Paralarvae of Octopus vulgaris Type II are stenohaline conformers: relationship to field distribution and dispersal

The degree of euryhalinity of cephalopod planktonic paralarvae potentially affects the dispersal capacity and distributional range of species. Herein, we tested the hypothesis that O. vulgaris Type II paralarvae would show some tolerance to seawater dilution, evaluated from integrated field and laboratory experiments. Paralarvae were collected from zooplankton samples (salinities of 30.3-32.6), in the Parana Bay Estuarine Complex (PBEC), Southern Brazil. In the laboratory, they were exposed to salinities of 28, 30, 33, 35 (control), and 37, from 15 min to 24 h. Survival ranged from 60 to 83%, 73-100%, and 100% at salinity of 28, 30, and 33-37, respectively. Body fluid osmolality (BFO), body water content (BWC), and ninhydrin-positive substances (NPS, amino acids) were quantified at salinities of 33, 35, and 37 (with 100% survival). BFO was always isosmotic to the external medium. BWC and the high NPS concentration remained unchanged in all salinities. Thus, these paralarvae tolerate some deviation from seawater salinity, displaying osmoconforming behavior, maintain body hydration, and show high ability of tissue volume/water regulation in short periods of time. Presumably, this physiological capacity of paralarvae might be one factor allowing their dispersal and survival along environments with moderate deviations from seawater salinity.