Population structure and reproductive migration of land crabs on Grand Cayman Island.

Abstract

Cardisoma guanhumi (white land crab) and Gecarcinus ruricola (black land crab) are found throughout Grand Cayman, and concern has been expressed regarding their possible declining populations and current distributions. The overall objective of this project was to gather biological data on C. guanhumi and G. ruricola, as well as document activity patterns for G. ruricola populations, to gain a better understanding of Grand Cayman's land crab population structure and reproductive migration patterns. Surveys occurred during the summers of 2016 and 2017 on Grand Cayman. The study area for G. ruricola was along a main highway in East End, while the study site for C. guanhumi took place at Barkers National Park, West Bay. A male-biased sex ratio was observed in the C. guanhumi population on Grand Cayman, specifically in the two larger carapace width size classes. Generally, C. guanhumi males reached a larger body size than females, which is a common trend in brachyuran crabs. Four color patterns were observed in C. guanhumi males and non-ovigerous females, while ovigerous females only displayed two of the patterns. The color patterns seem to be related to the development stage and maturity of the crab. Gecarcinus ruricola males also reach a larger maximum size than females, for all measured morphometrics on Grand Cayman. Similar to C. guanhumi, these growth differences are likely due to how each sex distributes their energy utilization. Overall, both sexes of G. ruricola on Grand Cayman are smaller in terms of body size than other populations in the Caribbean and it may be possible the populations on Grand Cayman reach sexual maturity at an earlier stage and smaller size than other geographic locations. The reproductive migration season for G. ruricola extends over three months on Grand Cayman, but the intensity of crab activity varies with time. In 2017, high roadside activity levels for G. ruricola shifted from inland to the coastal edge of Queen's Highway following the first migration peak in June. There were two mass migrations observed with each resulting in high numbers of crabs crossing the road and high roadkill numbers. Crabs were not randomly scattered along the highway; instead each sex showed a distinct clustering distribution which most likely resulted due to habitat changes from development. Ovigerous G. ruricola females exhibited randomness during the reproductive season nights, and those random nights of ovigerous female activity most likely coincided with spawning events. The number of roadkills generally reflected the migration intensity. Moonlight had a significant effect on the number of G. ruricola individuals on the road, specifically during the waning crescent moon phase, and rainfall enhanced overall activity on Grand Cayman. This project will provide government officials with baseline information and methodology they can use to monitor Cayman's land crab populations in the future. Lastly, this study will provide a greater overall understanding of land crabs and their reproductive migration.