Trophic interactions between two neustonic organisms: insights from Bayesian stable isotope data analysis tools

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. globosa 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. globosa 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. globosa, indicating a more variable diet but at a lower trophic level than J. globosa. 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. globosa displayed very low isotopic variability and very small isotopic niches. In contrast, there were no isotopic composition nor isotopic niche differences between J. globosa 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.

Isotopic niche of two coastal dolphins in a tropical marine area: specific and age class comparisons

Niche differentiation is the process by which species evolve different forms of resource use, and is used to explain the co-occurrence in a variety of habitats. The Bayesian framework of isotopic niche through quantitative niche metrics was applied to estimate and compare the niche breadth of two sympatric coastal dolphinsPontoporia blainvilleiandSotalia guianensisin a tropical marine area. The standard ellipse areas (SEAs) based on species were quite similar, but the SEAs based on age class showed that the matures’ niche space is larger than the immatures’ for both dolphins. A probabilistic comparison of SEAs indicated that specific differences are negligible compared with age class differences. Trophic level measures (δ15N range) indicated that the dolphins are comparable as top predators, and that immature specimens have a lower range of trophic levels than mature ones. In terms of variability of food sources (δ13C range),S. guianensisshowed a larger value thanP. blainvilleiand mature specimens had larger δ13C range than immatures for both species. In general,P. blainvilleiandS. guianensiswere similar in the niche metrics, with SEAs overlap of 52.1 and 39.7%. The immature specimens showed reduced isotopic niche overlap between species (<3%). In conclusion,Pontoporia blainvilleiandS. guianensisspecimens have similar isotopic niches, but pronounced differences between immature and mature specimens, both intraspecific and interspecific. Isotopic niche and quantitative metrics along with previous data on stomach contents provide a strong representation of species niche and their relationships.