Exotic herbivorous fish in Mediterranean ecosystems: biological causes and ecological consequences of an ongoing invasion – EXOFISHMED

Mediterranean coastal ecosystems are facing unprecedented anthropogenic disturbances that synergistically threaten their unique biodiversity and the ecosystem services they provide to human populations. The ongoing expansion of herbivorous fishes from the genus Siganus, which entered the Mediterranean Sea from the Red Sea after Suez Canal opening, is one of the most critical issues. Indeed, in the Oriental basin of the Mediterranean Sea (from Turkey to Tunisia) Siganus populations tend to exclude the single native herbivorous species (Sarpa salpa) and to cause a widespread depletion of macrophytes, including seagrass meadows that serve as habitat and/or food for many animal species and play key roles in nutrient cycles. As sea warming is accelerating, it is likely that these tropical exotic species will colonize the Occidental basin of the Mediterranean Sea and establish populations in coastal ecosystems of South-Western Europe in a near future. However, although some aspects of Siganus ecology have received attention, there are still knowledge gaps about the causes and consequences of this invasion. EXOFISHMED project will thus address two questions (1) which factors explain how exotic fishes are supplanting the native species ? (2) what are the consequences of this species replacement on the functioning of ecosystems ?

To answer these questions we will conduct a multidisciplinary approach to compare the biological attributes and ecological roles of native and exotic herbivorous fish species. We will collect this information on Siganus luridus and Sarpa salpa using two complementary approaches: (i) sample juvenile and adult individuals from 4 locations around the Mediterranean basin, and (ii) conduct a mesocosm experiment under controlled environmental conditions. Four biological features will be examined on these fish individuals: diet using stable isotope ratios, food acquisition strategy using morphology, growth rates using sclerochronology on otoliths, and nutrient storage in body using tissue nitrogen and phosphorus content. These data will permit to assess intraspecific variability of each feature between life-stages and between populations of each species, and to test for differences between the two species. In addition to these biological features, we will assess the diversity of microbial communities present in fish gut, which is a poorly known but a key feature of herbivorous fishes. More precisely, we will assess gut microbial diversity accounting for the three microbial domains (archaea, bacteria, eukaryotes) and for all their respective diversity facets (taxonomic, phylogenetic and functional) using high-throughput DNA sequencing methods and complementary biodiversity indices. Intra- and interspecific variability of gut microbial diversity will be measured and compared to intra- and interspecific variability of the four biological features to provide the first multi-faceted assessment of the differences between the two species.

Then, based on these fish data and measures during mesocosms experiment we will assess the contribution of native and exotic fish species to two key processes in coastal ecosystems: trophic control of macrophytes (macroalgae, seagrass) through grazing activity, and nutrient recycling through excretion of metabolic wastes. The unknown effects of these two roles of herbivorous fishes on the abundance and diversity of planktonic and benthic microbes (microalgae, bacteria, archaea, eukaryotes) will also be measured to provide the first overall assessment of the consequences of Siganus invasion on ecosystem functioning.

These objectives will be reached within 3 years thanks to the complementary skills of the research consortium. The outputs of the project will be communicated to scientists, policy makers and ecosystem managers and to the general public and will help set relevant prevention and mitigation measures against this invasion.