Lethal bacterial infections of a variety of hatchery-spawned bivalve mollusk larvae and juveniles have been studied. The symptoms of the disease and the course of the infection are described. Four biotypes and five antigenic types of bacteria, pathogenic for the larvae of five species of bivalve mollusks, were isolated and described in some detail. All are gram-negative motile rods. Comparative studies were made of a fairly large number of similar bacteria isolated from presumably normal marine fauna. None of these was pathogenic for the bivalve larvae nor did they have antigens in commoni with the pathogenic group. The four biotypes had a number of characteristics in common that rarely were present in other cultures from marine fauna. Several antibiotic preparations proved to be of value in the treatment and control of the infection.

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.

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Larvae of Mytilus cdulis aoteanus, Perna canaliculus, Xenosirobus pulex, and Modiolarca impacta were reared to settlement in the hatchery. A detailed comparative study, based on microscopy (optical and SEM) and morphometry, has been carried out on the four species. External features and the internal hinge structure of the larval shells of the four species show typical mytilid characteristics. Externally, umbo stage larvae of Modiolarca impacta are ovoid in the anterioposterior direction, shell heights are substantially less than lengths, umbones are low and broadly rounded, and the anterior end is more pointed than the posterior. Mytilus edulis aoteanus and Perna canaliculus umbo stage larvae are similar, initially ovoid in the anterioposterior direction and later more rounded with shell heights approximately equal to lengths, umbones broadly rounded. The shoulders of P. canaliculus larvae are higher and more angular than those of M. edulis aoteanus. Xenostrobus pulex larvae are round to ovoid in the dorsoventral direction. Shell heights are equal to or greater than lengths, umbones are knobby. Provinculum lengths and shapes are similar in Modiolarca impacta, M. edulis aoteanus, and P. canaliculus. Provinculum length is shorter and the shape distinctive in X. pulex. The provincular teeth number in larvae 200-299 |am long is 16-24 in Modiolarca impacta, 18-24 in P. canaliculus, 20-30 in M. edulis aoteanus, and 10-15 in X. pulex. The primary lateral hinge teeth in pediveliger stage larvae of P. canaliculus do not develop in the other three species.