Is larval dispersal a necessity for decapod crabs from the Amazon mangroves? Response of Uca rapax zoeae to different salinities and comparison with sympatric species

Salinity may play an important role in the larval Biology and Ecology of many brachyuran decapod crustaceans. Usually, ontogenetic changes in salinity tolerance represent a good indicator of larval dispersal in marine coastal ecosystems. In an experimental laboratory study, we investigated the effects of eight different salinities (Sal. 0–35) on zoeal development of the neotropical fiddler crab Uca rapax from a northern Brazilian estuary. In the study area the species reproduces year-round, including the wet season when low salinities (≤5) are frequent, resulting from heavy tropical rainfalls and input of freshwater mostly from the Amazon River. Larval survival and development duration from hatching to megalopa of U. rapax were significantly affected by salinity. All larvae died in Sal. 0–20, while they successfully developed through five zoeal stages to megalopa in higher salinities (Sal. 25–35). However, in Sal. 25 the survival rate was still significantly lower (33.8±4.8%) and the zoeal development duration was longer (13.7±0.5days) than in salinities 30 and 35, where >90% of the larvae successfully survived after an average of 11.8–12.4days of development, respectively. Our results strongly suggest that the early life-history stages of U. rapax perform ontogenetic migrations towards adjacent coastal or fully marine waters to develop in favourable high salinities. Comparison with two other sympatric crab species, e.g. Ucides cordatus and Uca vocator, suggests that the larval ‘export’ strategy is a convergent adaptation exhibited by decapod species in the Amazon region to avoid mass mortality within parental mangroves that are frequently subjected to strong salinity fluctuations caused by the specific meteorological and hydrological conditions.