Characteristics and implications of spongivory in the Knifejaw Oplegnathus woodwardi (Waite) in temperate mesophotic waters

This study has determined the diet of the demersal Knifejaw Oplegnathus woodwardi in depths of 50–200 m off the southern coast of Western Australia, where the benthic invertebrate epifauna is overwhelmingly dominated by sponges. The many fused teeth that form the parrot-like beak of O. woodwardi are used in conjunction with strong muscular plate-like jaws to shear/chop off pieces of sponge and crush the spicule-containing skeleton. Despite the potentially formidable physical and chemical defences (siliceous spicules and secondary metabolites) of sponges, these invertebrates constitute the main prey of O. woodwardi. Sponges were thus ingested by 44% of O. woodwardi and contributed 38% to the volume of the stomach contents across a wide length range of fish. The volumetric contribution of sponges was, however, far less in the stomachs than intestines, whereas the reverse was true for teleosts. This is presumably due to sponges undergoing less digestion than the externally soft-bodied teleosts as food passes through the gut, which is consistent with the large numbers of mainly intact spicules present in the intestine. Since the poriferan prey consisted almost exclusively of species of the Tetractinellida, even though there are three other speciose sponge orders in the region where O. woodwardi feeds in southern Western Australia, this predator apparently selects the sponge taxa it ingests. The length of the siliceous spicules of the Tetractinellida often exceeds 2000 μm, a value nearly ten times that which acts as a deterrent in another fish predator-sponge interaction. Thus, despite possessing formidable defence mechanisms, tetractinellinids are palatable to O. woodwardi. The next most important prey after sponges was teleost fish (among which clupeids were relatively abundant), being ingested by 35% of individuals and comprising 28% of the stomach contents. Other major invertebrate prey comprised hard-bodied crustaceans (mainly decapod crabs), echinoderms, bivalves and cephalopods (with their hard beaks and internal shells), all of which were probably associated with the particular habitat occupied by tetractinellid sponges. These hard-bodied prey are processed by O. woodwardi through the crushing action of its strong jaw mechanism. It is beneficial for O. woodwardi to feed on tetractinellid sponges because they are sessile, palatable and very abundant in the environment of this predator and thus do not incur a high energetic cost of foraging.