HOBSON: FEEDING RELATIONSHIPS OF FISHES 



Chaetodon occur together on Kona reefs — all very 

 similar in general body form, but with distinctive 

 differences in diet and related morphology. Obvi- 

 ously, such situations can exist only if, in addition 

 to having acquired adaptations suited to 

 specialized diets, closely related forms have also 

 acquired effective barriers to interbreeding. Cen- 

 tral to this is the ability of each individual to 

 recognize others of its own kind, which probably 

 relates to the circumstance that most species in 

 this category have highly visible species-specific 

 color patterns. 



It is logical that diurnal fishes would employ 

 visual cues to identify one another. But the dis- 

 tinctive nocturnal colorations of many chaetodon- 

 tids suggest that members of some species need to 

 recognize each other after dark as well. Nocturnal 

 colorations that occur among chaetodontids in 

 Kona tend to accentuate a contrast, thus making 

 them more visible at lower light levels (e.g. Fig- 

 ures 28a and b; 29a and b). Although the nocturnal 

 colorations of some fishes, such as those that be- 

 come mottled, make them more difficult to see in 

 the dark (Schroeder, 1964), certain chaetodontids 

 in Kona seem to be effecting a nocturnal display. 

 This phenomenon appears most pronounced 

 among fishes in the present category, but others 

 show it as well; for example, in Kona certain of the 

 nocturnal squirrelfishes, Holocentrus (Holocen- 

 tridae: Beryciformes) display characteristic 

 white bars or spots at night that are more 

 visible under reduced light than their daytime 

 colorations would be (e.g. Figure 12a and b). Prob- 

 ably one can generalize only to the extent that 

 distinctive day/night colorations in coral reef 

 fishes reflect distinctive day/night situations. 



Fishes Specialized to Feed on Vegetation 



Vegetation, which carpets much of the rocky sea 

 floor inshore, would seem ready food for fishes. Yet 

 relatively few species utilize this resource, even 

 though, as in Kona, they often predominate on 

 tropical reefs. The herbivorous habit is an ad- 

 vanced trait among marine fishes, a fact recog- 

 nized by Hiatt and Strasburg (1960). 



In general, herbivorous fishes on coral reefs 

 share many characteristics with the diurnal pred- 

 ators that are specialized to prey on benthic in- 

 vertebrates, discussed in the previous section. 

 Like the fishes grouped together in that category, 

 at least most coral reef herbivores are active by 

 day and relatively inactive at night; furthermore, 



they too tend to be colorful animals that have 

 small mouths which are part of highly evolved 

 feeding systems. In fact, several families of fishes 

 span both categories; for example, the Chaetodon- 

 tidae, Pomacentridae, Blenniidae, Balistidae, 

 Monacanthidae, and others include gradations of 

 species from some that are strictly carnivorous, to 

 others that feed on both plants and animals, to 

 still others that are strictly herbivorous. Within 

 these groups, which have favored plasticity in 

 feeding habits and structures, it seems that 

 characteristics adaptive to plucking benthic in- 

 vertebrates from the sea floor have been modified 

 in some species for grazing on plants. 



Nocturnal Activity Among Advanced Teleosts 



Not all the more advanced fishes are diurnal. 

 The chaetodontid Chaetodon lunula seems to be 

 nocturnal in Kona, and at least some of its 

 congeners — notably C. quadrimaculatus and C. 

 auriga — may feed to some extent after dark. But 

 these are exceptional cases in an overwhelmingly 

 diurnal group. As suggested above, nocturnal ac- 

 tivity in these instances may relate to competition 

 among the exceptionally large number of 

 Chaetodon species that cooccur on Kona reefs. 



Nocturnal habits cannot be regarded as excep- 

 tional where they occur among the diodontids, 

 however, because night feeding seems to be the 

 rule in this family. And these members of the 

 order Tetraodontiformes are among the most 

 highly evolved of all reef fishes. The prey of 

 Diodon hystrix and D. holocanthus in Kona 

 — large echinoids, gastropods, and pagurid 

 crabs — are more exposed at night than during the 

 day. And because they are relatively large and 

 move at least intermittently after dark, one can 

 predict they would be suitable quarry for noctur- 

 nal predators having means to crush heavy ar- 

 mour. These are large prey, so a predator must 

 carry its crushing mechanism in its mouth, rather 

 than in its throat — as do many of the labrids and 

 other predators that feed on smaller mollusks and 

 echinoids during the day. The highly evolved 

 diodontids accomplish this job with their powerful 

 crushing jaws, but the problem has also been 

 solved at a more primitive level by certain basal 

 percoids. In Kona, the nocturnal sparid Monotaxis 

 grandoculis, with its molariform dentition, has 

 feeding habits similar to those of the diodontids, 

 but with less emphasis on heavily armoured 

 forms. Clearly, the diodontids, with more powerful 

 jaws and heavier dentition, are better adapted 



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