Bonamiasis, first detected in the New Zealand dredge oyster Tjostrea lutaria (Hutton) during the southern autumn of 1986 was investigated through 2 surveys of oyster beds carried out over spring (Sep 1986) and summer (Jan 1987). In spring, a 'lighter' form of the parasite occurred most frequently, confined largely to the sub-epithelia1 tissue. In summer, a 'dense' form, nearly identical to the 'classic' European and American Bonamia ostreae, was the dominant type; it occurred in subsurface tissue around the digest~ve diverticula and gut, and ~n v a d e d gonadal follicles and gill filaments Other forms, such as a second type of the 'dense form' and what appeared to be intermediate stages between the 'lighter' and 'dense' types were also detected. The significance of these different types in the life cycle of the parasite is discussed. The New Zealand species shows distinct size and morphological differences from B. ostreae.

Detailed comparisons of the larvae of four species of New Zealand oyster (Saccostrea glomerata, Crassostrea gigas, Ostrea lutaria, and a new species of Ostrea) and one Chilean oyster (Ostrea chilensis) showed that larvae of species other than O. lutaria and O. chilensis share certain features. One conspicuous feature is a distinct tract, termed here the posterior dorsal sulcus, which runs from the posterior margin of prodissoconch I to the valve margin of prodissoconch II; other characteristic features are an umbonate, inequivalve shell and a provinculum bearing distinct teeth. The larvae of O. lutaria and O. chilensis, in contrast, lack the posterior dorsal sulcus, are non-umbonate, and have nearly equal valves with an edentulous provinculum. These distinctive features, together with other adult features, suggest that the two species are more appropriately included in a separate genus. The name Tiostrea is proposed for the new genus. Certain supraspecific groupings within the Ostreinae need to be re-examined in the light of breeding experiments.

The annual reproductive cycle and the cyclical changes in the gonad of the New Zealand rock oyster, Crassostrea glomerata (Gould, 1850), during the breeding period of 1970-71 and 1971-72 have been described. The gonad passes through an intermediate phase, after a post-spawning period, when sex is indeterminate. Gametogenesis begins in July and August, but follicles ripen mostly during the spring months of October and November. Maximum development is seen in November, and nearly all oysters are in spawning condition in December and January. Major spawning takes place in January or February depending upon water temperature, and spawning continues until the end of March. Larvae may be found in the-plankton until late in the season, to the end of April or even May. Following spawning, gonadial regression sets in, with leucocyte infiltration and phagocytosis of residual gonial cells. A greater percentage of oysters more than 1 y old are females. The majority of oysters of age 1 y and below are males.

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Crassostrea gigas (Thunberg), the Japanese oyster, is recorded for the first time in New Zealand. This exotic species may have been introduced accidentally.

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