) and S. elongata Zhang et al., 2012. A further four East Pacific species are described by Coan (1999) and these too would appear to be nestling taxa.

Indosphenia differs from Sphenia in being almost equilateral with a narrow, almost rostrate, posterior end and a very short pallial sinus. This reflects the infaunal lifestyle of I. kayalum . Some specimens of I. sowerbyi have the posterior end encrusted with epifauna while the anterior end is relatively clean, suggesting that this species lives in sediment with the anterior end at, or close to, the sediment surface.