Competition for mangrove leaf litter between two East African crabs, Neosarmatium meinerti (Sesarmidae) and Cardisoma carnifex (Gecarcinidae): a case of kleptoparasitism?

Many Indo-Pacific mangrove macroinvertebrates are herbivorous and heavily dependent on leaf litter. As a consequence, fallen leaves become a limiting factor and competition occurs among conspecifics and heterospecifics. Neosarmatium meinerti and Cardisoma carnifex are two of the most common herbivorous crabs of the landward mangrove belt in East Africa, often sharing an area and time of activity. This study used a series of field experiments to investigate their feeding habits when co-existing. The high density reached by both species led to overlapping of the individual feeding ranges: during the feeding excursions each crab inevitably encountered, and actively interacted with, other conspecifics and heterospecifics. Competition for food was high: N. meinerti lost approximately 50% of its leaves to theft by C. carnifex. Thus, approximately half of the leaves obtained by C. carnifex were stolen from a foraging N. meinerti. Considering the frequency of such thefts, C. carnifex may be regarded as a facultative kleptoparasitic species. This is the first time that a such strategy has been recorded for a crab species and for a vegetable resource.