Several different groups of fish have evolved parental care.
Different groups of fish have evolved a number of modified scales to serve various functions
To place the gut bacteria of fish into a broader evolutionary context, we constructed a maximum likelihood phylogeny of all representative sequences along with their top BLAST hits. Consideration of clade membership in the light of habitat source (detailed fully in ) revealed that over half of the species from 17/25 fish libraries had closest phylogenetic affinity to bacteria from vertebrate guts (blue bars in corresponding to categories 1–5). Three of four herbivorous gut communities were unique in that most of their representative bacterial species belonged to lineages consisting of microbes from the guts of other vertebrates—namely birds and mammals (categories 1 and 2). Bacteria from these three fish hosts grouped into a diverse range of gut-associated clades (17, 21 and 27 different vertebrate-associated clades across A. nigricans, P. sexstriatus and N. tong-anus, respectively), most commonly within the Bacteroidia, Clostridia and Verrucomicrobia. Many of these consisted only of gut bacteria from one to three of these fish plus microbes from various herbivorous and omnivorous mammalian counterparts. Another common class of fish gut associates (category 6) showed closest phylogenetic affinity to microbes isolated from other animals (mostly insects and corals) and nongut tissues of vertebrates. Free-living bacteria with environmental lifestyles (red, orange and yellow bars in ), however, were comparatively rare among the closest relatives of fish gut bacteria. In fact, only 2/25 studied fish communities harboured a majority species with affinities to environmental microbes (categories 7–10).
Elasmobranchs are a closely related group of fishes, differing from bony fishes by having cartilaginous skeletons and five or more gill slits on each side of the head. In contrast, bony fishes have bony skeletons and a single gill cover. Elasmobranchs include sharks, sawfishes, rays, and skates.
Fish Groups :: Florida Museum of Natural History
Three Classes of Fishes - Course World
However, no zoogeographer who utilizes the evidence of diverse groups can be familiar at first hand with all of them, and the difficulty facing such workers is that of seeking out the really dependable evidence in those groups he does not know well. Aside from the difficulty of selecting dependable authorities or systematic works, the zoogeographer desiring to use the evidence of fresh-water fishes has another troublesome matter to contend with. This is the differing tolerance of salt-water exhibited by different groups of fresh-water fishes. As one example, and one which has frequently troubled zoogeographers, we may mention the Galaxiidae, fresh-water fishes of Southern South America, Africa, Australia, and New Zealand, whose distribution has been held by some to be evidence for continental drift or southern intercontinental land-bridges. Ichthyologists now know that these fishes are, as a group, salt-tolerant and possibly either anadromous or catadromous, and that they are not really strong evidence for continental connections simply because it seems possible that they may cross ocean barriers. Much information upon salt-tolerance of fishes is to be found in studies on osmotic regulation (see KROGH, 1939, for a summary and bibliography) but most of this work has comparatively little bearing directly on the reasons for the differing salt-tolerance of systematic groups of fishes. The most important of these studies, from the zoogeographical standpoint, has been the work of HOMER SMITH and his associates on the physiology of excretion and the morphology of the kidney in fishes (see especially SMITH, 1932). However, there has yet been no thorough physiological study of systematic groups of fresh-water fishes to determine why they are confined to fresh water.