Abstract

In fishes, mouth gape and body size constrain food acquisition, and morphology often changes throughout ontogeny, potentially leading to covariation between shape and trophic ecology. We tested this hypothesis in the spotted drunk blenny Scartichthys variolatus, an insular species from Robinson Crusoe Island (Southeast Pacific Ocean), by integrating geometric morphometrics and stable isotope analysis. Specimens (n = 90) range from 4.66 to 24.01 cm total length (mean +/- SD: 12.14 +/- 5.14 cm) and from 0.43 to 131.33 g (24.94 +/- 29.55 g). The relative condition (Kn) was independent of size. Ontogenetic shape variation was characterised by a transition from individuals with relatively larger eyes, shorter snouts, and a relatively larger distance of dorsal fin insertion from the snout to specimens with smaller eyes relative to the head, vertically extended snouts, and more frontally inserted dorsal fins. The ontogenetic allometry was high (30.5%) and continuous across the size range, indicating substantial shape disparity among sizes. When interpreted within a locally constrained isotopic baseline defined by macroalgae consumed by the species, carbon stable isotope values (delta C-13) increased ontogenetically from -18.1 parts per thousand to -15.2 parts per thousand, whereas nitrogen stable isotope values (delta N-15) remained relatively constant across sizes (17.5 +/- 1.4 parts per thousand). Partial least squares analyses revealed a significant covariation between cranial shape, body size, body mass, and delta C-13 values, indicating an ontogenetic coupling between morphological change and shifts in the use of basal carbon resources, rather than changes in trophic position. Overall, our results highlight an ecomorphological pattern in which phenotypic plasticity mediates ontogenetic adjustments in basal resource use in an insular fish, emphasising the role of detrital pathways and local isotopic baselines in shaping trophic ecology.