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Xiphophorus is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. Xiphophorus species can be divided into 3 groups based on their evolutionary relationships: platyfish (or platies), northern swordtails, and southern swordtails.
The study of prehistoric fish is called paleoichthyology. A few living forms, such as the coelacanth are also referred to as prehistoric fish, or even living fossils, due to their current rarity and similarity to extinct forms. Fish which have become recently extinct are not usually referred to as prehistoric fish.
One of the most popular tropical aquarium fish, the green swordtail has been bred into various hybrid forms for the aquarium hobby due to its hardiness and suitability for community tanks. [7] X. hellerii is a common laboratory animal and has been used as a model organism in studies involving female mate preference [8] and male conflict. [9]
The tail is very long and has tapers at the end. They can reach a length of up to 2 m (6.6 ft). [1] The pectoral and pelvic fins are also very long and thin, almost spaghetti-like, used for gliding through the water. The newly hatched young have branched external gills much like those of newts.
Actinopterygii (/ ˌ æ k t ɪ n ɒ p t ə ˈ r ɪ dʒ i aɪ /; from Ancient Greek ἀκτίς (aktis) 'having rays' and πτέρυξ (ptérux) 'wing, fins'), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish [2] that comprise over 50% of living vertebrate species. [3]
The fish itself does not move much, letting the waves and current move it mostly, but with occasional brief twitching movements. This replicates the way a leaf would be blown around or carried away by the tide or a current. The line running from its eye, to its tail, is believed to replicate the midrib (midvein) of a leaf.
The marbled electric ray has a nearly circular pectoral fin disc and a muscular tail that bears two dorsal fins of nearly equal size and a large caudal fin. It can be identified by the long, finger-like projections on the rims of its spiracles , as well as by its dark brown mottled color pattern, though some individuals are plain-colored.
Stylephorus chordatus, the tube-eye or thread-tail, is a deep-sea ray-finned fish, the only species in the genus Stylephorus and family Stylephoridae. [ 1 ] [ 2 ] It is found in deep subtropical and tropical oceans around the world, living at depths during the day and making nightly vertical migrations to feed on plankton .