Isognomon Lightfoot, 1786

Mahapatro, Debasish, Nanda, S., Gowda, S., Tripathy, B., Mishra, S. S., Mishra, R. K. & Lenka, S., 2020, New record of an oyster species from Chilika Lagoon, Odisha, north-western Bay of Bengal, Records of the Zoological Survey of India 120 (4), pp. 313-321 : 315-316

Genus Isognomon Lightfoot, 1786

Species Isognomon ephippium Linnaeus, 1758

( Figure 2 & 3 )

1758. Ostrea ephippium Linnaeus, Syst. Nat , ed., 10: 700.

2017. Isognomon ephippium : Subba Rao, Rec. zool. Surv. India, Occ. Paper No. 375: 123, pl. 20, fig. 96.

Synonyms used: Ostrea ephippium Linnaeus, 1758 (original combination); Isogonum scapula Röding, 1798 ; Melina ephippium (Linnaeus, 1758) ; Melina periculosa Iredale, 1939 ; Perna argillacea Gould, 1850 ; Perna cumingii Reeve, 1858 ; Perna imbricata Reeve, 1858 ; Perna lamarckiana Clessin, 1890 ; Perna reeveana Clessin, 1890 ; Perna spathulata Reeve, 1858 .

Common Name: Saddle tree oyster or Jingle shells.

Diagnosis: Shell variable, irregularly rounded in outline ( Figure 3 ), with height about equal to length; dorsal margin straight and relatively short; not expanded posterior in a wing-like ear; on dorsal side anterior margin sharply sinuous; ventrally strongly convex and extending well forward of umbones; posterior margin slightly concave, forming an obsolete angulations with the rounded ventral margin; umbones small, pointing at anterior end of dorsal margin; outer surface covered with concentric lamellar processes, with very low radial ridge ending at postero-ventral angulation; ligamental area with a dozen transverse grooves; hinge dentition are well developed ( Figure 4 ); nacreous area of the inner side of shell surrounded by a broad, non-nacreous margin; colour: outside of shell horny to purplish brown; interior nacreous, with a broad dark brown margin ( Palomares & Pauly, 2019). Large number of miniature, undeveloped, semi-developed pearls are observed on the nacreous layer found in the inner lining of the bivalve.

Habitat: It is strictly a marine benthic bivalve having a sessile mode of life style since it gets attached to a variety of hard objects and mangroves plants by its strong byssus threads ( Figure 5 ). It can sustain in brackish water environment ( Thangavelu et al., 2010) too. This species

New record of an oyster species from Chilika Lagoon, Odisha, north-western Bay of Bengal is frequently observed in coral reef areas either found solitary or by forming colonies through firmly attached with each other by byssus threads ( Morton, 1983) .

Distribution: Available literature suggests that the common distribution of this round shelled saddle tree oyster is in the Indo-Pacific region from East Africa, to Melanesia; north to Japan and south to Indonesia ( Poutiers, 1998). This species is not observed earlier from the coastal region of peninsular India ( Subba Rao, 2017). However substantial information available regarding its occurrence is mostly confined to Andaman waters only.

Figures

Figure 2. External feature of Isognomon ephippium.

Figure 3. (A&B). Exterior of the Bivalve Isognomon ephippium, (C&D). Internal part of I. ephippium.

Figure 4. Ligament area, hinge dentition and white layer showing layer of mother pearl in I. ephippium.

Figure 5. A. Mangrove vegetation at outer channel area of Chilika lagoon. B. Large bivalve colony exposed during low tide period at mangrove vegetation of “Bhabakundaleswar” region, C. Occurrence of multiple colonies of Isognomon ephippium in the sea grass bed at “Arakhkuda”, D. Single colony of I.ephippium from mangrove vegetation.