Agnatha

Idealized bauplan of the Agnatha.

Agnatha is a chordate taxonomic group comprising the jawless fish and generally is placed as a superclass within in the subphylum Vertebrata. Agnathans have existed since the Cambrian and continue to be present now, with two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish.

Characterized by the absence of jaws derived from gill arches, agnathans are differentiated from the gnathostomes (superclass Gnathostomata), or "jawed vertebrates" (fish with hinged jaws, amphibians, reptiles, birds, and mammals). However, Agnatha generally is considered to be a paraphyletic grouping (Purnell et al. 2001; Nelson 1994).

Overview and classification

Vertebrates (subphylum Vertebrata) are generally classified into two groups, the Agnatha (jawless vertebrates) and the Gnathostomata (jawed vertebrates). The later group includes fish with hinged jaws and the tetrapods (amphibians, reptiles, birds, and mammals). Agnatha (Greek, "no jaws") includes the modern day lampreys (Petromyzontiformes) and hagfish (Myxiniformes) as well as several extinct orders.

In typical biological classifications, Agnatha and Gnathostomata are each considered a superclass of Vertebrata. However, there are different taxonomies, including ones in which Agnatha is considered a class, or Gnathostomata is not recognized as a taxon (ITIS 2001), or Agnatha is not recognized (Janvier 1981).

Hagfish, while generally classified in Agnatha and in the subphylum Vertebrata, actually lack vertebrae. For this reason, they sometimes are separated from the vertebrates. Janvier (1981) and others, for example, put hagfish as another subphylum, Myxini, along with the subphylum Vertebrata, under Craniata, recognizing these groups possessing a cranium (Janvier 1981). Others, however, use the terms Vertebrata and Craniata as synonyms, rather than different levels of classification, and retain the use of Agnatha (Nelson 1994).

The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage.

Agnatha generally is considered to by paraphyletic. Similarities between hagfish and lampreys appear to involve superficial or primitive characteristics that cannot support a hypothesis for a group composed of only hagfish and lampreys as a monophyletic group (Nelson 1994). On the other hand, there are a number of morphological and physiological similarities shared between lampreys and gnathostomes, but not hagfishes, that appear to be due to common ancestry, such as vertebral elements, highly differentiated kidney tubules, more than one semicircular canal, large exocrine pancreas, and so forth (Nelson 1994). Yalden (1985), however, does present an argument based on feeding mechanisms that hagfishes and lampreys do constitute a monophyletic grouping.

Description

For this reason, hagfish, which are also commonly known as "slime eels," are sometimes not considered to be fish. The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage. Hagfish are a staple food in Korea. They are classified in the order Myxini and the family Myxinidae. Both hagfish and lamprey have slimy skin without scales or plates. They also have a notochord that remains throughout life; circular, jawless mouths; and unpaired fins. Hagfish are found in the oceans and lampreys are found in both freshwater and ocean environments. Most lampreys are parasitic.

In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. There is 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, with a cartilaginous skeleton, and the heart contains 2 chambers.

Although they are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates, and modern classifications tend to place hagfish into a separate group (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) being more closely related to the jawed fishes.

Respiratory System

Agnathans are characterized by seven or more pairs of gill pouches. The bronchial arches supporting the gill pouches lie close to the body surface.

Metabolism

Agnathans are ectothermic or cold blooded, meaning they do not have to warm themselves through eating. Therefore, Agnathan metabolism is slow as well as the fact that Agnathans do not have to eat as much. They have no stomach.

Body covering

The only modern Agnathan body covering is skin. There are no scales. Extinct Agnathans had thick body plates (see below).

Appendages

Agnathans have no paired appendages, although they do have a tail and a caudal fin.

Skeleton

The internal skeleton of the Agnatha is not bony but rather cartilaginous (made up of dense connective tissue). Also, Agnathans have a notochord their whole life, a characteristic distinctive of the class. This notochord is the first primitive vertebral column.

Reproduction

Fertilization is external, as is development. There is no parental care.

Fossil agnathans

Haikouichthys is a fossil agnathan.

Cephalaspis is another fossil agnathan.

Although a minor element of modern marine fauna, Agnatha 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. Agnathans declined in the Devonian and never recovered.

Groups

• Myxini (hagfish)
• Hyperoartia
  • Petromyzontidae (lampreys)
• Pteraspidomorphi
• Thelodonti
• Anaspida
• Cephalaspidomorphi
  • Galeaspida
  • Pituriaspida
  • Osteostraci

References

ISBN links support NWE through referral fees

• Integrated Taxonomic Information System (ITIS). 2003. Agnatha ITIS Taxonomic Serial No.: 159693. Retrieved May 31, 2008.

• Integrated Taxonomic Information System (ITIS). 2001. Vertebrata ITIS Taxonomic Serial No.: 331030. Retrieved May 31, 2008.

• Janvier, P. 1981. The phylogeny of the Craniata, with particular reference to the significance of fossil "agnathans." J. Vertebr. Paleont. 1(2):121-159.

• Nelson, J. S. 1994. Fishes of the World, 3rd ed. New York: John Wiley & Sons. ISBN 0471547131.

last=Purnell|first=M. A.|authorlink=|editor=Derek E. G. Briggs and Peter R. Crowther|year=2001|title=Palaeobiology II|publisher=Blackwell Publishing|location=Oxford|isbn=0-632-05149-3|page=p401}

• Yalden, D. W. 1985. Feeding mechanisms as evidence for cyclostome monophyly. Zool. J. Linn Soc. 84:291-300.

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