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The occurrence of inbreeding was investigated in Celleporella hyalina, which has been reported as being capable of selfing. In total, 4 tetranucleotide microsatellite markers were scored In populations from sites within the Menai Strait (Wales) and the Yealm Estuary (Plymouth, England). All loci in all populat~ons were in Hardy-Weinberg equilibrium, with insignificant F,, values, which indicates that C. hyalina in these populations generally outbreed. No significant differences were found between populations up to 8 km apart, indicating sufficient gene flow on this scale to form a single breeding population despite geographic separation. The Plymouth population was significantly (p < 0.00001) deviant from the others, indicating possible isolation by distance at the scale of 900 km. C. hyaLina cultured in the laboratory were observed to spawn sperm into the surrounding seawater. Sib larvae were found to settle randomly with respect to each other. In isolation C. hyalina failed to produce offspring, though a small proportion brooded abortive embryos. Pairings between individuals of differing genotypes resulted in the production of offspring, all of which, when genotyped with respect to the parent colonies, were found to be the result of outcrossing.

Mature eggs were stripped from ripe adult specimens of Arctica islandica and exposed to a dilute solution of ammonium hydroxide for various lengths of time before addition of stripped sperm. Larval and early post-larval stages were cultured under experimental laboratory conditions using standard bivalve rearing techniques. Larval cultures were maintained at various controlled temperatures ranging from 8·5 to 14·5 °C. Minimum time to settlement was 32 days at a temperature of approximately 13 °C; at temperatures between 8·5 and 10·0 °C, settlement was not observed until approximately 55 days after fertilization. Larval growth rates were significantly faster at temperatures between 11·0 and 145 °C than at temperatures between 8°C. Morphometry of the larval shell and morphology of the larval hinge apparatus were independent of larval growth rates and experimental culture conditions.

The effects of copper on embryo development were found to differ between populations of the mussel Mytilus edulis L. (Mollusca: Bivalvia). Concentrations of copper which caused significantly increased rates of abnormality, and thus decreased yields of normal larvae, in populations from an unpolluted site (Menai Straits, Wales, UK) and to a lesser extent in an intermediately polluted site (Oosterschelde, The Netherlands) did not affect development of embryos from a polluted-site (Westerschelde, The Netherlands) population Crosses indicated that tolerance was mostly maternally determined, but there is also evidence of a paternal, i.e, genetic, effect. An appreciable quantity of background, non-treatment, embryo abnormality appeared to be sperm-mediated.