Abstract
The salinity tolerance of naupliar stages is a major factor in restricting the abundant calanoid copepod Acartia tonsa to estuarine and nearshore waters. There is significantly greater naupliar survival at salinities less than full strength seawater, and salinity-temperature interaction experiments indicate optimal conditions for A. tonsa nauplii are < 25 ppt and > 15 degree C. The early naupliar stages do exhibit some osmoregulatory ability, however. Demonstrations using fluorescein dye show that even the nonfeeding N1 stage can drink. While adult A. tonsa are good osmoregulators and, consequently tolerate a wide range of salinities, the restriction of this species to estuarine and coastal habitats may be a function of the physiology of the nauplii rather than the adult.