E-Trophic ecology of elasmobranchs

Sharks, as apex predators, play a fundamental role in most coral reef ecosystems by influencing communities directly via predation or via behaviourally mediated trophic interactions. They also create important linkages across ecosystems using resources from different habitats. The use of stable isotope analysis  and stomach contents as a tool to better understand the trophic ecology of numerous elasmobranchs evolving in a multitude of habitats has received an increased attention in the last decades. However, to date, few studies have been conducted on stable isotope analysis in reef sharks, and consequently the role reef sharks are playing in structuring the coral reef ecosystem remains poorly understood.

Although sharks are upper trophic level predators in many marine ecosystems, but there is considerable variation in diets and relative trophic position among species. It is now clear that not all sharks are keystone predators, but some species in some situations may be particularly important in maintaining ecosystem structure and function. However, studying the ecological importance of sharks is complex, and understanding their role in ecosystems requires a good understanding of their position in the food web as well as other information such as spatial and temporal variation of habitat use. Most ecosystem models lack of empirical ecological data on relative trophic position and trophic interactions of sharks is consequently needed to validate ecosystem models.

This axis of research aims to better understand the trophic ecology of reef-associated sharks and the  intrinsic or extrinsic factors that can influence it.

Two approaches are taken:

1) Direct observations of predation and interspecific interactions

Rare predation can be observed during underwater surveys providing direct observations of what sharks can eat. It therefore provide information of direct trophic interactions between predator and prey and can help in gaining insigth into the complexity of the marine food web. Some shark species which are believed to be apex predator can be also the prey of other larger sharks species which in turn make them mesopredators or lower level predators such as shown with the picture below with a great hammerhead shark hunting and predating a grey reef shark (Mourier et al. 2013).

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Smaller shark species can opportunistically target fish aggregation in order to easierly predate on their prey. This is the case in the picture below showing an aggregation of surgeonfish attracting a large number (> 50 individuals) blacktip reef sharks (Weideli et al. 2015).

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2) Use of stable isotope analysis

This section is conducted in collaboration with the Heithaus lab (Florida International University). Stable isotopes are naturally occurring biomarkers and provide tools to quantify temporal changes in animal diets. Stable isotope analysis is increasingly being used to investigate the trophic interactions of large-bodied marine predators, including sharks. We use differents tissues (plasma, skin, muscles) sampled from free-living reef sharks in an non-invasive way to investigate the trophic ecology of sharks at the community level but also at the species level including between sex and along the ontogeny of animals.

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Using isotope analysis taken from plasma samples (high turn over rate) of recaptured individuals provide valuable information on how individual shark diet change over time. The figure below shows shift in trophic interactions in juveniles of 3 different shark species as they grow (Matich et al. 2015).

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Associated papers:

Weideli OC, Mourier J, Planes S (2015) A massive surgeonfish aggregation creates a unique opportunity for reef sharks. Coral Reefs. PDF icon

 

Matich P, Kiszka J, Heithaus MR, Mourier J, Planes S (2015) Short-term shifts of stable isotope (δ13C, δ15N) values in juvenile sharks within nursery areas suggest rapid shifts in energy pathways. Journal of Experimental Marine Biology and Ecology 465: 83-91.

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