HOBSON: FEEDING RELATIONSHIPS OF FISHES 



Predators Specialized to Prey on 

 Benthic Invertebrates During the Day 



A wide variety of fishes prey on benthic inver- 

 tebrates during the day. They include most of the 

 labrids, chaetodontids, baUstids, canthigasterids, 

 monacanthids, ostraciontids, and tetraodontids, 

 as well as many of the pomacentrids, blenniids, 

 and others — all of them higher perciforms or tetra- 

 odontiforms. Their prey are among the more 

 prominent invertebrates on the reef, including 

 such sessile forms as sponges, coelenterates, and 

 tunicates; and also various slow moving animals 

 like echinoids and gastropods. Typically, these 

 prey are fortified with toxic or noxious compo- 

 nents, like spines, spicules, nematocysts, or tough, 

 fibrous components; or they are encased in heavy 

 armour. Because of these defensive features, 

 fishes that prey on such forms must have 

 specialized feeding structures or techniques — the 

 unspecialized feeding apparatus of generalized 

 predators is maladapted for this task. Also un- 

 available to generalized predators are the many 

 very small organisms whose capture requires 

 delicate manipulations or movements for which 

 large-mouthed fishes are unsuited. Moreover, 

 many of these prey are diurnal ly cryptic or secre- 

 tive, thus requiring still additional specializations 

 to capture them in daylight. 



Thus, fishes that successfully feed on most 

 benthic reef invertebrates during the day are ad- 

 vanced species whose evolution has been mostly 

 one of perfecting means to feed in daylight on prey 

 that are beyond the capacity of fishes with gen- 

 eralized feeding equipment. Certain mullids, 

 discussed above, are adapted to feed on many of 

 these prey, but mullids use nonvisual means, 

 whereas fishes considered here are primarily vi- 

 sual feeders. 



These are fishes that have passed the percoid 

 level of development. The evolution of the percoid 

 morphology, especially with its highly adaptive 

 feeding mechanism, gave fishes added potential to 

 adjust to a wide variety of feeding situations. But 

 although percoids appeared first during the Cre- 

 taceous (Patterson, 1964), not until modern reef 

 communities appeared during the Eocene 

 (Newell, 1971) does it appear they began to fully 

 realize this potential. 



Bakus (1964, 1966, 1969) concluded that the 

 secretive habits and defensive structures of many 

 benthic invertebrates on coral reefs today, includ- 

 ing sponges, didemnid tunicates, and others, are 



the result of predation pressures from fishes. 

 Whether or not this is so, certainly the array of 

 specialized feeding habits and structures that 

 characterize diurnal bottom-feeding fishes on 

 coral reefs are mostly adaptations which cope with 

 specific defensive characteristics of their prey. Be- 

 cause predation pressures lead to defensive ad- 

 justments in prey, and these in turn stimulate 

 further offensive modifications in predators, it is 

 not surprising that the diverse array of defenses in 

 benthic invertebrates today is matched in the 

 fishes that feed on them by an equally diverse 

 array of solutions. These solutions to invertebrate 

 defenses are manifest in the extremely varied 

 feeding structures and behaviors that occur 

 among diurnal fishes. Most diurnal fishes special- 

 ized for diets of benthic invertebrates have rel- 

 atively small mouths, but beyond this their feed- 

 ing morphologies have diverged widely. 



Sessile invertebrates seem to be significant prey 

 only during the day, perhaps because an animal 

 must move to be sensed by most predaceous fishes 

 at night (Hobson, 1968a). Thus, the few highly 

 specialized fishes that feed on sponges are strictly 

 diurnal. In Kona, the chaetodontid Holacanthus 

 arcuatus feeds on some of the larger sponges that 

 encrust in exposed locations on rocks, whereas the 

 zanclid Zanclus canescens uses its elongated 

 snout to feed on some of the smaller sponges that 

 are attached within crevices or depressions on the 

 reef. Randall and Hartman (1968), in studying 

 sponge-feeding chaetodontids and monacanthids 

 in the West Indies, noted that sponges cannot be 

 digested by most fishes, and concluded that these 

 organisms have become available as food for only 

 a few highly specialized teleosts in geologically 

 recent times. 



Some diurnal predators, for example Forcipiger 

 flavissimus, Chaetodon auriga, and C fremblii, 

 among chaetodontids in Kona, habitually tear off 

 pieces of larger sessile invertebrates, including 

 polychaetes, tunicates, and alcyonarians. The 

 analogy drawn above between the snout and jaws 

 of F. flavissimus and a pair of needle-nosed pliers 

 underscores the suitability of this fish's feeding 

 morphology for its feeding habit. 



One of the most obvious potential foods for car- 

 nivorous bottom-feeding fishes on coral reefs 

 would seem to be the corals themselves. Neverthe- 

 less only some of the most advanced teleosts ex- 

 ploit this resource. In Kona, coral eaters include 

 certain chaetodontids, pomacentrids, and blen- 

 niids (all higher Perciformes) and certain 



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