Reference: Biol. Bull. 175: 102-110. (August, 1988) Ontogeny of Osmoregulation and Salinity Tolerance in Two Decapod Crustaceans: Homams americanus and Penaeus japonicus G. CHARMANTIER1, M. CHARMANTIER-DAURES1, N. BOUARICHA1, P. THUET1, D. E. AIKEN2, AND J.-P. TRILLES1 1Laboratoire de Physiologie des Invertébrés, Université des Sciences et Techniques du Languedoc, PI. E. Bataillon, 34060 Montpellier Cedex, France, and ^Department of Fisheries and Oceans, Invertebrate Biology and Aquaculture, Biological Station, St. Andrews, New Brunswick EOG 2X0, Canada Abstract. Osmoregulation and salinity tolerance were studied in larvae and post-larvae of two species of crusta ceans, 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 20°C was about 17%» at metamorphosis, and about 10.5-12%« in stages IV and V. In P. japonicus, the 24 h LS50 at 25°C was about 25%« at metamorphosis, and about 7-10%o from the sixth post-larval stage onwards. In both species, larvae were hyper-osmoconformers and the osmoregulatory pattern changed after metamor phosis to the juvenile/adult type. In H. americanus, stages IV and V slightly hyper-osmoregulated in low sa linities. In P. japonicus, post-larvae hyper-hypo-regu lated, 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 with regard to their ecological and physiologi cal implications and to previous studies on other species. Introduction Most studies on crustacean Osmoregulation deal with adult forms (review in Mantel and Farmer, 1983), and only a few data are available on larval and post-larval Received 8 February 1988; accepted 29 April 1988. Osmoregulation. These are summarized in Table I. De pending on species, the osmoregulatory abilities can vary among the successive larval stages or remain unchanged. The adult type of regulation is also established at variable stages in different species. Numerous studies have been concerned with larval and post-larval salinity tolerance, but few have attempted to correlate the salinity tolerance of different developmental stages with their correspond ing osmoregulatory capabilities in a given species. The objective of this study conducted with the Ameri can lobster Homarus americanus H. Milne Edwards, 1837, and the shrimp Penaeus japonicus Bate, 1888, was to determine the salinity tolerance of larval and post-lar val stages of these species, to define the ontogeny of their Osmoregulation, and to attempt to correlate osmoregula tory abilities and salinity tolerance. H. americanus and P. japonicus are both economically important, and any knowledge of their larval and post-larval environmental tolerance and physiology can be valuable for their man agement and potential culture. Moreover, their patterns of post-embryonic development are different, offering the opportunity for comparisons of larval physiology. In the genus Homarus, larval development comprises one prelarva and three zoea or mysis larvae—stages I to III—before a metamorphosis leading to post-larvae— stage IV or megalopa—and then to juvenile stages. In H. americanus, Osmoregulation has been studied in adults (DalÃ-,1970) and juveniles (Charmantier et ai, 1981, 1984a); information on ionic regulation (Charmantier et al., 1984b) and preliminary data on Osmoregulation (Charmantier et ai, 1984c) are also available for the early post-embryonic stages of this species. Osmotic and ionic 102