A comparative study of photography and mud excavation methods for estimating population density, sex ratio, body size, and claw size of the fiddler crab Austruca perplexa (Brachyura, Ocypodidae)

Though fiddler crabs are considered ecosystem engineers in mangroves, accurately estimating their density, sex ratio, and body sizes is challenging. The conventional method of mud excavation is frequently employed to estimate these parameters, yet it proves destructive because it destroys crab burrows and stresses the crabs. In this study, we compared a non-destructive photography method with the mud excavation method in a fiddler crab, Austruca perplexa, population in southern Thailand. Ten 0.25 m2 quadrats were fixed, and 4 photographs of the crabs inside each quadrat were taken. Subsequently, the mud within the quadrat was excavated to 30 cm, and all crabs were collected. Parameters such as crab numbers, sexes, body sizes, male handedness (right/left major claws), and male major claw sizes were estimated. Afterward, the photographs were processed in GIMP (GNU Image Manipulation Program) to estimate the same parameters. These parameters were then compared between the two methods. We observed that the photography method was as effective as the mud excavation method in measuring most parameters, except female density, which was higher in the mud excavation method. This study reveals that the photography method could be used effectively instead of the mud excavation method in estimating several population parameters in fiddler crabs while acknowledging its limitation in accurately estimating female density.