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Some Studies on the New Zealand Oysters

[Introduction]

At the time of this account there is considerable difference of opinion concerning the systematic status of the New Zealand oysters, more especially the mud-oyster. This has arisen in the main because previous authors have relied entirely on shell characters and some species were described from single shell valves alone.

The first description of the genus Ostrea based on shell characters was that of Linnaeus, who included in the genus the true oysters and also the pectens, the limas and the amusiums. The work of Orton (1928) who based his classification on the shell characters but included also features of the life history, was a major step forward in the revision of the genus Ostrea. Orton separated living oysters into two major divisions which he referred to as 'type 1' and 'type 2,' and his classification has provided a basis for all following workers.

Type 1 was based on Ostrea edulis in which "the shell is subcircular; the egg is large; the adult larviparous; the individual is hermaphrodite; spawning occurs at medium temperatures round 15.0°C.; and the species flourish in temperate regions." The known species included by Orton in this type are: Ostrea edulis (the European oyster), Ostrea lurida (the British Columbia oyster) and Ostrea angasi (the South Australian mud-oyster). Type 2 was based on O. virginica, a species in which "the shell is elongated in the anterodorsal and posteroventral direction; the egg is small; the adult non-larviparous; the individual of one sex only; spawning occurs at moderately high temperatures, around 20.0°C; and the species flourish in sub-tropical or tropical regions." The known species included by Orton in this type are: O. virginica (the eastern North American oyster), O. angulata (the Portuguese oyster) and O. cucullata (of world-wide distribution in sub-tropical and tropical regions).

Further, Orton (1928, p. 321) suggested that 'type 1' oysters should belong to a new genus, Monoeciostrea and 'type 2' oysters to the new genus Dioeciostrea, but at the same time considered that these genera should not be adopted until definition could be made "founded on adequate information." Orton's two genera are invalid and have not been accepted since earlier names are available.

Nelson (1938, p. 1) in his review of Orton's work noted that the latter did not take into account the promyal chamber and that geographical limitations were unreliable where the American species O. equestris of type 1 is concerned because it ranges into sub-tropical water, and O. virginica of type 2 ranges from the sub-tropical to the more temperate waters of Nova Scotia. Nelson regarded O. denselamellosa of Japan and O. equestris as of Orton's type 1 (their spawning temperatures however, were unknown to him). In the main, Nelson agreed with Orton's divisions but recognised type 2 oysters as belonging to the genus Gryphaea Sacco and proposed to relate all deeply cupped oviparous oysters with the promyal chamber to this genus and the flat, larviparous oysters which lack the promyal chamber to the genus Ostrea.

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Lamy (1929) working on the extant oysters in the Paris Museum collections, used only shell characters for identification. He recognised only one genus, namely Ostrea, but within the genus included 12 subgenera and ' sections.' Lamy's classification is not generally accepted.

Ranson (1948) used the type of crenulations found on the larval shell (prodissoconch) when he reviewed the recent oysters and concluded that these were clearly recognisable as belonging to three genera, Pycnodonte Fischer de Waldheim, Gryphaea Sacco and Ostrea Linnaeus. The characters of the prodissoconch have proved to be completely reliable for generic identification and on occasion are also reliable for specific identification. However, in adult shells, the prodissoconch may be weathered away. Then only a preliminary diagnosis based on other shell and soft part characters can be given until the prodissoconch is finally observed either on adult shells or in the larva.

Gunter (1950, p. 442) reviewed shell characteristics and soft parts and extended the description of the characters given by Orton for type 1 and type 2 oysters. Gunter regarded type 1 oysters as possessing larger eggs, the upper limit produced at one spawning around 1,000,000; the eggs are retained; fertilisation and larval development take place within the mantle chamber; the gill ostia are large; the animals live in sea water of high salinity; the muscle scar is unpigmented and near the centre of the shell; the lower valve is flatter and fairly constantly of subcircular shape; the prodissoconch hinge is long with two crenulations at each end; the ligament is at the level of the cardinal plateau. In type 2 oysters, the eggs are shed into the water; fertilisation and larval development take place in the sea water; the eggs are small; the upper limit produced at one spawning around 50.000.000; gill ostia are small; promyal chamber present; live in estuaries and brackish water with low salinity; muscle scar pigmented; lower valve more cupped; shell shape more elongated and variable; prodissoconch hinge short with two crenulations at each end; ligament anterior and outside the cardinal plateau.

As a result of this work, Gunter (1950, p. 443) came to the same conclusion as Ranson (1948) regarding the generic status of oysters, namely that there are three genera, Ostrea, Pycnodonte and Crassostrea (=Gryphaea of older authors—now used only for fossil shells).

The genera Ostrea and Crassostrea only are known from New Zealand waters. Ostrea is represented by O. lutaria Hutton, the mud-oyster and O. heffordi Finlay, the southern rock-oyster. Both these species show clearly the characters described for the genus Ostrea by Orton and later workers. The genus Anodontostrea proposed by Suter (1917) for the New Zealand mud-oyster has been shown by Ranson (1948, p. 11) and Gunter (1950, p. 443) to be a synonym of Ostrea Linnaeus.

The genus Crassostrea is represented by the Auckland rock-oyster at present referred to Saxostrea glomerata Gould. Gunter (1950, p. 447) states that "Saxostrea differs from Crassostrea only in shell characteristics, which should not be considered valid grounds for separation ... therefore, Saxostrea is a synonym of Crassostrea." Furthermore, Powell (1957) notes that the Auckland rock-oyster is oviparous, a feature typical of the genus Crassostrea. It would seem possible that this oyster is conspecific with the Australian rock-oyster Crassostrea commercials Roughley and Iredale, 1933, but further investigations are needed before a decision can be made.