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Discussion

Reproduction and development in the New Zealand mud-oyster is essentially typical of the genus Ostrea being larviparous (incubatory) and hermaphroditic. Species of this genus are also held by some workers to be protandric, having a rhythmical alternation of female and male phases following the initial male phase and having the spermatozoa liberated in clusters. Certainly the New Zealand mud-oyster has spenn balls and appears to be protandric but further work is needed to determine the actual sequence of sexual phases, the duration of phagocytosis after female spawning and the length of time elapsing between the female phase and the following male phase. In other species of the genus Ostrea, spermatogenesis is well advanced and the oyster is capable of maturing as a male while larvae are still being held in the mantle cavity. Rapid proliferation of the spermatogonia while larvae were being liberated was not observed in Ostrea lutaria.

A comparison of O. lutaria with other incubatory and non-incubatory species of oysters shows several noteworthy contrasting features. The eggs and larvae of O. lutaria are considerably larger than those of other species. The larvae appear to be incubated for a much longer period than that recorded for other incubatory species and that they may be retained within the mantle cavity until they are fully developed. This is of great significance since the larvae of other incubatory species of oysters do not become fully developed until they have passed through a free swimming phase of several days.

An examination of the plankton of Evans Bay collected over a period of one year failed to identify any free swimming oyster larvae. The plankton did show, however, that several other bivalves were breeding all the year round in Wellington Harbour. It is therefore possible that the mud-oyster in Wellington Harbour may also breed for more than the eight months noted in this study.

Larvae are considered fully developed when they possess pigment spots, a well-developed foot and about seven gill filaments. Until this account, the fully-developed larvae of non-incubatory species of oysters such as C. virginica have always been found to be larger than those of incubatory species. The results of this study show that the larvae of O. lutaria are considerably larger and more fully developed than the larvae of any non-incubatory species of oyster. Fully-developed larvae of the New Zealand mud-oyster when liberated from the mantle cavity possess pigment spots, a well-developed foot and at least ten gill filaments. Furthermore, these larvae exhibit the characteristic exploratory phase immediately on liberation and settle within the following few days. Larvae of all other incubatory species of oysters do not exhibit the exploratory phase until they have been in the plankton for some considerable length of time. The pigment spots and seven gill filaments of such oysters are produced during the last 24 hours of the free swimming phase.

The attached larva (spat) of O. lutaria is consequently much larger than the metamorphosing spat of other incubatory and non-incubatory oysters. Other oyster spat of both types measure 2mm in length when the four gills are developed whereas the spat of O. lutaria are at least 5mm in length before the fourth gill (left outer demibranch) is developed. The young oysters of O. lutaria were never observed to be sexually mature before the second summer (this includes the summer during which they become attached). Thus it appears that even though the larvae are more fully developed and of a larger size at the time of settlement, they subsequently develop more slowly than all other incubatory and non-incubatory species of oysters.

It therefore seems that either O. lutaria follows a unique pattern of reproduction and development, or, it follows a pattern similar to that of many other invertebrate page 27 animals of south temperate regions in which the larva is large and the pelagic phase very short. If the latter is the case, then other incubatory species of oysters inhabiting latitudes greater than 40°S. could also be expected to possess large larvae and a short pelagic life. The oyster species known from these latitudes are O. angasi, the South Australian mud-oyster, O. heffordi, the Dunedin rock-oyster, and O. chilensis, the South American oyster. O. nomades, the Australian green oyster, extends from Port Jackson to the Gulf of Carpentaria and is only included in this list because of its southern range (where O. angasi also occurs).

No information was available to the author with regard to the reproduction and development of the South American oyster and very little is known about O. angasi. Thomson (1954, p. 145) notes that Ranson has investigated the prodissoconch of O. angasi but the results are not yet available. Roughley (1925, p. 15) states that the larvae of O. angasi are about 1/150th of an inch long when ready to be liberated. This is similar to other species of Ostrea.

During the present study brief observations were made on O. heffordi while specimens of this oyster were being studied with regard to its systematic status. Specimens of this oyster collected in winter had very young spat attached to the older oyster shells. In a total of about 100 specimens examined throughout the year, no incubatory specimens were observed.

Thus, the greater size of the larvae and shorter pelagic phase of O. lutaria is at the moment unique with regard to all other oysters known. Whether or not other south temperate incubatory oysters also have a similar reproduction and development yet remains to be determined.