On the Larvae of Fresh-Water Bivalves in the Lake Biwa-Ko

Up to this time, little has been known concerning the larvae of most fresh-water bivalves. To elucidate the larval history of fresh-water bivalves in the lake Biwa-ko, we had undertaken an investigation of the breeding season, the morphological type of larval mussel or glochidia borne in tne brood pouches of a fully grown female mussel. The results obtained are summarized as follows: 1. All of the bivalves except Corbiculidae belong to Unionidae, in which the brood pouch or marsupium consist of only the outer gills. 2. The bivalves of the lake Biwa-ko may be devided into three classes with respect to the breeding seasons-the long-term breeders, the moderate-term breeders and the short-term breeders (Table 1). 3. The spawning of Corbicula sandai takes place in early Summer, although in most mussels the spawning occures in early Spring. 4. The number of glochidia borne in the brood pouches of a fully grown female mussel varies in the different species from about 5, 000 to 350, 000. Hyriopsis schlegeli yields roughly 350, 000 glochidia. Cristaria plicata 400, 000, Anodonta lauta and Anodonta calipygos 180, 000 Inversidens reiniana 14, 000, Unio biwae 60, 000, Inversidens hirasei and Lanceolaria oxyrhycha 30, 000, Inversidens japanensis 15, 000, Inversidens baandti 5, 000. The number of eggs in the gonad at the spawning season of Corbicula sandai was estimated at about 60, 000 (Table 1). 5. The sex-ratio of these musseles are nearly 1: 1. 6. Two morphological types of glochidium were present, one, to which the majority of the family belong, with heavy triangular valves equipped with a stout spine at the ventral apex (so-called Anodonta type), and the other, a smaller group, generally of a coutour of a spoon bowel, without any spine (so-called Lampsilis type) (Plate 1).