Abstract
Assessing the trophic role and interaction of an animal is key to understanding its general ecology and dynamics. Conventional techniques used to elucidate diet, such as stomach content analysis, are not suitable for large threatened marine species. Non-lethal sampling combined with biochemical methods provides a practical alternative for investigating the feeding ecology of these species. Stable isotope and signature fatty acid analyses of muscle tissue were used for the first time to examine assimilated diet of the reef manta ray Manta alfredi, and were compared with different zooplankton functional groups (i.e. near-surface zooplankton collected during manta ray feeding events and non-feeding periods, epipelagic zooplankton, demersal zooplankton and several different zooplankton taxa). Stable isotope δ15N values confirmed that the reef manta ray is a secondary consumer. This species had relatively high levels of docosahexaenoic acid (DHA) indicating a flagellate-based food source in the diet, which likely reflects feeding on DHA-rich near-surface and epipelagic zooplankton. However, high levels of ω6 polyunsaturated fatty acids and slightly enriched δ13C values in reef manta ray tissue suggest that they do not feed solely on pelagic zooplankton, but rather obtain part of their diet from another origin. The closest match was with demersal zooplankton, suggesting it is an important component of the reef manta ray diet. The ability to feed on demersal zooplankton is likely linked to the horizontal and vertical movement patterns of this giant planktivore. These new insights into the habitat use and feeding ecology of the reef manta ray will assist in the effective evaluation of its conservation needs.
Highlights
Information on the diet and trophic position of an animal can improve ecological understanding of the underlying drivers of its movements and its role within the ecosystem
We reported the unusual fatty acid(s) (FA) profiles of the reef manta ray and the whale shark Rhincodon typus Smith 1828, both being dominated by omega-6 (v6) PUFA [44]
No assessment of the food source of the reef manta ray is available in the scientific literature apart from the single stomach content rudimentarily described for M. alfredi by Whitley [37]
Summary
Information on the diet and trophic position of an animal can improve ecological understanding of the underlying drivers of its movements and its role within the ecosystem. Such knowledge can support conservation plans for areas where the temporal and spatial abundance and distribution of prey are understood [1,2,3]. Stomach content analysis is the conventional approach used to assess a species’ diet [4] and has many advantages; it has several shortcomings This technique only provides a ‘snapshot’ of recent feeding and may not accurately reflect the composition of prey items that contribute most significantly to its general diet. Last, obtaining stomachs from large and threatened marine species is often difficult and killing animals for this purpose is ethically questionable
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