Agnatha

Agnatha (Greek,[3] ἀ-γνάθος "no jaws") is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. The group is sister to all vertebrates with jaws, known as gnathostomes.[4]

Recent molecular data, both from rRNA[5] and from mtDNA[6] as well as embryological data[7] strongly supports the hypothesis that living agnathans, the cyclostomes, are monophyletic.[8]

The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, comprising 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[5][6][9] and developmental[10] data, the group Craniata was created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fins; 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 mammals). 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 cartilaginous skeleton, and the heart contains 2 chambers.

While a few scientists still regard the living agnathans as only superficially similar, and argue that many of these similarities are probably shared basal characteristics of ancient vertebrates, recent classification clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.

Agnatha
Temporal range: 535–0 Ma[1][2]
"Lampetra fluviatilis"
Lampetra fluviatilis
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Agnatha
Cope, 1889
Groups included
Cladistically included but traditionally excluded taxa

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 homogenous throughout its length. Lampreys feed on other fish and mammals. Anticoagulant fluids preventing blood clotting are injected into the host, causing the host to yield more blood. Hagfish are scavengers, eating mostly dead animals. They use a row of sharp teeth to break down the animal. The fact that Agnathan teeth are unable to move up and down limits their possible food types.

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 (see below).

Appendages

Almost all agnathans, including all extant agnathans, have no paired appendages, although most do have a dorsal or a caudal fin. Some fossil agnathans, such as osteostracans and pituriaspids, did have paired fins, a trait inherited in their jawed descendants.[11]

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.[12] Most species are hermaphrodites. There is very little of the larval stage that characterizes the lamprey. Lamprey are only able to reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. 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. They also have a certain unusual form of reproduction.

Evolution

Evolution of jawless fish
Evolution of jawless fishes. The diagram is based on Michael Benton, 2005.[13]

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 agnathid from the same region is Haikouella. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.

Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the Ostracoderms, precursors to the bony fish and hence to the tetrapods (including humans)—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 thelodonts, osteostracans, and galeaspids, 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 conodonts.[14] Agnathans declined in the Devonian and never recovered.

Classification

Subgroups of jawless fish
Subgroup Example Comments
Cyclostomes Myxini Pacific hagfish Myxine
hagfish
Myxini (hagfish) are eel-shaped slime-producing marine animals (occasionally called slime eels). They are the only known living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils; hagfish are basal to vertebrates, and living hagfish remain similar to hagfish 300 million years ago.[15] The classification of hagfish has been controversial. The issue is whether the hagfish is itself a degenerate type of vertebrate-fish (most closely related to lampreys), or else may represent a stage which precedes the evolution of the vertebral column (as do lancelets). The original scheme groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving clade of vertebrates alongside gnathostomes (the now-ubiquitous jawed-vertebrates). An alternative scheme proposed that jawed-vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduces the category craniata to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme.[8]
Hyperoartia Eudontomyzon mariae Dunai ingola
lamprey
Hyperoartia is a disputed group of vertebrates that includes the modern lampreys and their fossil relatives. Examples of hyperoartians from early in their fossil record are Endeiolepis and Euphanerops, fish-like animals with hypocercal tails that lived during the Late Devonian Period. Some paleontologists still place these forms among the "ostracoderms" (jawless armored "fishes") of the class Anaspida, but this is increasingly considered an artificial arrangement based on ancestral traits. Placement of this group among the jawless vertebrates is a matter of dispute. While today enough fossil diversity is known to make a close relationship among the "ostracoderms" unlikely, this has muddied the issue of the Hyperoartia's closest relatives. Traditionally the group was placed in a superclass Cyclostomata together with the Myxini (hagfishes). More recently, it has been proposed that the Myxini are more basal among the skull-bearing chordates, while the Hyperoartia are retained among vertebrates. But even though this may be correct, the lampreys represent one of the oldest divergences of the vertebrate lineage, and whether they are better united with some "ostracoderms" in the Cephalaspidomorphi, or not closer to these than to e.g. to other "ostracoderms" of the Pteraspidomorphi, or even the long-extinct conodonts, is still to be resolved. Even the very existence of the Hyperoartia is disputed, with some analyses favoring a treatment of the "basal Hyperoartia" as a monophyletic lineage Jamoytiiformes that may in fact be very close to the ancestral jawed vertebrates.
Ostracoderms Pteraspidomorphi
(extinct)
Larnovaspis stensioei
Pteraspidomorphi is an extinct group of early jawless fish. The fossils show extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water.

The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida.

Thelodonti
(extinct)
Thelodonti Thelodonti (nipple teeth) are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids[16]) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group,[17] although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.[18]:206 "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.[19][20] The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They occupied a large variety of ecological niches, with a large amount of species preferring reef ecosystems, where their flexible bodies were more at ease than the heavily armoured bulks of other jawless fish.[21]
Anaspida
(extinct)
Anaspida Anaspida (without shield) is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods.[22] They are classically regarded as the ancestors of lampreys.[23] Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian, and flourished until the Late Devonian extinction,[24] where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Cephalaspido-
morphi

(extinct)
Cephalaspis Lyellii Cephalaspidomorphi is a broad group of extinct armored agnathans found in Silurian and Devonian strata of North America, Europe, and China, and is named in reference to the osteostracan genus Cephalaspis. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, as lampreys are sometimes thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the early Silurian period through the Mesozoic, and into the present day. Cephalaspidomorphi were, like most contemporary fish, very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of the innards. The body was in most forms well armoured as well. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed on the top of the head. There was no proper jaw. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite.[25] Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina," include the classes Osteostraci, Galeaspida, and Pituriaspida

Groups

Phylogeny based on the work of Mikko Haaramo and Delsuc et al.[26][27]

Vertebrata/
Agnatha

Hyperotreti/Myxini (hagfishes)

Petromyzontomorpha (lampreys)

Cyclostomata

?†Conodonta

Pteraspidomorpha

Myopterygii
Anaspidomorphi

?†Jamoytiiformes

?†Euphanerida

Anaspida

Thelodonti

Cephalaspidomorpha

Galeaspida

Pituriaspida

Osteostraci

Gnathostomata

Craniata

See also

References

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Adaptive immunity in jawless fish

Jawless vertebrates, which today consist entirely of lampreys and hagfish, have an adaptive immune system remarkable similar to that found in jawed vertebrates. The cells of the agnathan AIS have roles roughly equivalent to those of B-cells and T-cells. At first, two lymphocyte lineages—VLRA and VLRB—were identified, while a third, VLRC, was discovered in 2013:

VLRA (most similar to α/β T cells)

VLRB (most similar to B cells)

VLRC (most similar to γ/δ T cells)

Amphirhina

Amphirhina are animals, a phylogenetic classification within the subphylum vertebrata. They are more commonly known as the Branch Gnathostomata, and are described as having double nasal chambers, or nostrils, and jaws. The parallel branch in this naming system is Monorhina (more commonly Agnatha), which possess a single nostril and a circular mouth without jaws. The ears of all animals within Amphirhina possess three semicircular canals.

Areyongalepis

Areyongalepis is a genus of prehistoric jawless fish that lived during the Ordovician period.

The genus was originally named Areyonga by Australian palaeontologist Gavin C. Young, but it turned out that the name had been preoccupied by Areyonga, a modern genus of wasp. Young therefore renamed the fossil fish Areyongalepis in 2000.

Craniate

A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the much more numerous Gnathostomata (jawed vertebrates).The clade was conceived largely on the basis of the Hyperoartia (lampreys and kin) being more closely related to the Gnathostomata (jawed vertebrates) than the Myxini (hagfishes). This, combined with an apparent lack of vertebral elements within the Myxini, suggested that the Myxini were descended from a more ancient lineage than the vertebrates, and that the skull developed before the vertebral column. The clade was thus composed of the Myxini and the vertebrates, and any extinct chordates with skulls.

However recent studies using molecular phylogenetics has contradicted this view, with evidence that the Cyclostomata (Hyperoartia and Myxini) is monophyletic; this suggests that the Myxini are degenerate vertebrates, and therefore the vertebrates and craniates are cladistically equivalent, at least for the living representatives. The placement of the Myxini within the vertebrates has been further strengthened by recent anatomical analysis, with vestiges of a vertebral column being discovered in the Myxini.

Cyathaspididae

Cyathaspididae is an extinct family in the heterostracan order Cyathaspidiformes.Cyathaspididae contains most of the genera originally contained within Cyathaspididae, as well as those genera contained within Irregularaspididae, and Poraspididae. In addition to the type genus, †Cyathaspis, Cyathaspididae contains the following genera: †Americaspis, †Archegonaspis, †Capitaspis, †Dikenaspis, †Dinaspidella, †Homaspidella, †Irregulareaspis, †Nahanniaspis, †Pionaspis, †Poraspis, †Ptomaspis, †Seretaspis, †Steinaspis, †Torpedaspis and †Veronaspis.

Cyclostomata

Cyclostomata is a group of agnathans that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth, and branchial arches that are internally positioned instead of external as in jawed fishes. The name Cyclostomata means "round mouths".It was named by Joan Crockford Beattie

Ichthyology

Ichthyology (from Greek: ἰχθύς, ikhthys, "fish"; and λόγος, logos, "study"), also known as fish science, is the branch of zoology devoted to the study of fish. This includes bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). While a large number of species have been discovered, approximately 250 new species are officially described by science each year. According to FishBase, 33,400 species of fish had been described by October 2016.

Lampricide

A lampricide is any chemical designed to target the larvae of lampreys in river systems before they develop into parasitic adults. One lampricide is used in the headwaters of Lake Champlain and the Great Lakes to control the sea lamprey (Petromyzon marinus), an invasive species to these lakes.TFM (3-trifluoromethyl-4-nitrophenol) is the main chemical used for this purpose. As it is hydrophobic, it passes through biological membranes.

TFM is a metabolic uncoupler—that is, TFM separates the electron transport chain from ATP synthesis, resulting in the failure of the aerobic respiration process. It accomplishes this by disrupting the electrochemical gradient that powers ATP synthase—as an acid, it donates H+ ions to the mitochondrial matrix. The electron transport chain is not affected and continues using oxygen, without producing ATP.

While the general opinion is that TFM typically does not harm other fish (due to the relationship between true fish and lampreys), lampricide can be problematic for many amphibians, such as mudpuppies (genus Necturus) which often share the same habitats. Also, some more "primitive" species of fish, such as the sturgeon in the Great Lakes are sensitive to chemicals such as TFM.

List of animal classes

The following is a list of the classes in each phylum of the kingdom Animalia. There are 107 classes of animals in 33 phyla in this list. However, different sources give different numbers of classes and phyla. For e.g, Protura, Diplura, and Collembola are often considered to be the three orders in the class Entognatha. This list should by no means be considered complete and authoritative and should be used carefully.

List of chordate orders

This page contains a list of all of the classes and orders that are located in the Phylum Chordata.

Lists of organisms by population

This is a collection of lists of organisms by their population. While most of the numbers are estimates, they have been made by the experts in their fields. Species population is a science falling under the purview of population ecology and biogeography. Individuals are counted by census, as carried out for the piping plover; using the transect method, as done for the mountain plover; and beginning in 2012 by satellite, with the emperor penguin being first subject counted in this manner.

Longodus

Longodus is an extinct genus of thelodont, placed in its own family – Longopdidae – which existed in what is now Estonia during the Ludlow epoch of the upper Silurian period. The type and only species is Longodus acicularis.

Metaspriggina

Metaspriggina is a genus of chordate initially known from two specimens in the Middle Cambrian Burgess Shale and 44 specimens found in 2012 at the Marble Canyon bed in Kootenay National Park.

Whilst named after the Ediacaran organism Spriggina, later work has shown the two to be unrelated. Metaspriggina is considered to represent a primitive chordate, possibly transitional between cephalochordates and the earliest vertebrates, albeit this has been questioned because it seems to possess most of the characteristics attributed to craniates. It lacked fins and it had a weakly developed cranium, but it did possess two well-developed upward-facing eyes with nostrils behind them.

Metaspriggina also possessed a notochord, along with seven pairs of pharyngeal bars, possibly made of cartilage. Surprisingly they were not formed from a singular bone, but they were formed of multiple separate pairs of bones, along with first two of them that were enlarged compared to the others and that seemed to not support any gills, all of these characteristics suggesting a "distant link to gnathostomatans". The largest specimens are 10 centimetres (3.9 in) in length. Originally believed to be free-swimming but occasionally found on the sea floor, the fossils from Marble Canyon showing the presence of eyes and their placement suggests it lived as a filter-feeder swimming above the sea floor.

The exceptional preservation at Marble Canyon also preserved muscle detail, showing that the animal moved with a side-to-side swimming motion. In Metaspriggina the myomeral configuration has an additional ventral chevron, and a clear dorsal bend which defines a W-shaped arrangement that is directly comparable to fish.The discovery of pharyngeal bars (gill bars) makes Metaspriggina the oldest known animal to have this feature. The first pair of pharyngeal bars later evolved to form the upper and lower jaws of vertebrates. The second pair evolved to form the hyoid arch. In vertebrates this supports the jaws and the hyoid bone anchors the base of the tongue.

The discovery of Metaspriggina makes the origins of gnathostomatans a little more confusing, as it was roughly contemporary with Pikaia. As Pikaia did not have gill bars, unlike Branchiostoma, there are two possible explanations for this. One is that the Chordate phylum split in four before Metaspriggina lived, with Metaspriggina and the other craniates (both gnathostomatans and Agnatha) grouped with Branchiostoma and the cephalochordates, and Pikaia out on a side branch. Metaspriggina is here a direct ancestor of all gnathostomatans, with the Agnatha the most closely related group. In this explanation, Pikaia is not a close relative of Craniates at all, nor of cephalochordates, but something even more primitive, and the defining feature of the craniate-cephalochordate group is their gill bars.

The other explanation is that Metaspriggina was the ancestor of all gnathostomatans, again closely related to the Agnatha to form the Chordata. However, instead of being a very primitive relative, Pikaia was the ancestor of all cephalochordates, and the gill bars evolved convergently in them somewhere between Pikaia and Branchiostoma. This might explain why Branchiostoma has such a different number of gill bars to chordates (or at least their embryos).

Neoptera

Neoptera is a classification group that includes most parts of the winged insects, specifically those that can flex their wings over their abdomens. This is in contrast with the more basal orders of winged insects (the "Palaeoptera" assemblage), which are unable to flex their wings in this way.

Ostracoderm

Ostracoderms (Ancient Greek, ὄστρακον+δέρμα "shell-skinned") are the armored jawless fishes of the Paleozoic. The term does not often appear in classifications today because it is paraphyletic or polyphyletic, and has no phylogenetic meaning. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.

An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gills used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow moving prey into their mouths.

The first fossil fishes that were discovered were ostracoderms. The Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeons but later realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms" which means "shell-skinned".Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth-like structures." Neil Shubin writes: "Cut the bone of the [ostracoderm] skull open…pop it under a microscope and…you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull--one of the earliest in the fossil record--is made entirely of little teeth. Teeth originally arose to bite creatures (see Conodonts); later a version of teeth was used in a new way to protect them."Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.

It was long assumed that pteraspidomorphs and thelodonts were the only ostracoderms with paired nostrils, while the other groups have just a single median nostril. But it has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs.After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates, however, that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years.The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.

Silver lamprey

The silver lamprey (Ichthyomyzon unicuspis) is a lamprey commonly found in the Northern and Central United States, as well as a large part of southern Canada. Its binomial name means "fish to suck" in Greek and "one point" in Latin. The silver lamprey is a member of the Class Agnatha, sometimes referred to as cyclostomes (round-mouths). Other common names include: Bloodsucker, Blue Lamprey, Hitch-hiker, Lamper, Lamprey Eel. The silver lamprey should not to be confused with the sea lamprey (Petromyzon marinus), which has caused considerable damage to native fish populations in the Great Lakes region.

TFM (piscicide)

TFM (3-trifluoromethyl-4-nitrophenol) is a common piscicide, i.e., a fish poison used to combat parasitic and invasive species of fish.The substance was discovered in 1958 when researching means to combat sea lampreys and it currently remains the primary lampricide (lamprey-killer) in the Great Lakes area.TFM toxicity has not been thoroughly investigated for humans, but is considered an irritant, respiratory irritant, and toxic by the manufacturer. Toxicity studies of other mammals have generally found it to be non-toxic at concentrations expected to be found in treated areas . Impact on other fish species may be controlled by selective application during the larvae season for lampreys and other management of its concentration. TFM does not accumulate, since it breaks down within several days.

Variable lymphocyte receptor

Variable lymphocyte receptors (VLRs) belong to the Leucine-rich repeat (LRR) family and mediate adaptive immune responses in the jawless vertebrates, lampreys and hagfish.

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