Agnatha (;[Shorter Oxford English Dictionary] ) or jawless fish is a paraphyletic infraphylum
Sequencing, both from rRNA
The oldest fossil agnathans appeared in the Cambrian. Living jawless fish comprise about 120 species in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular
Metabolism
Agnathans are
ectothermic, meaning they do not regulate their own body temperature. Agnathan 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. Lampreys feed on carrion, as well as other fish and marine mammals, although some species are non-carnivorous.
fluids preventing blood clotting are injected into the host, causing the host to yield more blood. Hagfish are scavengers, eating mostly dead animals, although they have also been observed
predation.
They use a row of sharp teeth to break down the animal. Because agnathan teeth are unable to move up and down it limits their possible food types.
Morphology
In addition to the absence of
Fish jaw, modern agnathans are characterised by absence of paired
Fish fin; the presence of a
notochord both in larvae and adults; and seven or more paired
gill pouches. Lampreys have a light sensitive pineal eye (homologous to the
pineal gland in
). All living and most extinct Agnatha do not have an identifiable
stomach or any
appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are
ectothermic or cold-blooded, with a
Cartilage skeleton, and the
heart contains 2 chambers.
Body covering
In modern agnathans, the body is covered in skin, with neither dermal or epidermal scales. The skin of
hagfish has copious slime glands, the slime constituting their defense mechanism. The slime can sometimes clog up enemy fishes' gills, causing them to die. In direct contrast, many extinct agnathans sported extensive exoskeletons composed of either massive, heavy dermal armour or small mineralized scales.
Appendages
Almost all agnathans, including all
extant taxon, have no paired appendages, although most do have a dorsal or a
caudal fin. Some fossil agnathans, such as
and
pituriaspida, did have paired fins, a trait inherited in their
Gnathostomata.
[Alfred Romer. & Parsons, T.S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia.]
Reproduction
Fertilization in lampreys is external. Mode of fertilization in hagfishes is not known. Development in both groups probably is external. There is no known parental care. Not much is known about the hagfish reproductive process. It is believed that hagfish only have 30 eggs over a lifetime.
There is very little of the larval stage that characterizes the lamprey. Lamprey are only able to reproduce once. Lampreys 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.
Evolution
Although a minor element of modern marine fauna, agnathans were prominent among the early fish in the early
Paleozoic. Two types of Early
Cambrian animal apparently having fins,
vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of
China:
Haikouichthys and
Myllokunmingia. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathan from the same region is
Haikouella. A possible agnathan that has not been formally described was reported by Simonetti from the Middle Cambrian
Burgess Shale of
British Columbia.
, a class of agnathans which arose in the early Cambrian,
remained common enough until their extinction in the
Triassic that their teeth (the only parts of them that were usually fossilized) are often used as
from the late Cambrian to the Triassic.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the , precursors to the bony fish and hence to the tetrapods (including )—are known from the middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as , , and , were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including .
Approximately 500 million years ago, two types of recombinatorial adaptive immune systems (AISs) arose in vertebrates. The jawed vertebrates diversify their repertoire of immunoglobulin domain-based T and B cell antigen receptors mainly through the rearrangement of V(D)J gene segments and somatic hypermutation, but none of the fundamental AIS recognition elements in jawed vertebrates have been found in jawless vertebrates. Instead, the AIS of jawless vertebrates is based on variable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences.
Classification
| + Subgroups of jawless fish |
|
|
|
|
|
|
|
|
|
Phylogeny
Phylogeny based on the work of Mikko Haaramo and Delsuc et al.
While the "Agnatha" Conodonta was indeed jawless, if it would have continued to live, its descendants would still be closer related to e.g. humans than to lampreys, and also contemporary it was closer related to the ancestor of humans. Due to such considerations, Agnatha can not be consolidated into a coherent grouping without either removing any non-Cyclostomata, or by including all Vertebrata thus rendering it into a junior synonym of Vertebrata.
The new phylogeny from Miyashita et al. (2019) is considered compatible with both morphological and molecular evidence.
See also
