Do the habitats, mouth morphology and diets of the mullidsUpeneichthys stottiandU. lineatusin coastal waters of south-western Australia differ?

Abstract

Two benthic carnivorous goatfish (Mullidae), Upeneichthys stotti (max total length, L(T) = 179 mm) and U. lineatus (max L(T) = 257 mm), were trawled from the inner continental shelf waters of Western Australia. U. stotti was found almost exclusively offshore at 20-35 m depth, while U. lineatus was most abundant inshore at 5-15 m depth. Smaller individuals of both species ate small, thin-shelled mysids, tanaids and amphipods, while larger fish ate large, hard-bodied isopods, carid decapods and brachyuran crabs. Classification and ordination of the mean volumetric percentage contributions of the prey (dietary samples) of both species from all sites and in each season did not lead to a clear separation between these two species. However, ordination demonstrated that the dietary samples of the two species were distinct when the two species were found together, and when fish of sequential 20-mm length intervals were used. The interspecific size-class differences were consistent with the results obtained using Schoener's overlap index, for which values >0.6 were recorded for only two of the 45 possible interspecific pairwise comparisons. Since these interspecific differences occurred despite virtually identical mouth sizes and morphologies, the two species must feed in a slightly different manner and/or in different microhabitats. While most comparable-sized fish ate tanaids and amphipods, U. stotti ate more relatively mobile epibenthic mysids, cumaceans and carid decapods, whereas U. lineatus ate more larger and slower-moving burrowing bivalves, onuphid polychaetes and brachyuran crabs. The partial partitioning of habitats, when combined with differences in the diet, would help facilitate the co-existence of these two mullids when they are abundant.