Ecomorphology of six goatfish species (Mullidae) from Toliara Reef, Madagascar

Despite the role that goatfishes play in reef ecosystems, knowledge of their ecomorphological diversity remains scarce. Here, we explore the ecomorphology of six species of goatfishes living in sympatry at Toliara Reef (South-West of Madagascar) by using a combination of morphometric and isotopic (δ¹³C, δ¹⁵N and δ³⁴S) data. The shape of cephalic region was quantified by landmark-based geometric morphometrics and linear measurements, permitting to distinguish each genus and species according to head elongation, cheek size and pectoral fins insertions. Isotopic ratios, used as trophic niche proxy, allowed to highlight clear trophic niche segregation among species built on C and S isotopic ratios, possibly linked to variation in selected prey type and feeding areas. Interspecific variation of these two ratios could also be linked to a cross-reef distribution with species living in the lagoon relying on local primary production and outer-shelf dwellers depending on oceanic inputs. The absence of variation in δ¹⁵N values suggests that the six species share similar trophic position. Our comparative analyses revealed a significant relationship between isotopic and morphometric data while taking phylogenetic relationships into account. These results allowed the formulation of hypotheses regarding differences in goatfishes feeding strategies: “long head” species mainly search for preys in anfractuosities or deep in the substrate while “short head” species use their head to dig in superficial soft bottoms. Overall, we highlight ecomorphological partitioning among sympatric goatfishes based on their morphology and feeding habits, and we show that head shape could be used as a trophically relevant trait in Mullidae.