Males of the seasonal fishes are aggressively territorial, defending their breeding sites against other males. In aquaria, where retreat is limited, males may fight until one or both die from their injuries. Aggressive territorial behavior is common in the Cyprinodontidae and is known in the Fundulidae (Fun-dulus catenatus, F. diaphanus, and Lucania goodei). Aggression sometimes extends to nonbreeding females. Male agonistic behavior is very common among the killifishes and some live-bearers and is not necessarily territorial. In the cyprinodontids aggression is associated with the defense of breeding territories; otherwise they move about in peaceful schools. In one cyprinodontid species, Jordanella floridae, the male defends a territory, builds a nest, and fans the eggs—a rare case of cyprin-odontiform male parental care. The long-term defense of a breeding territory by most male Cyprinodon likewise confers a degree of protection to the eggs deposited there.

The Poeciliinae, Anablepidae, and Goodeidae are active, gregarious, and sometimes scrappy. When they are not occupied by feeding activities, males posture and display as they seek to mate. Female receptivity behavior is complex. Among the poeciliines, a male sometimes rushes in quickly, thrusting his gonopodium, and then beats a hasty retreat, particularly in those species where the female is much larger. Poeciliine females release a pheromone-like substance, thought to be estrogen, which stimulates males into a mating frenzy. Among the goodeids, members of the genus Allodontichthys behave much like North American darters. In Africa the nonseasonal killifishes inhabit swamps, trickles, very small streams, and occasionally rivers, but usually they occur in vegetation-choked portions at the edges. Here they are distributed singly in small pockets of open water in the weedy margins, under the vegetation itself, or sometimes under the leaf litter on the bottom but never out in the open. Interestingly, one of these species, Aphyosemion franzwerneri, also behaves like a darter. In aquaria, males of all these species range from peaceful to ferociously aggressive toward each other. Those species that exhibit schooling behavior occur exclusively in the suborder Cyprinodontoidei in the families Fundulidae, Cyprinodonti-dae, Anablepidae, and Poeciliidae. Whether a single species or a mix of species, these schools sometimes are composed of massive numbers of individuals.

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