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Fish are the largest vertebrate group with more than 30,000 species. They show an amazing diversity, coming in many shapes and sizes, but are united by their aquatic lifestyle (although independently many have become air-breathing, and so too a number of species are capable of terrestrial excursions) that has led to the evolution of features such as gills for breathing and fins for locomotion. Many different groups are generally included in the fish, but two major subgroups can be identified. These are the cartilaginous fish (e.g. sharks and rays) and the bony fish (Actinopterygia), which include the teleosts, that account for the majority of fish species. Other important groups include the jawless fish (Agnatha) such as hagfish and lampreys, and lobe-finned fish (Sarcopterygii) such as coelocanths and lungfish (relatives of which evolved into tetrapods).
Despite the range of forms and behaviours, convergent evolution is rampant in this group. Perhaps the classic example is the striking similarity between tuna and the lamnid (and alopiid) sharks, which involve remarkably similar fusiform bodies, a special form of locomotion called thunniform swimming that employs a complex system of ligaments and endothermy ("warm-bloodedness") that has also evolved independently in various other fish (notably the marlin and swordfish). The tuna/lamnid convergence is only one of many examples of the strikingly parallel emergence of ecomorphs, i.e. fish with very similar shapes that have evolved in different parts of the world. Amongst the most notable instances are eel-like (angulliform) and pike morphologies, but many other examples are known, especially from freshwater assemblages. Cichlids in particular provide many fascinating examples.
In the marine realm, there is a whole series of fascinating convergences amongst the sharks and the related rays (collectively the elasmobranchs), in addition to the tuna/lamnid convergence already mentioned. In the former, one remarkable convergence involves reproduction, not only with the development of viviparity but also the evolution of a placenta. Nor is this convergence restricted to the sharks, but both viviparity and the placenta have evolved independently in many other groups of fish. In the jaws of rays, we find a surprising parallel to trabecular bones. Both sharks and rays are capable of electroreception, which has also independently evolved in bony fish (at least twice) and, more remarkably, monotreme mammals.
The repeated evolution of the swim bladder represents an interesting example of an anatomical convergence. This organ is not unique to fish but has also evolved in at least one species of cephalopod. In addition, the swim bladder has been repeatedly lost in fish, which has interesting implications relating to change in the developmental pathways responsible. The function of the swim bladder is dependent on the counter-current exchange system of the rete mirabile, which is itself rampantly convergent, and gas-proofing by guanine crystals that dramatically decrease the permeability of the swim bladder wall also find a very different role in the reflective surfaces of the eye/tapetum.
Many more fascinating convergences can be found among the fish, for example with respect to the lateral line system (analogues of which have evolved in e.g. cephalopods and crustaceans), the red/white muscle system (that is similar in squids) and feeding. Teeth have evolved at least twice and lungfish have evolved complex 'molariform' tooth plates analogous to those of certain reptiles and mammals. Some fish (e.g. damselfish) have independently evolved agriculture and maintain algal "gardens", reminiscent of those maintained by the marine mammalian herbivores, dugongs. Regarding locomotion, of course, fish principally swim, but some fish, such as the eel Moringua, have converged on burrowing forms and many groups of fish have actually learnt to walk. There is a striking example of a ray that has effectively evolved legs to engage in bipedal locomotion. In this context it is important to recall that the tetrapods evolved from the sarcopterygian fish in the Devonian, which is in itself a fascinating story of evolution, introducing further examples of convergence such as the independent evolution of fingers in a group known as the rhizodonts. In fish there is also evidence for laterality, social intelligence and personalities, so it is not surprising to find additional convergences in many aspects of behaviour, including socio-sexual systems (e.g. monogamy and parental care).