On the reproduction and growth of some endemic gastropods of Lake Baikal

Reproduction and ontogeny of some Baikalian gastropods inhabiting the littoral were studied. To detect phases of enhanced hatching, growth and production of fresh egg capsules, SCUBA divers lifted stones from the littoral of the western shore of Lake Baikal and from River Angara monthly. All egg capsules and gastropods were recorded. Four different developmental phases of embryos were distinguished. In March the percentage of egg capsules with fully developed embryos is very high, but in April-June the percentage rapidly decreases due to enhanced hatching and a high production of newly laid capsules. From June to October the proportion of older developmental stages increases again. Freshly laid capsules were found at any time, but in March-April, the percentage is low while it is high from May to July. Embryogenesis does not take a few weeks as in most Palaearctic species but several months. Although embryos were found fully developed in autumn-winter, most hatching occurs in spring. In order to determine growth rate and time to maturity for different endemic gastropods, juveniles were kept in rearing cages in situ. The cages were lifted monthly and the snails measured. Results showed growth is very low from November to May and maximal in the period August to October. Thus, shell growth is not steady but cyclical. Compared with relatived species inhabiting smaller Palaearctic water bodies, the ontogeny of Baikalian gastropods is prolonged. It appears that in Lake Baikal Megalovalvata needs 2 years and Choanomphalus 2-3 years to become mature, while the Baicaliidae need at least 4 years to mature and 5 years to be fully grown. Growth rates are higher in the Angara River than in the Lake Baikal littoral.