Physical characteristics

The earliest known killifish "description" is a 600-year-old piece of mother-of-pearl jewelry, one inch long, produced by the Native American Mogollon culture in the recognizable shape of Cyprinodon tularosa, endemic to New Mexico. As a general rule, killifishes and live-bearers are sexually dimorphic and dichromatic. In 1881 Steindachner described male and female Cynolebias bellottii as two different species, the female named appropriately Cynolebias maculatus. Cyprinodon-tiform males and females differ in shape and color and sometimes in numbers of anal and dorsal fin rays, as noted by Steindachner. There is a gestalt to the Cyprinodontiformes that is difficult to describe (because they are so variable) but which makes them instantly recognizable. Few fishes can be mistaken for a killifish or live-bearer, but some of the very few are the mudminnows of the Umbridae. In fact, in 1843 Umbra pygmaea was described by Ayres as a killifish (Fundu-lus fuscus). Ayres was perhaps the first person to document his confusion about this resemblance.

The Cyprinodontiformes vary greatly in length, from 0.4 in (1 cm), which meets the formal definition of a miniature fish, to nearly 13 in (33 cm). Many are basically cylindrical in shape, with tapering around the caudal peduncle. Some are sleek, pikelike predators and others elongate and flat-topped (the top minnows of the order), with mouths designed for surface feeding. Some are laterally compressed and elongate for fast movement in streams or in pelagic conditions, and others that occupy ben-thic ecological niches may or may not be compressed but tend to be deep-bodied. Some Orestias have "chunky" body proportions akin to those of various fancy goldfish.

Cyprinodontiformes possess only one dorsal fin, which has its origin anywhere from far forward of the first anal fin ray to a point over the last few anal fin rays. The dorsal fin is never completely ahead and rarely entirely behind the anal fin; there almost always is an overlap. The origin of the anal fin ranges from about the midbody to three-fourths of the way from the snout. Fin rays are soft; Cyprinodontiformes do not have spines. Unpaired fins are rounded, truncated, pointed, elongated, or a combination of these shapes. Caudal fins are sometime lyre-shaped. The unpaired fins may carry very elaborate extensions or filaments, which in some cases extend beyond the caudal fin. Males of most species have contact organs, that is, bony outgrowths along the outer margins of the scales, along the fin rays, or on the snout. These organs help initiate spawning or position the males during spawning. In the live-bearers and some killifishes, the anal fin of the male becomes a gonopodium, which is used as an intromittent organ.

Pelvic fins sometimes are a prominent feature, but mostly they are small, tiny, or absent. Pelvic fin position varies, though usually it is close to the origin of the anal fin; sometimes it is far forward and close to the pectoral fins. There is no lateral line system along the sides, although in some species neuro-masts protrude through the scales, running along what normally would be the course of the lateral line. The lateral line system is present around the head, with the cephalic neuromasts either totally exposed or in canals or a combination of the two states. Derived states of the cephalic lateral line system are very useful in taxonomic studies. Some species use the cephalic lateral line system to locate surface prey by its vibrations.

In 1949 Gosline developed an elaborate and very useful classification and numbering system for the sensory canals and pores of the cyprinodontiform head. The anterior naris is tubular in the aplocheiloids and in the cyprinodontoid genera Cubanichthys and Anableps. Among the cyprinodontoids this is considered to be independently derived. Overall squamation is complete, partial, or absent. There has been some attempt, mainly among those studying aplocheiloids, to use the pattern of scales on the head as a taxonomic tool. Upper and lower jaw teeth are spatulate, unicuspid, bicuspid, or tricuspid or have various combinations of those tooth forms. Sometimes teeth

The largescale foureyes (Anableps anableps) sees well both above and below the water because its eyes have two regions of retina, one for seeing above water and the other for underwater. It feeds on both insects from the air and small fish from the water. (Photo by Dr. Paul A. Zahl/Photo Researchers, Inc. Reproduced by permission.)

The largescale foureyes (Anableps anableps) sees well both above and below the water because its eyes have two regions of retina, one for seeing above water and the other for underwater. It feeds on both insects from the air and small fish from the water. (Photo by Dr. Paul A. Zahl/Photo Researchers, Inc. Reproduced by permission.)

are present on the vomer. Jaw teeth are used to seize food items; teeth on the pharyngeals do the chewing. Mouths are protrusile. In some cases the lower jaw has a marked upward turn, sometimes almost perpendicular to the body axis. Some species have thickened lips to facilitate the eating of algae.

Body proportions and fin lengths and shapes are different in the sexes. Females typically have a more rounded appearance. Aquarists never have problems determining the sex of the Cyprinodontiformes. In other groups, this is not the case. Color differences are always noticeable and, in many cases, dramatic. The females generally are plain—perhaps silvery, olive drab, or brownish—whereas males may be brightly colored in crimson, iridescent greens and blues, bright yellow, bright blue, or a combination constituting a veritable riot of colors.

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