Abstract
Mangrove tree crabs are considered ecosystem engineers to mangrove forests impacting energy flow by forming links between trophic levels in mangrove food webs. Although they are common throughout southwest Florida, relatively little is known about their natural history and role in mangrove ecosystems. Climate change effects on mangroves are relatively well understood however, the same cannot be said for the most common macrofauna found in mangrove ecosystems. The aim of this study was to identify wet season variations of mangrove tree crab (Aratus pisonii) population characteristics along a salinity gradient between the Imperial River and Estero Bay, Bonita Springs. Crabs were observed and captured from June 2021 to October 2021. A total of 537 individuals were captured, comprising 226 females, 188 males and 123 egg-bearing females. In this study, salinity was most closely related to population variation compared to air and water temperature, however this may be due to the small spatial scale of the Imperial River-Estero Bay ecosystem sites chosen. No crabs were found in the freshwater river sites, whereas the highest number of crabs observed and captured occurred in the site closest to the Gulf of Mexico. Salinity was significantly correlated with carapace width, sex ratio, number of observable crabs and number of ovigerous females found. A site in close proximity to the Gulf of Mexico was the only location with a male biased population and another site with one of the largest salinity ranges saw significant sexual dimorphism with statistically larger gravid females, both of which are uncommon patterns in crab species. Using the number of ovigerous females where they occur through the summer, established an egg-bearing female seasonality pattern that peaked in September, and drastically decreased to almost zero in October. Carapace width variation increased both spatially and temporally and was assumed to be related to crab growth overtime. All the results from this study indicate variations in mangrove tree crab population morphology on a relatively small spatial scale. Understanding the relationship of mangrove tree crabs and local salinities will help determine how climate-induced salinity changes will alter mangrove tree crab population sizes and distribution.