Evaluating reasons for biased sex ratios in Crustacea

Influence of environmental patterns on the population structure and secondary production of the fiddler crab Uca maracoani (Latreille) in the Amazon mangroves

Uca maracoani is a fiddler crab found in estuaries along the western Atlantic coast, with a notable preference for euhaline environments. This study aimed to analyze the population structure and dynamics of this species in an estuary on the North Coast of Brazil, specifically in an area of the upper estuary where seasonal rainfall fluctuations result in significant changes in salinity. Monthly crab samples were taken from December 2013 to November 2015, together with measurements of environmental variables, such as water and climate parameters. The population maintains a balanced sex ratio; however, males are generally larger, with lower mortality rates and longer lifespans than females. Reproduction is continuous but mainly takes place in the dry season when salinity levels are higher (above 12‰). Higher crab densities have been observed during the rainy season when, despite lower salinity levels (below 10‰), the conditions for survival (food availability and milder climate) seem to be more favorable. The estimated average annual biomass and production for the population were 2.62 g AFDM m-2 and 5.43 g AFDM m-2 year-1, respectively, characterized by a high turnover rate (P/B = 2.10 year-1). Our results suggest that U. maracoani has thriving populations in the Amazon coast's mangroves, benefiting from the vast muddy intertidal zone and the high organic content delivered by the estuaries.

Impacts of pollution, sex, and tide on the time allocations to behaviours of Uca arcuata in mangroves

Fiddler crabs (Uca) are ecosystem engineers in coastal ecosystems. Many anthropogenic and natural factors can affect the time allocated to various behaviours in Uca. However, the behaviour of U. arcuata, a widely distributed fiddler crab in Asia, has not been studied in mainland China. Here, we used binoculars to record the time budget of ten behaviours of U. arcuata to investigate the potential effects of sex, tides, and pollution on these behaviours. We found that the crabs spent 42.3%, 27.0%, and 10.6% of their time on feeding, feeding while walking, and stationary respectively. The crabs spent <1.5% of their time on copulation and grooming. The total foraging time (feeding + feeding while walking) did not differ among the three polluted sites. However, crabs spent more time on feeding but less time on feeding while walking. The feeding rate and probability of burrowing and grooming decreased while the possibility of locomotion and stay in burrow increased with increasing nutrient concentration. Females spent 13.9% more time on feeding and fed 54.9% faster than males. Males had a higher tendency to grooming and combat while they were less likely to walk than females. Regarding to the influence of tide, fiddler crabs fed 11.2% faster at ebb tides than at flood tides, and they were more likely to walk and stay in burrows at flood tides than at ebb tides. Our results indicated that nutrient pollution had stronger impacts on the behaviours of crabs than sex and tide. In polluted mangroves, increasing nutrient concentration reduced the quantity of sediment processed by fiddler crabs due to their smaller feeding area, slower feeding rate, and reduced frequency of burrowing activities. These results imply that mitigating nutrient pollution in mangroves may benefit the restoration and management of coastal ecosystems through the enhanced engineering functions of fiddler crabs.