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Osmoregulation and salinity tolerance were studied in larvae and post-larvae of two species of crustaceans. Homarus americanus and Penaeus japonicus, that have different types of embryonic development.In both species, salinity tolerance decreased through the larval stages, was at a minimum at metamorphosis, and increased in post-larval stages. In H. americanus, the lethal salinity for 50% of the animals (24 h LS50) at 20C was about 1 7%o at metamorphosis, and about 10.5-12%o in stages IV and V. In P. japonicus, the 24 h LS50 at 25C was about 25%o at metamorphosis, and about 7-10% from the sixth post-larval stage onwards.In both species, larvae were hyper-osmoconformers and the osmoregulatory pattern changed after metamorphosis to the juvenile/adult type. In //. americanus, stages IV and V slightly hyper-osmoregulated in low salinities. In P. japonicus, post-larvae hyper-hypo-regulated, and their regulatory capacity increased up to the fifth post-larval stage.In young stages of H. americanus and P. japonicus, Osmoregulation and salinity tolerance appear correlated, and are modified at metamorphosis. These results are discussed iih regard to their ecological and physiological implic i md to previous studies on other species.

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