A chordate is an animal of the phylum Chordata. All chordates possess 5 synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other taxa. These 5 synapomorphies include a notochord, dorsal hollow nerve cord, endostyle or thyroid, pharyngeal slits, and a post-anal tail. Chordates get their name from their characteristic “notochord”, which plays a significant role in chordate structure and movement. Chordates are also bilaterally symmetric, have a coelom, possess a circulatory system, and exhibit metameric segmentation.
Vertebrates comprise all species of animals within the subphylum Vertebrata. Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. Vertebrates comprise such groups as the following:
Hagfish, of the class Myxini and order Myxiniformes, are eel-shaped, slime-producing marine fish. They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae. Along with lampreys, hagfish are jawless; they are the sister group to jawed vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago.
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct species. The group is sister to all vertebrates with jaws, known as gnathostomes.
A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish, making living craniates synonymous with living vertebrates.
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.
The Gulf hagfish is the only known species with the ability to enter brine pools, or pools of extremely high salt content, unharmed. These pools resemble lakes on the ocean floor. They exist in both the Gulf of Mexico and the Mediterranean Sea. The high salt content, approximately 200 ppt compared to 35 ppt for standard seawater, creates a buoyant surface which renders oceanic submersibles unable to descend into the pool. Often, other species can be seen floating lifelessly on the surface. It is believed that the pools only support bacterial life.
The broadgilled hagfish or New Zealand hagfish is a hagfish found around New Zealand and the Chatham Islands as well as around the south and east coasts of Australia, at depths between 1 and 900 metres.
The Pacific hagfish is a species of hagfish. It lives in the mesopelagic to abyssal Pacific ocean, near the ocean floor. It is a jawless fish and has a body plan that resembles early paleozoic fish. They are able to excrete prodigious amounts of slime in self-defense.
Eptatretus is a large genus of hagfish.
The Caribbean hagfish is a species of hagfish. It is a scaleless, eel-like fish found in Caribbean waters that feeds off material from the surface that drifts down. It is rarely seen as it lives in very deep water from 2,300-4,950 ft and likes to burrow into the mud. Their bodies are grey with contrasting white heads. They have seven internal gills connected to a single opening on each side of the body.
Eels are ray-finned fish belonging to the order Anguilliformes, which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage, and most are predators.
The inshore hagfish is a hagfish found in the Northwest Pacific, from the Sea of Korea and across eastern Japan to Taiwan. It has six pairs of gill pouches and gill apertures. These hagfish are found in the sublittoral zone. They live usually buried in the bottom mud and migrate into deeper water to spawn. The inshore hagfish is the only member of the Myxinidae family having a seasonal reproductive cycle.
Eptatretus bischoffii is a common hagfish of the genus Eptatretus. Its maximum length is 55 centimetres (22 in). It lives in a demersal, non-migratory, marine habitat with its depth range between 8–50 m. It can survive in only temperate zones. These organisms are found in the South Pacific, mainly, Chile. It is harmless to humans.
Myxine glutinosa, known as the Atlantic hagfish in North America, and often simply as the hagfish in Europe, is a species of jawless fish of the genus Myxine.
The black hagfish is a species of hagfish.
Eptatretus hexatrema, the sixgill hagfish or snotslang, is a species of marine fish in the hagfish family of order Myxiniformes. It is native to the South Atlantic Ocean and southwestern Indian Ocean.
Eptatretus polytrema, the fourteen-gill hagfish or Chilean hagfish, is a demersal and non-migratory hagfish of the genus Eptatretus. It is found in muddy and rocky bottoms of the southeastern area of the Pacific Ocean near the coast of Chile between Coquimbo and Puerto Montt, at depths between 10 and 350 m. This hagfish can reach a length of 93 cm. It is only known from a few specimens and has not been recorded since 1988.
The short beard codling is a species of marine bony fish in the family Moridae. Native to the western Atlantic Ocean, it is found on the continental slope at depths between 50 and 1,620 m.
