Streams and rivers on the Vanuatu islands of Espiritu Santo, Pentecost, Éfate and Tanna were sampled for gastropods and thephysical and chemical characteristics of the water were investigated between 25/11/95 and 13/12/95. Twenty three species of neritidand forteen of thiarid prosobranch gastropods were found, together with the pulmonate Physastra nasuta and an opisthobranchspecies Strubellia. Six thiarid, two Clithon and one Neritina species, which are present in the Solomon Islands and SouthEast Asia, but which are absent further south on New Caledonia and the Fiji islands, were found on the Vanuatu islands. Onthe other hand, Septaria bougainvillei and Clithon pritchardi, which are present on the Fiji islands and New Caledonia, werefound no further north than the southern Vanuatu island of Tanna. This suggests that Vanuatu is a transition zone for both northernand southern freshwater snails. Éfate had many more species (26) than the other islands (16-19-. This was partly explainedby its central geographical position and the higher ion content of its streams.

In the south-west lagoon of New Caledonia, the first sexual maturity size is 45 mm for Annachlamys flabellata (Bernadi), 60 mm for Comptopallium radula (L.) and 50 mm for Mimachlamys gloriosa (Reeve). The global sex-ratio was not significantly different from 1:1 for C. radula unlike A. flabellata andM. gloriosa (1.3:1 for two species). The significantly greater mean size of females suggests a protandric sex reversal. The reproductive cycle was studied monthly for two years for M. gloriosa and one year for C. radula and A. flabellata, then weekly over periods of 12 months for M. gloriosa and 6 months for C. radula. Reproduction of the three species occurs throughout the year with a maximal activity during the warm season. M. gloriosa and C. radula are capable of spawning several times a month. Fluctuations of reproductive intensity are related to temperature and salinity fluctuations.

Factors influencing the spatial distribution of the bivalves Atactodea striata. Gafrarium tumidum and Anadara scapha were analysed on the south-west coast of New Caledonia, using a multivariate approach (correspondence analysis). A. striata is abundant on sandy beaches with a high coarse sand content. G. tumidum is preferentially distributed on substrates located close to the lower limit of neap tide and seems to be independent of sediment granulometry. A. scapha adults occur at lower bathymetric levels, with sediment composed of 40% medium, fine and very fine sand, and 30% very coarse and coarse sand. For this species, juvenile migration is suggested by the differential distribution of young and adults.

Different random samplings were used to assess the biomass of intertidal bivalves on two main biotopes of the south west coast of New Caledonia. Over the 46.5 km line of sandy beach, the total bivalve biomass is 4.3 ± 1.8 t and the mean biomass of Atactodea striata is about 3.2 ± 1.2 t. Over the 9.85 km2 sandy-muddy shore of the study area, bivalve stocks (1.050 ± 392 t) are dominated by Gafrarium tumidum and Anadara scapha; their mean biomasses are respectively 270 ± 132 t and 418 ± 308 t. This resource is expected to meet the demand of the domestic market through an artisanal fishery.