Reference: Biol. Bull. 182: 270-277. (April, 1992) Developmental Changes in Ionic and Osmotic Regulation in the Dungeness Crab, Cancer magister A. CHRISTINE BROWN' AND NORA B. TERWILLIGER Oregon Institute of Marine Biology, University of Oregon, Charleston, Oregon 97420, and Department of Biology, University of Oregon, Eugene. Oregon 97403 Abstract. The ontogeny of osmoregulation and specific ion regulation was studied in the megalopa, 1st instar ju-venile, 5th instar juvenile and adult of Cancer magister. Hemolymph Na + , Cl~, K + , Mg ++ , and Ca ++ concentra-tions and osmolality were measured after 8-h exposure to 100%, 75%, and 50% seawater at 10C and 20C. The ability to hyperosmotically regulate is present in the megalopa, and ontogenic changes occur in both ionic and osmotic regulation. First instar juvenile crabs, which are exposed to the greatest extremes of salinity and temper-ature in the field, are less able to osmoregulate than are the other three stages examined. Changes in Na + , Cl~, and K + concentrations parallel total osmolality in all four stages. Hemolymph Mg ++ concentrations in megalopa and juveniles acclimated to 100% seawater are more than twice that of the concentration in the adult; after 8 h in 50% seawater, the megalopa and juvenile Mg ++ concen-trations decrease to the level of the strongly regulated adult Mg ++ concentration. Ca ++ is strongly regulated by mega-lopas and adult crabs exposed to reduced salinity com-pared to the two juvenile stages. Diminished predation pressure and high food availability are proximate factors that may outweigh short-term osmoregulatory stress en-countered on the tideflats during development of the ju-venile crab. Introduction Estuarine invertebrates vary greatly in their abilities to deal with changes in ambient salinity. The effects of en-vironmental salinity on the internal osmolality and spe-cific ion regulation of adult estuarine crustaceans have Received 26 August 1991; accepted 21 January 1992. 1 Present Address: Department of Biology. Lake Forest College, Lake Forest. IL 60045. been investigated in numerous studies (for review, see Mantel and Farmer, 1983). Ontogeny of osmoregulation and ion regulation has been comprehensively studied in branchiopod crustaceans, especially the anostracan brine shrimp, Anemia (for review, see Conte, 1984). Compa-rable information about larval, post-larval, and juvenile decapod crustacean osmoregulation is relatively limited (Kalber, 1970; Foskett, 1977; Young, 1979; Read, 1984; Rabalais and Cameron, 1985; Charmantier el ai, 1988; Charmantier and Charmantier-Daures, 1991), and there are almost no data available regarding specific ion regu-lation during decapod crustacean development (Char-mantier et al, 1984a,b,c; Feldere/a/.. 1986). The Dungeness crab, Cancer magister, inhabits the cold waters of the Pacific Northwest coast of North America and uses different portions of the estuarine and nearshore waters during its life cycle. Along the Oregon coast, em-bryos hatch from December through March (Reed, 1969; Lough, 1976). The newly hatched larvae go through five zoeal stages, all of which are planktonic in ocean waters, moving as far as 200 miles offshore. The transitional stage, an actively swimming planktonic megalopa, reenters the coastal and estuarine waters from mid April through early July (Lough, 1976). The megalopas then metamorphose into 1st instar juveniles that join the benthic community. Throughout the summer, juvenile crabs in the estuary are found in high numbers on the tideflats, while the adult crabs occur mainly in the deeper channels. Summer tidal changes in salinity and temperature, extending over a pe-riod of 6-8 h, are much greater on the tideflats than in the estuarine channels. Adults of C. magister do not mi-grate up into brackish waters for long periods as does Cal-linectes sapidus, the East and Gulf coast blue crab, but remain in the lower half of the bay, moving back and forth into nearshore waters. 270
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