Atanystoma

From Coriapedia, the alien wikiAtanystoma ( 'without long mouths' ) is a superclass of jawless optifish in the phylum Cucumichordata, subphylum Thalassochordata, consisting of both present (discostomes) and extinct (coronatodonts and ostracichthyans) species. The group is sister to all thalassochordates with jaws, known as the tanystomes.

The oldest fossil atanystomes appeared in the Walesian, and two groups still survive today: the skinsuckers and the spooky optifish, comprising about 126 species in total. In addition to the absence of the Tullimonstrum/Opabinia style mouths, modern atanystomes are characterised by absence of paired fins; the presence of a 'protochord' both in larvae and adults; and 7 or more paired gill pouches. Skinsuckers have a light sensitive pineal eye (analogous to the Terran pineal eye in vertebrates). All living and most extinct Atanystoma do not have an identifiable stomach or any appendages. Fertilization and development are both external, reproducing by broadcast spawning and from eggs. There is largely no parental care in the Atanystoma class with the exception of the broodmother optifish. The atanystomes are ectothermic or cold-blooded, and the heart contains 3 chambers.

Metabolism
Atanystomes are ectothermic, meaning they do not regulate their own body temperature. Atanystome metabolism is slow in cold water, and therefore they do not have to eat very much. They have no distinct stomach, but rather a long gut, more or less homogeneous throughout its length. Skinsuckers feed on other optifish and galactognathans. Anticoagulant fluids preventing blood clotting are injected into the host, causing the host to yield more blood. Spooky optifish are scavengers, eating mostly dead archozoans. They use a row of sharp teeth to break down the animal. The fact that their teeth are unable to move up and down limits their possible food types.

Body covering
In modern atanystomes, the body is covered in skin, with neither dermal or epidermal scales. The skin of spooky optifish has copious and electricity-generating slime glands, the slime constituting their defense mechanism. The slime can either clog up enemy fishes' gills, causing them to die or shock the would-be predator to death. In direct contrast, many extinct species sported extensive exoskeletons composed of either massive, heavy dermal armour or small mineralized scales.

Appendages
Almost all atanystomes, including all extant atanystomes, have no paired appendages, although most do have a dorsal or a caudal fin. Some fossil atanystomes, such as ostracichthyans did have paired fins, a trait inherited in their jawed descendants.

Reproduction
Fertilization in skinsuckers is external. Mode of fertilization in spooky optifishes is not known. Development in both groups is external. There is very little to no parental care. Not much is known about the spooky optifish reproductive process. It is believed that spooky optifish only have 40 eggs over a lifetime. Most species are hermaphrodites. There is very little of the larval stage that characterizes the lamprey. Skinsuckers are only able to reproduce once. After external fertilization, the skinsucker's cloacas remain open, allowing a parasitic dhusia to enter their intestines, killing them. Skinsuckers reproduce in freshwater riverbeds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment. The resulting hatchlings go through four years of larval development before becoming adults. They also have a certain unusual form of reproduction.

Evolution
See also: Evolution of optifish

Although a minor element of modern marine fauna, atanystomes were prominent among the early optifish in the early Amphizoic. Two types of Early Walesian archozoans apparently having fins, spine musculature, and gills are known from the early Walesian in Coiana: Walesianichthys and Walesia.

Many Medusian, Poseidonian, and Novacorian atanystomes were armored with heavy bony-spiky plates. The first armored atanystomes—the Ostracichthyans, precursors to the derived tanystomes are known from the middle Medusian, and by the Late Poseidonian they had reached the highest point of their evolution. Most of the ostracichthyans were more closely related to the tanystomes than to the surviving atanystomes, known as discostomes. Discostomes apparently split from other atanystomes before the evolution of fins and 'bones', which are present in many fossil atanystomes, including coronatodonts. Atanystomes declined in the Novacorian and never recovered.