Stable isotopic composition of anguilliform leptocephali and other food web components from west of the Mascarene Plateau

Abstract

Leptocephali are the poorly-understood transparent larvae of elopomorph fishes that live in the ocean surface layer throughout the world’s tropical and subtropical oceans. Their feeding ecology has been difficult to understand because they appear to primarily feed on particulate organic material (POM), which contains few identifiable objects, and there have been few studies on their diets or trophic positions. This study presents the first comparative results on the stable isotope ratios of 7 families of leptocephali in relation to the ratios of 30 taxa of other marine animals and POM samples. The carbon and nitrogen stable isotope ratios were analyzed using 50 specimens of leptocephali, 354 specimens of mesozooplankton, cephalopods, fishes, and POM collected west of the Mascarene Plateau in the western Indian Ocean. Nitrogen and carbon isotopic ratio analyses indicated that the 12 taxa of DNA barcoded leptocephali (⩾15 species) could be separated into 2 groups of species with either higher (Group 1: 9 taxa of 7 families, 25–91mm) or lower (Group 2: 3 taxa of 2 families, 43–275mm) δ15N ranges. Group 2 exclusively included species that reach much larger sizes of >150–200mm (Nemichthys and Avocettina, 3 species of Ariosoma-type), whereas Group 1 included Anguilla bicolor bicolor, Serrivomeridae, Muraenidae, Congridae (3 species), Chlopsidae, Ophichthidae (2 species), and Thalassenchelys. Differences in feeding depths, the types of POM ingested by preference or because of different jaw morphology, or the transport of larvae from other regions with different isotopic signatures are possible reasons for the differences between the two groups. The isotopic signatures of 14 taxa of copepods had higher but slightly overlapping δ15N and δ13C signatures compared to leptocephali and most crustaceans and other mezozooplankton, cephalopods and mesopelagic fish taxa had even higher values. The δ15N and δ13C signatures and composition of POM were variable spatially and with depth and may have been influenced by particulates originating from the shallow banks of the Mascarene Plateau. The two apparent isotopic groups of leptocephali should be examined in relation to their consumption of POM, which can include various proportions of prokaryotes, phytoplankton, protozoans, discarded appendicularian houses and other materials, by conducting further studies in different regions and using a variety of techniques.