Inferring feeding habits of the whale shark (Rhincodon typus) using fatty acids, in the Northern Mexican Caribbean

Abstract

Background In the Mexican Northern Caribbean, whale sharks feed on abundant plankton. Assessing the trophic role of this species is key to understanding its general ecology. Non-lethal sampling combined with biochemical methods such as analysis of fatty acids (FA) are promising approaches for investigating the assimilated diet of the whale shark. Approach During 2010 and 2011, a total of 68 whale shark samples (dermal tissue) were collected from free-swimming individuals while they were feeding. Zooplankton samples were collected near feeding whale sharks at the surface. These samples were divided in two categories: mixed zooplankton - several groups of zooplankton, and fish eggs - more than 95% of sample components were fish eggs. FA profiles were assessed from biopsies and zooplankton. ANOSIM was performed to identify the level of significant difference among the different groups of FA composition. Non-metric multi-dimensional scaling (MDS) plots were used to visualize grouping among whale sharks, zooplankton and fish eggs FA profiles. Results The FA profiles of R. typus and their potential prey were dominated by saturated FA. Whale shark FA profiles were significantly different from mixed zooplankton items (ANOSIM R value= 0.87). Fish egg and whale shark FA profiles formed separate groups with overlapping values (ANOSIM R value= 0.67). Whale shark and fish egg FA profiles registered high levels of oleic acid. Arachidonic acid (ARA; mean + SE=8.3+0.8) was the polyunsaturated fatty acid present at the highest level in the whale shark FA profile. These levels did not match the levels of mixed zooplankton and fish eggs. Conclusions The FA profile of the whale shark differs from other zooplanktivore animals whose profiles are usually dominated by polyunsaturated fatty acids. Results of the FA analysis suggest that R. typus is feeding mainly on near surface zooplankton. This is supported by whale shark behavioral observations and low levels of bacterial FA in R. typus profiles. The fact that levels of ARA registered in superficial zooplankton did not explain the ARA content found in biopsies suggests that the whale shark could have other complementary feeding sources, such as demersal zooplankton, in this aggregation area.