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Some Studies on the New Zealand Oysters

1. The Systematic Status of the New Zealand Oysters

1. The Systematic Status of the New Zealand Oysters

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.

The Genus Ostrea Linnaeus, 1758 In New Zealand
(Pls. 1, 2 and 3)

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.

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.

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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.

Ostrea lutaria Hutton, 1873 (Pl. 2)

  • 1873 (non) Ostrea purpurea Hanley. Hutton, p. 84.
  • 1873 Ostrea lutaria Hutton. Hutton, p. 84.
  • 1873 (non) Ostrea virginica Lamarck. Hutton, p. 84.
  • 1880 (non) Ostrea edulis Linnaeus. Hutton, p. 175.
  • 1880 (non) Ostrea discoidea Gould. Hutton, p. 175.
  • 1884 (non) Ostrea edulis purpurea Hanley. Hutton, p. 533.
  • 1884 (non) Ostrea edulis angasi Hutton. Hutton, p. 533.
  • 1884 (non) Ostrea discoidea Gould. Hutton, p. 533.
  • 1904 (non) Ostrea angasi Sowerby. Hutton, p. 93.
  • 1904 (non) Ostrea purpurea Hanley. Hutton, p. 93.
  • 1913 (non) Ostrea angasi Sowerby. Suter, p. 888.
  • 1913 (non) Ostrea hyotis Linnaeus. Suter, p. 889.
  • 1923 (non) Ostrea angasi Sowerby. Oliver, p. 182.
  • 1928 (non) Ostrea sinuata Lamarck. Finlay, p. 264.
  • 1928 Ostrea charlottae Finlay. Finlay, p. 265 (nom. nov. for O. hyotis Suter non Linnaeus).
  • 1957 (non) Ostrea sinuata Lamarck. Powell, p. 78.
  • 1957 Ostrea charlottae Finlay. Powell, p. 78.

Description: Shell shape generally subtriangular in outline ranging from a subcircular form when growing in uncrowded conditions to an elongate form when growing in crowded conditions. Oysters found in shallow beds have thinner shells than those found in deeper beds, the shells being imbricate and frilled when growing recumbent on a shifting substrate such as mud or sand. Right upper valve flat, sometimes domed just under hinge; distally the valve bears dark brown imbricating prismatic scales which are broad and not crowded together. The scales extend to the outer margin of lower valve so that both distal margins meet. Edges of valves not known to be crenulated. Internally valve colour is white or greenish-yellow and there may or may not be a darker brown band on the distal margin of the valve. Ligament triangular, slightly raised from surface of valve and dark brown in colour. Posterior muscle scar is central in position, broadly lunate, anteroventral region extending further forward. Impression of 'catch' muscle at least 50 per cent or more larger in area than impression of 'quick' muscle area.

The left lower valve is the valve by which the oyster is attached, but in adult specimens the attachment is usually lost and the oyster lies free on the substrate. Valve moderately convex, usually smooth but low distinct radiating ribs and prominent imbricate frills may be present. Beaks usually blunt but may sometimes be more acute; very common on lower left valve whereas in right upper valve they tend to be quickly worn away. In Wellington Harbour oysters, colour of left valve is greenish-white whereas in oysters from Foveaux Strait valve is yellow to dark brown. Internally valve is nacreous or dull, greenish-white. Muscle impression may be slightly larger or smaller than that found in right valve. Ligament is winged laterally so that it is triangular, also elevated.

Denticles occasionally present in young specimens and in oysters that have been growing attached to wharf piles in a crowded condition. Denticles vary in number from three to seven and are on one or both sides of right valve only. Pits corresponding to denticles are present on lower left valve.

Palps and gills creamish-white; posterior suture well dorsal. Tentacles of the mantle fold are generally unpigmented. Middle fold has two rows of tentacles; an outer row with tentacles three to four times longer than their basal width and page 8 an inner row of tentacles twice as long as they are broad. There are at least three to five smaller tentacles to each large tentacle. The inner fold has regular tentacles four to six times longer than their basal width and very closely placed. There are fewer and smaller tentacles towards the hinge region. There are 11–13 bars per plica of gill.

Mature oyster larviparous and hermaphroditic, spawning for at least eight months of the year between August and March at temperatures as low as 10.0°C; is probably protandric with regular alternating female and male phases; becomes hermaphroditic after second or third breeding season. Fecundity about 1,000,000; egg size at liberation, 200μ–270μ. Larvae are incubated for about 20 days; are ready to settle when they measure approximately 0.44mm × 0.34mm. The free swimming larva has not yet been identified in plankton collections. Settling larvae exhibit an exploratory phase; spat measuring 5.5mm in length have total complement of gills present.

Type Locality: Pelorus Sound; Catlin River (= Catlins River).

Distribution: Ostrea lutaria Hutton is recorded from Port Pegasus, Stewart Island; Waitangi Beach, Chatham Islands; Foveaux Strait, East Coast and Golden Bay, South Island; Wellington Harbour, Pauatahanui Inlet, Waitarere Beach, Patea, Napier, Firth of Thames, Auckland Harbour and Bay of Islands, North Island.

Vernacular Names: Stewart Island oyster; Foveaux Strait oyster; Bluff oyster; mud-oyster; deep-water oyster; dredge oyster.

Hutton (1800, p. 175, 1884, p. 533, 1904, p. 93) with some doubt referred the Dunedin rock-oyster to O. reniformis Sowerby and gave on each occasion Sowerby's description. The systematic status of the Dunedin rock-oyster was queried by Suter (1913, p. 889) who referred it to O. tatei and another oyster from Auckland Harbour to O. reniformis Sowerby. Oliver (1923, p. 182) recognised the O. reniformis of Suter as a synonym of O. corrugata Hutton non Brocchi and said that the "use of the name O. reniformis shows how persistently names once introduced into a fauna are retained. Hutton used it for the Dunedin rock-oyster but Sowerby's description does not agree with any New Zealand species. The locality from which it was collected is unknown and the name is best rejected as indeterminable". Oliver (1923, p. 182) further recognised the Dunedin rock-oyster as an intertidal variant of the mud-oyster. However, this view has never been accepted by other systematists.

Suter (1913, p. 889) referred the Dunedin rock-oyster to O. tatei, a new name for O. hippopus Tate non Lamarck, an Australian shell present only in the Eocene, not applicable to a recent New Zealand shell and as Finlay (1928, p. 265) has said, the name O. tatei can only be used as a substitute for O. hippopus Tate non Lamarck. However, the description is still valid and Finlay (1928, p. 265) renamed the oyster O. hefferdi (incorrect spelling for Hefford) and selected as his neotype a shell from Dunedin Harbour. Powell (1957) followed Finlay in keeping the Dunedin rock-oyster separate from the mud-oyster as also does this account.

Ostrea heffordi Finlay, 1928 (Pl. 3. figs. 3-8)

  • 1880 (non) Ostrea reniformis Sowerby. Hutton, p. 175.
  • 1884 (non) Ostrea reniformis Sowerby. Hutton, p. 533.
  • 1904 (non) Ostrea reniformis Sowerby. Hutton, p. 93.
  • 1913 Ostrea tatei Suter (in part). Suter, p. 889.
  • 1928 Ostrea hefferdi. Finlay, p. 265. nom. nor. for O. tatei Suter (in part).
  • 1957 Ostrea heffordi Finlay. Powell, p. 78.
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Description: Shell shape regularly subtriangular, valves thin and delicate. Right upper valve flat, sometimes domed beneath hinge. Distally the upper valve bears dark, concentrically laminated prismatic scales. Three to six dark bands are sometimes present crossing the valve to the outer margin. Ligament raised, triangular and winged laterally. Umbos are blunt on young shells and usually absent on older shells. Muscle scar lunate with anteroventral region extending further forward and lying alongside ventral margin of valve rather than centrally; slightly excavated on posteroventral margin. Colour of internal surface generally white, sometimes pink or yellowish-green; a conspicuous blue or purple band extends around distal posterior free margin.

Left lower valve is the valve by which the oyster is attached to the rock and the attachment scar occupies one half of the total area of the valve. Distally from the region of attachment, the valves turn upwards so that shell becomes either erect or lies at an angle to the rock. The distal free area is smooth with occasional low radiating ribs extending to the margin, ribs never projected into frills: distal shell margin moderately crenulated. External surface of left valve is grey to bluish-purple, occasionally pale brown. Internally the ligament is slightly raised, triangular in shape and winged laterally. Muscle scar identical in position to that of right valve, but not always the same size. Internally the free margins have a broad bluish-purple band, slightly wider than in upper valve. A few shells examined had no marginal colouration at all. Denticles occur regularly, there being between one and eight placed on either side of valve just posterior to the hinge; pits corresponding to the denticles are present on the upper valve.

Type: Suter's type (Ostrea tatei) and Finlay's neotype (Ostrea heffordi) are both held at the Geological Survey, Lower Hutt.

Distribution: Ostrea heffordi is known from the rocks at Dunedin and Lyttelton.

Vernacular Names: Dunedin rock-oyster; Southern rock-oyster.