Laboratory trials reveal that exposure to extreme raining events prior to metamorphosis affect the post-settlement performance of an estuarine crab

• Survival and metamorphosis of Carcinus maenas megalopae is poorly affected by salinity.
• Deleterious effects of osmotic stress can cascade from megalopae to early benthic life.
• Crabs from early settling megalopae with no osmotic stress were the biggest.
• Crabs from late settling megalopae exposed to osmotic stress were the smallest.
• Smaller juveniles are more vulnerable to predation which may shape adult populations.

Meteorological forcing can impact planktonic communities, with extreme raining events promoting salinity decreases and triggering larval mortality in estuarine plankton. The present study evaluated how exposure to low salinities prior to metamorphosis of Carcinus maenas megalopae (last larval stage) may affect its ability to metamorphose and the post-metamorphosis performance of juvenile crabs. An extreme raining event that promoted a generalized decrease in salinity (from 25 to 10) in the whole water column of one of the main channels of a coastal lagoon was mimicked in the laboratory. Wild megalopae of C. maenas were collected and kept individually without any food at salinities of 10 or 25 (S10 or S25) until they either died or metamorphosed to the first crab instar (C1). Specimens metamorphosing in 5 days or less following their collection were labeled as early settlers (ES10 and ES25), while those taking more than 5 days were labeled as late settlers (LS10 and LS25). All newly metamorphosed crabs were kept individually until C5 at a salinity of 25 and fed ad libitum, with their intermolt periods and carapace width (CW) being recorded. Osmotic stress did not affect the survival or ability to metamorphose of C. maenas megalopae, with 89% of all larvae in both salinities being able to metamorphose. This result is supported by the ability of this larval stage to hyper-regulate. Nonetheless, an exposure of late settling megalopae to low salinities prior to metamorphosis promotes the occurrence of juvenile crabs with a smaller CW. The deleterious effects of exposing late settling megalopae to low salinities appears to be magnified during early benthic life, with C5 originating from treatment LS10 displaying a significantly smaller CW (4.87 ± 0.28 mm) and lower wet weight (WW) (28.95 ± 4.62 mg). On the other side, C5 originating from ES25 exhibited a significantly higher CW (5.90 ± 0.33 mm) and WW (50.89 ± 8.14 mg). The nutritional vulnerability experienced by megalopae starved for longer periods (late settlers) may have been magnified for specimens exposed to a lower salinity, with the energetic costs associated with hyper-regulation negatively affecting the growth performance of juvenile crabs. Osmotic stress experienced by late settling megalopae can shape adult populations of C. maenas by promoting the occurrence of smaller juveniles, thus more vulnerable to predation and cannibalism. Phenotypic links must be incorporated in the study of marine invertebrates, namely when life stages are vulnerable to metereological forcing (e.g., extreme rainfall) at critical periods of their life-cycle (e.g., metamorphosis).