Symbiosis is the living together of two unrelated organisms. In parasitism one member of a pair benefits and the other suffers a reduction in fitness, in mutualism both members of a pair benefit, and in commensalism one benefits and the other is neither harmed nor helped. Mutualistic relationships are the most interesting because they indicate a relatively long period of co-evolution between pair members. Fishes form mutualistic relationships with other fishes and with a variety of invertebrate species.
The abundance and diversity of external and internal parasites of fishes is tremendous and beyond the scope of this discussion (e.g., Dogiel et al. 1961; Sinderman 1990; Gabda 1991; Bush et al. 2001; Benz & Bullard 2004). Three families of fishes – synaphobranchid eels, trichomycterid catfishes, and carapid pearlfishes – include members that are internal parasites on fishes and other animals (carapids are discussed below under Mutualism and commensalism). The Snubnose Parasitic Eel, Simenchelys parasiticus (Synaphobranchidae), burrows into the flesh of bottom-living fishes such as halibut and has even been found in the heart of Mako Sharks (see Subdivision Elopomorpha). The trichomycterids are particularly insidious because, in addition to feeding on tissue and blood in a host’s gill cavity, the diminutive, eel-like Candiru (Vandellia, 2.5 cm) of South America occasionally enters the urethra of human bathers and wedges itself there with its opercular spines, requiring surgical removal (Spotte 2002; Subdivision Otocephala, Superorder Ostariophysi).
Many fishes, including marine catfishes, characins, tigerperches (Theraponidae), carangids, sea chubs (Kyphosidae), sparids, cichlids, blennies, and spikefishes (Triacanthodidae) fall into a category somewhere between parasitism and predation by removing scales or taking pieces out of fins of other fishes (Losey 1978; Noakes 1979; Zander et al. 1999). Populations of scale-eating cichlids from Lake Tanganyika (Perissodus) contain even numbers of individuals whose mouths twist left or right, facilitating scale removal from the right or left sides of their prey, respectively (Hori 1993). Cookie cutter sharks, Isistius spp. (Squalidae), remove plugs of flesh and blubber from tunas and cetaceans, and lampreys rasp through the skin of numerous fishes and feed on tissues and body fluids. At least two species, the Cutlips Minnow,Exoglossum maxilingua, of North America, and the Eyebiter Cichlid, Dimidiochromis compressiceps, of Lake Malawi, commonly remove eyeballs from unsuspecting prey (although neither species’ behavior has been studied adequately). Access to food sources for many of these partial consumers depends on deceit; small piranhas resemble and school with other characins and then bite the tails off their schoolmates; several juvenile carangids (e.g., Scomberoides, Oligoplites) resemble their silverside and anchovy schoolmates, whose scales they remove; sabretooth blennies mimic cleanerfishes (see below); and cookie cutter sharks may mimic bioluminescent invertebrates that live in the deep scattering layer of the mesopelagic region (Losey 1978; Sazima & Machado 1990; The deep sea).