Abstract The mangrove crab Ucides cordatus (Linnaeus, 1763) is a burrowing crab with an important role in mangrove nutrient cycling. The species holds major socioeconomic importance, generating yield for traditional and low-income populations. Despite its ecological and economic importance, there are few experiments in Brazil applying stable isotope tools to trophic crab classification, and even fewer considering females in the reproductive period. Females have different energy demands than males; studies examining C and N can reveal details regarding the differences. Hence, the present study is the first analysis of the δ 13C, δ 15N, and the C/N ratio in ovigerous females of two populations from Southeastern Brazil (Caceribu River - Guanabara Bay and Gargaú mangrove - Paraiba do Sul River secondary estuary). The Caceribu mangrove is larger and is located in one of the most impacted bays in the world. In the Paraiba do Sul River region, the mangrove swamp is about ten times smaller, has a substantially lower population in the surrounding area, and is more influenced by agricultural activities. The δ 13C analysis confirmed mangrove leaves as their main food source. The significant δ 13C variation between the Guanabara and Paraiba do Sul estuary populations can be related to differences in food availability and nutritional value. However, the δ 15N values can be related to the ingestion of other food items during the breeding season as well as the influence of anthropogenic mangrove degradation. At both study sites, the δ15N values were higher than those observed in other mangrove crab populations and other herbivorous species. Significant (p < 0.05) isotopic differences were found among populations, suggesting variations in biogeochemical cycles that may be related to different environmental conditions between the sites.

Abstract Crustacean decapod larvae have developed several adaptations to maximize their survival, including swimming behavior. Swimming improves larvae dispersal and contributes to their response to environmental stresses, including variations in abiotic factors, presence of predators, and search for food. Data on larval behavior can also assist in studies on conservation and in species management in aquaculture. Unfortunately, swimming behavior remains unstudied in many species. The present work aims to analyze the swimming behavior in newly hatched larvae of six common decapod crustaceans in coastal Southeast Brazil (Macrobrachium acanthurus, Macrobrachium olfersii, Emerita brasiliensis, Ucides cordatus, Minuca rapax, and Callinectes danae). The analysis included tests with larval responses to four isolated stimuli: gravity (geotaxis), vertical salinity gradients, low-pressure zones, and presence of the planktivorous fish, Atherinella brasiliensis (only for C. danae and M. rapax). The first two and the last two stimuli were used as indicators of dispersal and defensive behaviors, respectively. All larvae preferred water depth with higher salinity levels. Additionally, negative geotaxis was observed in M. olfersii, M. acanthurus, U. cordatus, and C. danae, being stronger in the last. Both Macrobrachium larvae responded to low-pressure zones, promptly moving away. Callinectes danae and M. rapax larvae avoided contact with the predatory fish by creating avoidance zones around it, even in the absence of light. This is an active defensive behavior that has never been reported for the newly hatched larvae of these two species.