Hyotissa imbricata ( Lamarck, 1819 )

Lam, Katherine & Morton, Brian, 2004, The Oysters Of Hong Kong (Bivalvia: Ostreidae And Gryphaeidae), Raffles Bulletin of Zoology 52 (1), pp. 11-28 : 25

Hyotissa imbricata ( Lamarck, 1819)

Common name: Imbricated oyster ( Figs. 6 A-C)

Ostrea imbricata Lamarck, 1819: 213 .

Ostraea imbricata Sowerby, 1871 , sp. 36, Pl. 17, Figs. 36a-b.

Dendostrea imbricata Habe, 1951: 93 .

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

Hyotissa hyotis forma imbricata Stenzel, 1971 : N961, N1026, Figs. J5 , J49.

Hyotissa hyotis imbricata Habe & Okutani, 1975: 195 .

Parahyotissa imbricata Harry, 1985: 130 .

Material examined. – Type not studied. Probably in MNHN or in the Geneva, Switzerland, museum .

Description. – Shell large, up to 150mm in height. Outline either orbicular or semicircular, equivalve. Hinge line is long and straight and approximately equal to shell length in smaller individuals. Shell relatively thin and light in smaller individuals but becomes thicker and heavier in larger ones. Both valves have dichotomous ribs radiating from the umbo and which bear prominent growth squamae that increase in length to become hyote spines. In older, eroded individuals, hyote spines are obsolete but growth squamae are still prominent at the newly-grown ventral margin. Attachment area varies from small and limited at the umbones, to large and encompassing the entire left valve. The shell margins of both valves are undulate, fitting into each other and, thus, reflecting the positions of the radial ribs. External coloration of both valves varies from white to cream in younger individuals to pink and purple with dark purple bands in older ones. The interior of the shell is white with pearly opalescent patches within the pallial area. The large commissural shelf is dull in texture because of the vesicular shell structure. The ligament is short and the ligamental area is usually not elongate. Umbonal cavity shallow. Chomata are absent. The adductor muscle scar is large, white or pink, approximately circular and positioned postero-dorsally from the centre of the valve.

Distribution. – This species can be obtained only by trawling in the southern and western waters of Hong Kong, e.g. near Po Toi Island and northern Lantau Island. It is an Indo-West Pacific species which occurs in the Philippines, Australia and South China Sea to the Boso Peninsula, Japan.

Remarks. – This species is distinguished from Hyotissa sinensis in having a white shell interior, obvious hyote spines and radiating ribs on both valves.

Figures

Fig. 6. Hyotissa from Hong Kong showing external and internal views of the right valves and the internal views of left valves. A-C. H. imbricata (SBMNH 345718). D-F. H. sinensis (NHM 20030494). Scale bar = 20 mm.

Fig. 5. Dendostrea and Ostrea from Hong Kong showing external and internal views of the right valves and the internal views of left valves. A-C. D. folium (SBMNH 345717). D-F. D. crenulifera (NHM 20030493). G-I. O. denselamellosa (AM 108985). Scale bar = 20 mm.