Fish behavior is as diverse as fish morphology. Some species travel in schools, while others are highly territorial. Interesting commensal relationships exist with other fishes and other animals. Fishes are adapted to a wide variety of foods. Some are specialized or highly adapted to feed on such items as zooplankton, snails, and coral. Almost all classes of animal and some plants can serve as food. A few species have a parasitic mode of feeding on other species or on the female of their own. Some produce venom, electricity, sound, or light. Most fishes are ectotherms, but some sharks and some scombrids have evolved endothermy for at least part of their body. Internal fertilization occurs in certain species, and females of some of these species provide nutrients to developing embryos. Some exhibit parental care for their offspring, and others scatter millions of eggs to the hazards of predation. Whereas most fishes are gonochoristic (fixed sexual pattern), many are hermaphroditic. Most of the latter are protogynous (vs. protandrous) sequential (vs. synchronous) hermaphrodites, as in labrids, where females change to males. Some fishes have a larval stage and undergo metamorphosis.
Lifespan in fishes may vary from a little over 1 year to about 120 years. A few die relatively soon after a single spawning period (a phenomenon termed semelparity), but individuals of most species normally reproduce for more than one season (iteroparity). Fewer than 1% of fishes are semelparous, and these tend to be diadromous species. Semelparity is known primarily in petromyzontiforms, anguillids, some Pimephales (a cyprinid), some populations of osmerids, some galaxioids (e.g., some Retropinna), five species of Pacific salmon (Oncorhynchus), Labidesthes sicculus (an atherinopsid), and a few gobiids. Fishes in all types of aquatic environments may migrate phenomenal distances and use various homing mechanisms, a subject rich in questions for researchers. The larvae and early juveniles of some oceanic species (e.g., fly-ingfishes and dolphinfishes) regularly inhabit shore waters, whereas the larvae of many shore fishes inhabit oceanic waters. In freshwater, Oncorhynchus keta and O. tshawytscha migrate 3,000 kilometers (km) up the Yukon River to their spawning grounds without feeding. Other fishes are known to live out their lives in very restricted areas.
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