ONTOGENY OF OSMOREGULATION AND SALINITY TOLERANCE IN THE ISOPOD CRUSTACEAN SPHAEROMA-SERRATUM

Osmoregulation was studied in embryos, juveniles and adults of the iso pod Sphaeroma serratum (Fabricius, 1787). Salinity tolerance was studi ed in juveniles and adults. Juveniles and adults were euryhaline. Sali nity tolerance was wide at birth in stage I Juveniles and it increased up to a maximum in stage IV and adults. The lower and upper lethal sa linities for 50% of the animals (96 h LS(50)) at 20 degrees C varied f rom about 13 parts per thousand and 59 parts per thousand, respectivel y, in stage I, to about 2 parts per thousand and 74 parts per thousand , respectively, in stage IV and in adults. Early and late embryos had, respectively no or weak osmoregulatory capabilities. They were isosmo tic and slightly hyperosmotic to the fluid of the incubating pouches i n which they developed. The adult type of osmoregulation was partly pr esent in stage I juveniles and was fully acquired at stage II. The osm oregulatory capacity increased in the following stages and the adult o smoregulatory efficiency was established in stage IV. In juveniles and adults, osmoregulation and salinity tolerance appeared to be correlat ed. The ability of S. serratum to live in a habitat subjected to large variations of salinity is achieved through 2 sequential adaptations: (1) embryos, which develop in incubating pouches, are protected in the m against osmotic stress; and (2) juveniles are able to hyperregulate at birth and, therefore, after birth, all stages are euryhaline.