Manta birostris , predator of the deep? Insight into the diet of the giant manta ray through stable isotope analysis

Katherine B Burgess,Andrea D Marshall,Scarla J Weeks,Anthony J Richardson,Michael B Bennett,Lydie I E Couturier

doi:10.1098/rsos.160717

Abstract

The characterization of diet for the giant manta ray Manta birostris has been problematic given their large-scale movement patterns and the difficulty in obtaining stomach contents from this species. The large majority of existing information is based on observational data limited to feeding events at the sea surface during daylight. Recently discovered aggregation sites for the giant manta ray off mainland Ecuador are some of the most accessible to date and provide a unique opportunity for researchers to gather much needed information on this elusive species. To assess how important surface zooplankton is to giant manta ray diet, we conducted stable isotope analysis (15N and 13C) on M. birostris muscle and surface zooplankton. Trophic position estimates placed M. birostris overall at a secondary consumer level of approximately 3.4 but there was large variation in δ15N and δ13C values among individuals. Manta birostris muscle tissue δ13C values were also not consistent with this species feeding predominantly on surface zooplankton and suggest that the majority of dietary intake is of mesopelagic origin. Given the conservative life history and fisheries pressure on large planktivores, knowledge of their trophic role and foraging strategies is essential to better understand their ecology and develop effective conservation measures.

Highlights

Manta rays are large filter feeding elasmobranchs, but despite considerable study many aspects of their biology and ecology remain enigmatic
There was a high degree of overlap in isotopic niche space between M. birostris and other co-occurring vertebrates from the eastern equatorial Pacific
There are no quantitative estimates of dietary composition for M. birostris and difficulties exist in assigning pre-2009 observations of manta ray feeding to the species involved due to the taxonomic revision of the genus Manta in 2009 [32]

Summary

Introduction

Manta rays are large filter feeding elasmobranchs, but despite considerable study many aspects of their biology and ecology remain enigmatic. Dietary information for Manta species is based mostly on observational data, primarily gained from near-surface feeding events during daylight. Non-lethal, minimally invasive biochemical methods, such as bulk stable isotope analysis (SIA), have proved useful in the examination of dietary intake of large, mobile and difficult-to-observe elasmobranch species [1,2]. The ratio of heavy to light isotopes of carbon (δ13C) and nitrogen (δ15N) can provide information on dietary sources [3] and trophic position, respectively [4]. Considerable variability in δ13C exists between benthic and surface water marine algae, and consumers of benthic carbon sources are enriched in 13C compared with pelagic surface feeders [5]. As 14N is lost more rapidly than 15N during the processes of metabolism and excretion, increasing values of δ15N are found as animals attain higher trophic positions [6]

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Conclusion