Distributions, Size Compositions and Diets of Two Abundant Benthic Ambush-Feeding Teleosts in Coastal Waters of South-Western Australia

Abstract

The scorpaenid Maxillicosta scabriceps (Teleostei) and the platycephalid Platycephalus longispinis (Teleostei) were trawled seasonally for a year from shallow (5-15 m) and deeper (20-35 m) waters in four distantly-located regions of the inner continental shelf of south-western Australia. The former species was more abundant in southern regions, which is consistent with its temperate distribution, while the latter species was more abundant in deep than shallow waters. Unlike M. scabriceps, P. longispinis with total lengths < 110 mm were rarely caught, indicating that this latter species only moves on to the sandy substrate of the inner shelf when it reaches a certain size. As M. scabriceps increased in size, the contributions of mysids, amphipods and oxyrhyncan crabs to the diet declined, while those of carid decapods and brachyrhyncan crabs increased. The invertebrate fauna ingested by the smallest length-class of P. longispinis, 110-139 mm, was similar to that of the same and largest length-class of M. scabriceps. However, unlike M. scabriceps, this length-class of P. longispinis consumed teleosts, despite having a relatively smaller mouth. The contribution of teleosts subsequently increased as P. longispinis increased in size, a feature reflected by the relatively low dietary breadth of large fish. The marked shift in the types of prey ingested by P. longispinis as it increased in size accounted for the fact that, in contrast to M. scabriceps, there was limited dietary overlap between the larger and smaller members of this platycephalid. The ability of P. longispinis to ingest other fish is presumably related to the fact that platycephalids lie just under the substrate surface and are thus concealed from their potential prey, and that they are able to emerge rapidly and pursue their prey. Unlike several other local demersal fish species, M. scabriceps and P. longispinis ingested only a small amount of sediment, implying that these ambush predators target their prey very precisely, a foraging mode that would be facilitated by the epibenthic and conspicuous nature of their prey. The absence of marked differences in the dietary compositions of these two species in different regions, water depths and seasons is consistent with a specific mode of foraging by these species. INTRODUCTIONDemersal species of teleosts are essentially restricted to feeding on those prey that are found either in or close to the substrate and which typically comprise polychaetes, molluscs, crustaceans and small benthic teleosts (Gibson & Ezzi, 1987; HarmelinVivien et al., 1989;Meyer & Smale, 1991). However, the dietary compositions of the various species within demersal fish communities sometimes differ quite markedly, reflecting the different modes of feeding that are associated with a range of morphological and behavioural adaptations (e.g. Blaber & Bulman, 1987; Gibson & 588 M.E. PLATELL AND I.C. POTTER Ezzi, 1987;Fujita et al., 1995;Motta et al., 1995). Yet it is still frequently possible to group different species within a demersal f...

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Active acoustic tracking suggests that soft sediment fishes can show site attachment: a preliminary assessment of the movement patterns of the blue-spotted flathead (Platycephalus caeruleopunctatus)

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