| Anamniotes | |
|---|---|
 | |
| Trout spawning showing typical anamniote external fertilization | |
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| Anamniotes have a distinctlarval stage, such as in thesmooth newt. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Vertebrata |
| Informal group: | Ichthyopsida Huxley, 1863 |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
Theanamniotes are aninformal group ofcraniates comprising allfish andamphibians, which lay theireggs inaquatic environments. They are distinguished from theamniotes (reptiles,birds andmammals), which can reproduce ondry land either by layingshelled eggs or by carryingfertilized eggswithin the female. Older sources, particularly before the 20th century, may refer to anamniotes as "lower vertebrates" and amniotes as "higher vertebrates", based on the antiquated idea of theevolutionary great chain of being.
The name "anamniote" is aback-formation word created by adding the prefixan- to the wordamniote, which in turn refers to theamnion, anextraembryonic membrane present during the amniotes'embryonic development which serves as a biochemical barrier that shields theembryo from environmental fluctuations by regulating theoxygen,carbon dioxide andmetabolic waste exchanges and secretinga cushioning fluid. As the name suggests, anamniote embryos lack an amnion during embryonic development, and therefore rely on the presence of external water to provide oxygen and help dilute and excrete waste products (particularlyammonia) viadiffusion in order for the embryo to complete development without being intoxicated by their own metabolites.[1] This means anamniotes are almost always dependent on an aqueous (or at least very moist) environment forreproduction[2] and are thus restricted tospawning in or nearwater bodies. They are also highly sensitive to chemical and temperature variation in the surrounding water, and are also more vulnerable toegg predation andparasitism.
During theirlife cycle, all anamnioteclasses pass through a completely aquatic egg stage, as well as an aquaticlarval stage during which allhatchlings aregill-dependent and morphologically resemble tinyfinless fish (known as afry or atadpole for fish and amphibians, respectively), beforemetamorphosizing intojuvenile andadult forms (which might beaquatic,semiaquatic or eventerrestrial), thus indicating theirphysiologicalhomology.
The group is characterized by retaining the primitive vertebrate condition in several traits:[3][4]
The features unifying the anamniotes was first noted byThomas Henry Huxley in 1863, who coined the phraseIchtioid orIchthyopsida ("fish-face") for the group.[5][failed verification] It is ataxonomic classification just below the level ofVertebrata, though Huxley presented the Ichthyopsida as an informal unit and never ventured to forward aLinnaean rank for the group. The term Ichthyopsida meansfish-face orfish-like as opposed to theSauropsida orlizard-face animals (reptiles andbirds) and themammals.[6]The group representing anevolutionary grade rather than aclade, the term anamniote is now used as an informal way of denoting the physical property of the group, rather than as a systematic unit.