Influence of salinity on the larval development of the fiddler crab Uca vocator (Ocypodidae) as an indicator of ontogenetic migration towards offshore waters

Larvae of many marine decapod crustaceans are released in unpredictable habitats with strong salinity fluctuations during the breeding season. In an experimental laboratory study, we investigated the influence of seven different salinities (0, 5, 10, 15, 20, 25 and 30) on the survival and development time of fiddler crab zoea larvae, Uca vocator, from northern Brazilian mangroves. The species reproduces during the rainy season when estuarine salinity strongly fluctuates and often reaches values below 10 and even 5. Salinity significantly affected the survival rate and development period from hatching to megalopa, while the number of zoeal stages remained constant. In salinities 0 and 5, no larvae reached the second zoeal stage, but they managed to survive for up to 3 (average of 2.3 days) and 7 days (average of 5.1 days), respectively. From salinity 10 onwards, the larvae developed to the megalopal stage. However, the survival rate was significantly lower (5–15%) and development took more time (average of 13.5 days) in salinity 10 than in the remaining salinities (15–30). In the latter, survival ranged from 80–95% and development took 10–11 days. Given the 100% larval mortality in extremely low salinities and their increased survival in intermediate and higher salinities, we conclude that U. vocator has a larval ‘export’ strategy with its larvae developing in offshore waters where salinity conditions are more stable and higher than in mangrove estuaries. Thus, by means of ontogenetic migration, osmotic stress and resulting mortality in estuarine waters can be avoided.