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Trophic interactions between two neustonic organisms: insights from Bayesian stable isotope data analysis tools
Lepoint, G.; Bernard, L.; Gobert, S.; Michel, L.N. (2016). Trophic interactions between two neustonic organisms: insights from Bayesian stable isotope data analysis tools. Belg. J. Zool. 146(2): 123-133
In: Belgian Journal of Zoology. Koninklijke Belgische Vereniging voor Dierkunde = Société royale zoologique de Belgique: Gent. ISSN 0777-6276; e-ISSN 2295-0451, more
Peer reviewed article  

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    Janthina globosa Swainson, 1822 [WoRMS]; Velella velella (Linnaeus, 1758) [WoRMS]
Author keywords
    Janthina globosa; Velella velella; Mediterranean Sea; neuston; SIBER;SIAR

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    The by-the-wind sailor Velella velella (Linnaeus, 1758) and its predator, the violet snail Janthina globosa (Swainson, 1822) are both floating neustonic organisms. Despite their global oceanic distribution and widespread blooms of V. velella in recent years, many gaps remain in our understanding about prey/predator interactions between these two taxa. Using stable isotope ratios of carbon and nitrogen, we aimed to study the trophic relationship between V. velella and J. globose and investigate diet variation of V. velella and J. globosa in relation to individuals’ size. Bayesian approaches were used to calculate isotopic niche metrics and the contribution of V. velella to the J. globose diet. Our data showed that the isotopic niche of V. velella differed markedly from that of J. globosa. It was larger and did not overlap that of the J. globose, indicating a more variable diet but at a lower trophic level than J. globose. The isotopic niche of V. velella also varied according to the size class of the individual. Small individuals showed a larger isotopic niche than larger animals and low overlap with those of the larger individuals. J. globose displayed very low isotopic variability and very small isotopic niches. In contrast, there were no isotopic composition nor isotopic niche differences between J. globose of any size. This very low isotopic variability suggested that J. globosa is a specialist predator, feeding, at least in this aggregation, principally on V. velella. Moreover, outputs of a stable isotope mixing model revealed preferential feeding on medium to large (> 500 mm2) V. velella colonies. While our isotopic data showed the trophic relationship between V. velella and J. globosa, many questions remain about the ecology of these two organisms, demonstrating the need for more fundamental studies about neustonic ecosystems.

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