Indosphenia kayalum Oliver, Hallan & Jayachandran, gen. n. et

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 : 37-39

Indosphenia kayalum Oliver, Hallan & Jayachandran, gen. n. et sp. n.

Type material.

Holotype (Fig. 10 a–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

Type Locality.

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

Description.

Shell (Fig. 10 a–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, 7 j–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, 7 l–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. 11 a–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.

Etymology.

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.

Habitat.

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.

Remarks.

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.