Hyotissa imbricata ( Lamarck, 1819 )

Lam, Katherine & Morton, Brian, 2009, Oysters (Bivalvia: Ostreidae And Gryphaeidae) Recorded From Malaysia And Singapore, Raffles Bulletin of Zoology 57 (2), pp. 481-494 : 490

Hyotissa imbricata ( Lamarck, 1819)

Common name: Imbricated oyster ( Fig. 6 )

Ostrea imbricata Lamarck, 1819: 213 .

Ostraea imbricata – Sowerby, 1871, sp. 36, Pl. 17, Fig. 36 a, b.

Dendostrea imbricata – Habe, 1951: 93.

Pretostrea imbricata – Habe & Kosuge, 1967: 138, Pl. 51, Fig. 14.

Hyotissa hyotis forma imbricata – Stenzel, 1971: N961 & N1026, Fig. J5 & J49.

Hyotissa hyotis imbricata – Habe & Okutani, 1975: 195.

Parahyotissa imbricata – Harry, 1985: 130.

Material examined. – Type not studied. Probably in the Geneva, Switzerland Museum .

Voucher: [ NHM] 20090293, “ Dredged from off Pelangi Beach, ~ 2–3 m, Pulau Langkawi, Malaysia ”, coll. B. Morton, 7 Feb.2004, two individuals preserved in alcohol .

Description. – Shell semicircular or orbicular, up to 45 mm in height, 50 mm in length and 15 mm in width, equivalve. Hinge line long and straight and approximately equal to shell length in smaller individuals. Shell relatively thin and light in smaller individuals but becoming thicker and heavier in larger ones. Both valves with dichotomous ribs bearing prominent growth squamae that increase in length to become hyote spines. In eroded individuals hyote spines obsolete but growth squamae still prominent at the newlygrown ventral margin. Attachment area from small and limited at the umbones, to large and encompassing the entire left valve. Shell margins of both valves undulate, fitting into each other and, thus, reflecting the positions of the radial ribs. External coloration of both valves from white to cream in younger individuals to pink and purple with dark purple bands in older ones. Shell interior white. Commissural shelf large, dull in texture because of the vesicular shell structure. Ligament short, ligamental area usually not elongate. Umbonal cavity shallow, chomata absent. Adductor muscle scar large, white or pink, approximately circular and positioned postero-dorsally from the centre of the valve.

Distribution and habitat. – An Indo-West Pacific species, occurring from the Philippines, Australia and South China Sea to the Boso Peninsula, Japan. This subtidal species is usually obtained by trawling and diving (Lam & Morton, 2004).

Remarks. – Morris & Purchon (1981) recorded two gryphaeids, Hyotissa hyotis ( Linnaeus, 1758) and Hyotissa numisma ( Lamarck, 1819) by trawling off the West coast of Malaysia. Our collected Hyotissa is distinguished from these two in having an irregular and rounded shell margin, crenulations and a white shell interior. Hyotissa hyotis has regular, strong and acute shell margin crenulations and H. numisma has white shell exterior and slightly violet or largely white interior.

Figures

Fig. 6. Hyotissa imbricata (Lamarck, 1819) from Pulau Langkawi, Malaysia (NHM20090293). A, external view of the right shell valve, B, external view of the left shell valve, C, internal view of the right shell valve and D, internal view of the left shell valve.

Fig. 5. Dendostrea folium (Linnaeus, 1758) from Pulau Langkawi, Malaysia (NHM20090292). A, external view of the right shell valve, B, external view of the left shell valve, C, anterior view of an individual, showing the “graspers” developed to hold on to a Modiolus micropterus shell, D, internal view of the right shell valve and E, internal view of the left shell valve.