Ontogenetic variation in the foraging strategies of the painted ghost crab Ocypode gaudichaudii (H. Milne Edwards & Lucas, 1843) at two sandy beaches in Panama
Authors:   , Yong Adeline Yean Pin 
Published: November 2024

Growth can lead to the development of critical life history traits (i.e., behaviour, physiology, morphology) and alter the resource use patterns of organisms. The changes in ecological interactions can also affect the structure and dynamics of populations, communities and ecosystems. This dissertation examined the feeding strategies of the painted ghost crab, Ocypode gaudichaudii at two life stages (i.e., juvenile and adult crabs). The resource use patterns of the crabs at two Panamanian sandy beaches with different levels of wave exposure were determined by laboratory and field studies. Seven environmental variables at the two beaches were determined and related to the 1) zonation patterns, 2) diet, 3) foraging habits, and 4) activity budgets of the juvenile and adult crabs. Three experiments were performed to determine the effects of food densities on the feeding modes, the plasticity of feeding behaviour, and the occurrence of food hoarding in O. gaudichaudii. Isotopic signatures of the crabs were used as a proxy to indicate their ecological roles over ontogeny. The sheltered Culebra Beach had a higher abundance and diversity of prey, but a lower abundance of crabs as compared to the semi-exposed beach (Playa Venao). At low tide, the larger crabs at Playa Venao will move out of the nutrient-poor burrow zone to forage at the nutrient-rich sand flats in droves. Birds were commonly seen around Culebra Beach during the day, but at Playa Venao, a nocturnal species of ghost crab Hoplocypode occidentalis that preys on O. gaudichaudii was commonly present instead. Ocypode gaudichaudii showed interpopulation and ontogenetic variations in their distribution, diet, foraging habits and activity budget in response to the different levels of food and predation risk at the two beaches. The results of the feeding experiment showed that both the juvenile and adult crabs exhibited functional responses to the increasing densities of food (i.e., diatoms and rove beetles). In the transplantation experiment, the crabs that were transplanted from Playa Venao altered their feeding habit and fed like their conspecifics at Culebra Beach. The results of the food hoarding experiment showed that the crabs at Playa Venao hoarded food in their burrows when food pellets were supplemented towards the end of their feeding cycle or when the pellets required a long handling time. Since food hoarding only occurred at the beach with low food availability, it could be a strategy that O. gaudichaudii used to overcome food scarcity. Results of the stable isotope analyses of the crabs showed an ontogenetic isotopic niche shift in both populations and indicated the varying ecological roles of the two ontogenetic stages. These findings suggest that O. gaudichaudii is an opportunistic omnivore with short-term and long-term feeding strategies to cope with the spatial and temporal variations in food availability to maximise the energy gained during foraging. These strategies could account for the persistence of the two ghost crab populations with different food resources in their habitats.