Diversity of diet of herbivorous fishes in the Lesser Antilles (Scaridae and Acanthuridae), using isotopic and microbiologic approaches.

Supervisors: Charlotte Dromard, Malika Rene-Trouilefou (Université des Antilles).
Coral reefs are one of the most diverse and productive ecosystems in the world. Algal compartment (microalgae, turf and macroalgae) is a major constituent in coral reefs as it is located at the basis of the trophic food-web. Development of algae are mainly controlled by environmental factors such as nutrient concentrations in surrounding waters and are regulated by grazing pressure of herbivorous fish and urchin. Since 1980, due to the decline of the sea urchin Diadema antillarum, and the increase of anthropogenic pollution, coral communities began to be replaced by macroalgae benthic communities, which is known as “coral algal phase shift” phenomenon. Such phenomenon increases the importance of fish grazing, but herbivorous fish are threatened by fisheries, anthropogenic pollution. Facing such phenomenon, fish grazing and conservation of herbivorous fishes are crucial research areas, to understand coral reef resilience. In the present study, the trophic ecology of two Caribbean herbivorous fishes, Acanthurus coeruleus and Sparisoma viride, was studied on Guadeloupe reefs. Firstly, isotopic analyses were conducted in order to compare isotopic ratio between muscle and fins (pectoral and dorsal). Results of this analysis showed a positive correlation  between the two tissues and allow the sampling of fins instead of muscle for future isotopic ratio measurement, avoiding the death of the fish and leading to the conservation of individuals. Isotopic measurement were also used to compare fish diets, isotopic niches and estimate the proportions of assimilated food sources, using mixing models. These analyses showed similar proportions of assimilated food resources between the two fish species, but a small overlap between their isotopic niches, indicating an ecological complementarily of Acanthurus coeruleus and Sparisoma viride. Secondly, microbiological analyses were made on bacteria communities respective to the diet habit of each fish species. The specific approach was based on the culture of bacteria colonies present on the microbiome culturable part. The culture was focused on gastrointestinal tract: the wall and the content (feces) of the hindgut, and it gave specific information concerning cultivated bacteria. Bacterial colonies obtained were described and compared between culture medium, fish species and tissues. The total count of bacteria obtained revealed a higher number of CFU/mL in feces than in hindgut wall. We obtained 44 pure clones for Acanthurus coeruleus and 77 for Sparisoma viride species. These, pure clones were described and frozen in order to be determined, and compared with metagenomic approach.