Genotype (by subsequent designation Schmidt, 1818)

Ostrea edulis Linnaeus, 1758.

"Shell subcircular, relatively constant in shape within the species; lower valve shallow not recessed under the hinge; muscle scar subcentral, upper valve may be flat and opercular, page 4

Text-fig. 1.—General systematics of O. angasi and O. lutaria. Fig. A—General diagram of O. lutaria. Fig. B—Variation of the posterior adductor muscle scar in O. lutaria. Fig. C—General diagram of O. angasi. Fig. D—Variation of the posterior adductor muscle scar in O. angasi.

Abbreviations: a.s., anterior suture; c., catch muscle; exh.c., exhalent chamber; l.m., left mantle notch; p., palps; per., pericardium; p.s., posterior suture; q., quick muscle; v.m., visceral mass.

page 5 or domed and subequal to the lower, and it may or may not have denticles along the margin laterally to the hinge which fits into pits in the lower valve. Prodissoconch with a long hinge, two denticles at each end, of which the anterior pair are reduced; ligament at the centre or displaced slightly anteriorly. Without a promyal chamber, gill ostia and the eggs are relatively large, fecundity relatively low; auricles of the heart unite broadly, the ventricle not penetrated by the rectum; 'catch' area of the adductor muscle more than 50% of the area. Incubatory forms. Usually totally submerged or baring only on spring tides, in high saline waters free from turbidity."—Thomson (1954, p. 141).

O. lutaria Hutton, 1873 (Pl. 2) the mud-oyster and O. heffordi Finlay, 1928 (Pl. 3), the southern rock-oyster are here recognised as the only representatives of the genus Ostrea in our waters. Hutton (1880, 1884, 1904), Cox (1883), Suter (1913), Oliver (1923), Finlay (1928), and Thomson (1954) regard the New Zealand mud-oyster as conspecific with the South Australian mud-oyster. Iredale (1924, p. 191) referred the South Australian mud-oyster to O. sinuata Lamarck and Finlay (1928) referred the New Zealand mud-oyster to this genus also. Until the present paper the New Zealand mud-oyster has been diagnosed as O. sinuata. Lamy (1924, p. 84) clearly shows that O. sinuata Lamarck is a species of Crassostrea with a recess under the hinge, a feature not possessed by the Australian or New Zealand mud-oyster. Furthermore, the New Zealand mud-oyster is not recognised here as a synonym of the South Australian mud-oyster. Reasons for regarding the South Australian mud-oyster as distinct from the New Zealand mud-oyster are based on Sowerby's original description of O. angasi (1873, pl. 13) and examination of shells and soft parts of O. angasi from Port Phillip and Cronulla, Australia.

In his description Sowerby made no reference to the ligament, muscle shape and the presence or absence of denticles and his illustration does not show these structures. Nonetheless, it can be seen from the illustration that the lower valve extends beyond the upper valve and that both valves are fluted and crenulated. New Zealand shells are distinctive in that both valves meet around the entire free margin and the edges of the valves may sometimes be waved but are never crenulated. Taken together, the short description and illustration of O. angasi Sowerby, demonstrate that the New Zealand mud-oyster is not to be regarded as conspecific with O. angasi.

Moreover, oyster shells from Port Phillip show that the muscle impression is distinctly different from the New Zealand mud-oyster. The impression of the 'quick' area of the posterior adductor muscle is always 50% larger than the impression of the 'catch' area in Port Phillip shells, whereas in New Zealand shells, it is the 'catch' area impression that is always 50% or more larger than the 'quick' area. There is also a very definite notch between the 'catch' and 'quick' muscle area on the ventral surface of the muscle both in the meat and in the impression on the shell, but in New Zealand mud-oyster shells, the constriction, if present, is minute and only discernible on examination of the meat (Text-fig. 1. Band D).

The upper valve of the South Australian mud-oyster shell has narrow scales in close imbricating rows (Pl. 1, fig. 2) whereas the scales on the New Zealand shells (Pl. 2, fig. 2) are broader in comparison and less closely packed together. The Australian shells are in general much thicker and heavier than the New Zealand shells. The rate of calcium deposition, however, may be controlled by environmental conditions and too much reliance should not be placed on this latter feature for specific identification.

The South Australian shells are blue-grey to deep purple. In contrast, the New Zealand shells are dark brown or dark grey and shells from less than three fathoms are a pale green-ochre.

Several features of the gross anatomy of the South Australian mud-oyster are also seen to be different from those in the New Zealand mud-oyster. In shape the page 6 soft parts are elongate in the former, in contrast to the round form found in the New Zealand oysters (Text-fig. 1, A and C). The mantle lobes are more translucent, suggesting histological differences in the mantle tissue. The mantle hood covering the palps is more open and loose whereas in the New Zealand oysters the mantle hood completely encloses the palps. In some specimens of the South Australian mud-oyster, the visceral mass was not fused to the anterior suture. Further discernible differences found in the mantle structure are the arrangement of the tentacles on the middle and inner folds. The middle fold in the South Australian mud-oyster has two rows of tentacles: an outer row of small bulbous tentacles closely placed to each other and an inner row of tentacles which are 2–3 times longer than their basal width, with 1 or 2 smaller tentacles of the outer row to every larger tentacle. In the New Zealand mud-oyster the larger tentacles are much further apart, there being at least 3–5 smaller tentacles to each larger tentacle.

The inner fold in the South Australian oyster consists of regular bulbous tentacles, 4 times longer than their basal width, regularly arranged and separated one from the other by a space equal to their width, or even further apart, whereas the tentacles of the inner fold in the New Zealand oysters are 6 times longer than their basal width and are never more than their own basal width apart.

In all 48 specimens of the South Australian mud-oysters examined, there were no well-developed pseudofaecal tracts. However, this could be a reflection of the feeding conditions at the time of collection and more observations on the tract in the living animal will be necessary to give a decision on the permanent presence of this feature. Accessory hearts are present in the New Zealand mud-oyster but have not been observed in any specimens examined from South Australia.

The posterior suture is greatly displaced from the posterior adductor muscle in the Australian mud-oyster (Text-fig. 1, C) so that the opening of the supra-branchial chamber into the exhalent chamber is very wide, in fact twice as wide as in the New Zealand mud-oyster. In the latter species, the gills follow very closely the posteroventral curvature of the adductor muscle and terminate at the posterior suture which is much closer to the muscle, so that the opening of the suprabranchial chamber into the exhalent chamber is narrower than in the Australian mud-oyster (Text-fig. 1, A).

All these readily recognisable differences of the shell and soft parts of the New Zealand mud-oyster show this oyster to be a distinct species here referred to O. lutaria Hutton, 1873, the first valid name available for the New Zealand mud-oyster. The first cited locality from which this species was taken is Pelorus Sound.

O. charlottae was proposed by Finlay (1928, p. 265) as a new name for O. hyotis Suter non Linnaeus. This species includes all those New Zealand mud-oysters that possess lateral and marginal frills on the lower valve. Dell (1960, p. 143) using shell characters only, recognises oysters with frills as a subspecies of the mud-oyster which he refers to O. sinuata charlottae.

The present study of approximately 2,000 shells from a wide range of localities has shown that the possession of imbricate marginal and lateral frills is not sufficient to distinguish this sub-species from the mud-oyster, O lutaria. Frilled shells are more characteristic of deeper water where they are found free on the substrate which is usually composed of mud or sand. Shells with well-developed frills are especially common in depths greater than 20 fathoms off Otago, Kaikoura, and in Tasman Bay (pers. comm. Dr R. Dell, Mr J. Graham and Mr J. H. Choat). Non-frilled shells are more common in the shallower water but a complete gradation from frilled shells to non-frilled shells has been found in every bed examined.

The anatomy of the soft parts and the life-history of oysters with frilled shells characteristic of O. sinuata charlottae also support the view that this is a growth form of O. lutaria Hutton.