Scavengers, detritivores, and herbivores
Many fishes scavenge on dead and dying animals. A few species obtain most of their nutrition through scavenging (e.g., hagfishes) or detritivory (e.g., some minnows and suckers, curimatids, prochilodontids, mullets, some Old World cichlids), whereas others supplement predation and omnivory with scavenging (e.g., catfishes, anguillid eels). Importantly, most predators will not pass up freshly dead prey (otherwise bait would not work in hook-and-line fisheries) and most scavengers and herbivores will take advantage of easily captured live prey. In essence, although dietary specializations certainly exist, fishes are highly opportunistic and will eat available prey of the appropriate size. At Johnston Atoll in the tropical Pacific, discarded doughnuts are eaten readily at the surface by such carnivores as snake eels, butterfl yfishes, and flounders, and by such herbivores as damselfishes, parrotfishes, and surgeonfishes (D. A. Mann, pers. comm).
For scavenging animals, the predation cycle is usually shortened to search, wait, manipulate, and handle, whereas for detritivores and herbivores the waiting is eliminated. The major task befalling detritivores is one of separating edible, fine particulate organic matter from any refractory, inedible sediments ingested. Ridges in the mouth and a maze of passageways associated with the gill rakers and epibranchial organs accomplish this in characoids. A winnowing process occurs in the orobranchial chambers as fishes pick up a mouthful of bottom material, sift it in the mouth, and expel inedible sediments back out the mouth or out the gill openings. Detritivores have some of the longest or most complexly folded intestines of any fishes, attesting to the resistance of detritus to enzymatic digestion (Bowen 1983).
Herbivory occurs less commonly in fishes when compared to mammals and birds. Non-teleostean fishes are exclusively carnivorous, with the possible exception of limited herbivory in the Australian Lungfish, Neoceratodus forsteri. In teleosts we find the evolution of pharyngeal mills and gizzards – mechanisms for rupturing cell walls and digesting plant matter. The most diverse freshwater fish taxa include substantial numbers of herbivorous species (characoids, minnows, catfishes, cichlids), and herbivores on coral reefs are among the most abundant fishes there (e.g., halfbeaks, parrotfishes, blennies, surgeonfishes, rabbitfishes). Temperate waters are relatively lacking in herbivores, although some marine families (porgies, sea chubs, Aplodactylidae, Odacidae, pricklebacks) feed heavily on plant matter (Horn 1989).
Herbivory requires accurate search and efficient handling. Herbivores, particularly those that browse on upright macroalgae and do not graze on finer algal turfs, appear to use visual cues for selecting edible versus inedible species. Herbivory is consequently a primarily daytime activity. Targeted search is necessary because plants defend themselves by being tough or by producing chemicals, often in the form of halogenated terpenoids. Herbivorous fishes show strong preferences among algal types, feeding preferentially on species that lack structural and chemical defenses, while avoiding limestone-encrusted species or algae that contain deterrent chemicals. Some of these chemicals can slow growth or cause death in fishes (Horn 1989; Hay 1991).
Specializations for handling plants relate to the difficulty with which cell walls are disrupted, cellulose is digested, or defensive structures and chemicals are overcome. Herbivorous fishes typically have long guts, high ingestion rates, and rapid gut transit times. Large quantities of plant matter are passed through the gut and relatively little nutrition is assimilated from each ingested fraction. Cell walls are broken down in pharyngeal mills or lyzed in highly acidic (pH as low as 1.5) stomachs, although con- clusive evidence of enzymes capable of digesting cellulose (i.e., cellulase) is lacking. Unlike insects and many herbivorous vertebrates, fishes also generally lack endosymbiotic bacteria and other microbes that aid in the digestion of plant matter. The exceptions include surgeonfishes, which contain bacteria, flagellates, and peculiar protist-like organisms, and sea chubs (Kyphosidae), which possess a unique digestive tract morphology and a hindgut microfl ora that aids in digestive fermentation (Fishelson et al. 1985; Rimmer & Wiebe 1987). Interestingly, some sea chubs feed heavily on brown algae that are avoided by most other herbivores (Horn 1989; Kramer & Bryant 1995).
Herbivory on coral reefs is intimately linked to both shoaling and territoriality. Most herbivores either defend exclusive territories (e.g., damselfishes, adult parrotfishes, blennies, surgeonfishes) or roam about the reef in monospecific or heterospecific shoals (sea chubs, parrotfishes, surgeonfishes, rabbitfishes). Territorial defense is very successful against solitary foragers but less so against grouped foragers. Individuals in large groups sustain fewer territorial attacks and have higher feeding rates than solitary foragers or members of small groups. Hence territoriality by some fishes promotes aggregation behavior in others (Robertson et al. 1976; Foster 1985).