(here designated) Indosphenia kayalum sp. n.

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.

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.

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

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.

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.

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.

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”.

Shell (Fig. 5a–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.

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. 6a–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.

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).

Corbula chilkaensis. Type material was not available for study.

Cuspidaria annandalei. Syntypes, 3 shells, Lake Chilka [this can be interpreted as 19°42.9'N, 85°18.6'E]. Ex Preston, NMW.1955.158.18922-18923.

Corbula chilkaensis. “Rambha, S. end of Lake Chilka” [this can be interpreted as off Rambha approximately 19°31.2'N, 85°6.3'E].

Cuspidaria annandalei. “Lake Chilka, 4–9 miles E. by S.1/2 S. of Patsahanipur, 4–5 ft” [this can be interpreted as off Patasanipur approximately 19°42.9'N, 85°18.6'E].

Examination of shells (Fig. 5k–m) of Cuspidaria annandalei shows these to be identical with small specimens of the typical Port Canning form, including the presence of radial striations on the prodissoconch. Lake Chilka, some 55 km in length, is situated in the northern part of the Bay of Bengal approximately 400 km south-west of Port Canning and is largely isolated from the Bay of Bengal by a long shore bar, only connected by a channel less than 2 km wide. For the moment, and based on significant morphological similarities, we regard all of the taxa from the northern part of the Bay of Bengal to be a single species, taking the name of Indosphenia abbreviata (Preston, 1907), with the Lake Chilka population herein deemed a subspecies. However, we note that future molecular study may suggest genetic divergence not readily apparent based on shell morphology.

Syntypes, 5 shells, Ariancoupar near Pondicherry [this can be interpreted as the Chunnambar River mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E]. NHMUK 1893.3.16.6-10.

4 shells, NMW 1955.158 15753 and 50+ shells, NHMUK 1953.1.7, all from Adyar, Madras [this can be interpreted as the Adyar River Mouth, Adyar, S of Chennai], 13°0.8'N, 80°15.4'E, all Winckworth Collection, March 1931 to June 1936.

50+ shells, Cooum River, Madras [this can be interpreted as the Cooum River mouth, Chennai, 13°4.3'N, 80°16.1'E]. Winckworth Collection, September 1931.

Ariancoupan, Pondicherry [this can be interpreted as the Chunnambar River Mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E].

The sample from Adyar most closely matches the form of the syntypes (Fig. 8l–o) from Ariyankuppam, and is described here as typical. Shell (Fig. 8a–e) to over 12 mm in length, inequivalve, left valve slightly smaller; slightly inequilateral, beaks slightly to the posterior (PL/AL = 0.83–0.97); moderately inflated (L/T = 1.9– 2.2). Beaks prosogyrate, directed anteriorly. Outline subovate-sub-rostrate, anterior end broadly rounded, ventral margin curved, L/H = 1.6–1.8; posterior end much narrower than anterior end, sub-rostrate, demarcated initially by sharp carina running from beak to ventral margin of posterior end, but this carina rapidly becoming obsolete; posterior dorsal margin concave; posterior end rounded, often eroded and obscured by periostracum extending beyond shell. Sculpture of weak commarginal lines and dense, thin raised lamellae - these more prominent on angle of rostrum. Exterior of shell with persistent, thin, straw-coloured periostracum. Prodissoconch (Fig. 9c) small P1 (75 µm) much larger P2 (189 µm); P1 with punctate micro sculpture, P2 with commarginal ridges. Hinge myid, with chondrophore in left valve. Chondrophore (Fig. 7e) with proportionately wide ligament insertion plate and short posterior flange with rounded margin; ridge between ligament area and flange prominent. Right valve with very weak pseudo cardinal tooth (Fig. 7i) commonly rising dorsally to obscure beak. Muscle scars poorly defined. Anterior adductor muscle scar elongate and placed medially on anterior face. Posterior adductor muscle scar subcircular, placed close to dorsal margin. Pallial sinus a shallow depression, more sinusoidal than U-shaped (Fig. 8d).

Comparisons of shells from the Adyar and Cooum rivers show some significant difference in shape, despite the rivers being only 6 km apart. The shells from the Cooum River (Fig. 8g–i, k) are shorter posteriorly, thus the beaks are closer to the posterior end, and the height and tumidity are proportionately greater. The shells from the Cooum River are more robust and about 20% are tinged with pale purple (Fig. 8g). Although the tissues are dry, the blotched pattern (Fig. 8j) on the mantle has been preserved and it matches that present in both I. abbreviata and I. kayalum.

has a thicker shell than either I. abbreviata or I. kayalum and possesses a much more prominent sculpture. The beaks lie closer to the posterior end, which is the opposite of both I. abbreviata and I. kayalum. Furthermore, the posterior carina is not developed, whereas in I. abbreviata it is finely developed. Additionally, prodissoconch 2 shows no radial sculpture, as is present in I. abbreviata, making it more similar to I. kayalum in this character.

Holotype (Fig. 10a–c), 1 specimen in 70% ethanol, Length = 7.6 mm. Anterior length = 3.4 mm, Height = 5.4 mm, Tumidity = 3.6 mm. Ezhupunna region of Cochin Backwater, Vembanad Lake 9°50'43.9"N, 76°17'17.2"E. Coll. Philomina Joseph, 3 March 2016. ZSI M-31827/8. Paratypes 20 specimens, as holotype, ZSI M-31828/8. Voucher specimens associated with GenBank sequences MH644188-644191, as holotype, ZSI M-31829/8 to M-31833/8

Ezhupunna region of Cochin Backwater, Vembanad Lake 9°50'43.9"N, 76°17'17.2"E.

Shell (Fig. 10a–k) small, to 8.2 mm in length, thin, translucent, fragile, slightly inequivalve - left valve slightly smaller and overlapped by right valve, tumid across umbonal region (length/tumidity ratio = 2.26 but variable, some specimens more tumid, some much less, range from 1.8 to 2.6. Outline inequilateral, beaks slightly in front of midline with average ratio of posterior length/anterior length = 1.25, but variable due to degree of posterior extension, more equilateral in small shells; overall subovate, broadly rounded anteriorly, narrowed posteriorly, sub-rostrate. Poorly defined weak ridge demarcates narrow posterior dorsal area, best seen in juveniles. Sculpture of commarginal growth lines and weak (but raised) threads. Prodissoconch (Fig. 9d) small P1 (55 µm) much larger P2 (145 µm); P1 with punctate micro sculpture, P2 with commarginal ridges. Hinge myid with chondrophore in left valve. Chondrophore projecting, laminar (Figs 3d, 7j–k), with shallow triangular depression in front; ligament attachment in narrow, deep groove, extending over anterior part of chondrophore; ligament separated from posterior flange by weak ridge. Chondrophore extending posteriorly as narrow flange, with median flexure; posterior rounded and outer face slightly sinuous in juveniles (Fig, 7j), rounded in adults (Fig. 7k). Right valve (Fig. 3d, 7l–m) with resilifer, depressed subumbonally accommodating left valve chondrophore. Anteriorly small projecting pseudo- tooth appears as extension of anterior margin. Muscle scars poorly defined; anterior adductor scar elongate, placed medially on anterior face; posterior adductor scar subcircular, close to dorsal margin. Pallial sinus (Fig. 10j, k) shallow depression, more sinusoidal than U- shaped.

Anatomy (Fig. 11a–d). Adductor muscles proportionately not large; posterior rounded in section, with posterior pedal retractor inserted immediately to its dorsal; anterior adductor muscle elongate, with anterior pedal retractor inserted at its dorsal edge. Mantle thin, distinctly patterned with brown radiating spots (Fig. 11a) visible through thin shell. Margins of mantle mostly fused, small anterior-ventral pedal gape present. Siphons (Fig. 11c, d) fused, rather short, with outer ring of 12–14 tentacles surrounding both inhalant and exhalant apertures; exhalent aperture with simple retractile tube, while inhalant aperture has inner ring of ten tentacles, the anterior-most larger and dull orange in colour. Foot small, distinct heel producing fine, multi-threaded byssus. Gills (Fig. 11b) large, inner and outer demibranchs present, fully reflexed; those of outer demibranch reflected dorsally. Labial palps very small with few sorting ridges. Only detail of alimentary canal readily observable is large style sac penetrating deep into posterior of foot.

The specific name kayalum is from “kayal” the Malayalam (South Indian language) name for the backwaters of Kerala state in which this species lives. The name is intended as a noun in apposition.

The specimens were found attached to the filamentous alga Microspora sp. that was growing on wooden poles in the shallow (1 m) channel in the backwater. The bottom substrate consisted of silty-sand and the measured salinity was 5‰ (oligohaline). Occurring with I. kayalum were the bivalves Mytilopsis sallei and Arcuatula along with a large number of amphipods and polychaetes.

In overall shell shape, Indosphenia kayalum is most similar to the typical form of I. abbreviata, but it differs in being less tumid and with a less well-defined rostrate posterior end on which the carina is weak to obsolete. Prodissococh 2 in I. abbreviata has radial lines, whereas in I. kayalum P2 has only commarginal lines. In size, the prodissoconchs are similar, as is also the punctate sculpture of P1. Indosphenia sowerbyi is much larger, and more robust, such that the sculpture is of commarginal raised ridges, especially over the posterior area. In outline, I. sowerbyi is more tumid, less deep and the beaks are behind the midline. The sculpture of the larval shells is the same. Indosphenia cochinensis has a heavier shell that is rather narrow, it has very prominent ridges, and is distinctly rostrate posteriorly.

Cuspidaria cochinensis (Fig. 12a–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.

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 habitats. 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 populations 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.