Coelacanth s a rare order of
fish that includes two extant species: West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis).
They follow the oldest known living lineage of Sarcopterygii (lobe-finned fish and tetrapods), which makes more closely related to lungfish, reptiles and mammals than to the common ray-finned fishes. They are found along the coastlines of the Indian Ocean and Indonesia.
Both are threatened, and the West Indian Ocean coelacanth is a critically endangered species.
Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish that are related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms, rhizodonts, and Panderichthys.
The coelacanths, which are related to lungfishes and tetrapods, were believed to have been extinct since the end of the Cretaceous period. More closely related to tetrapods than even the ray-finned fish, coelacanths were considered to be transitional species between fish and tetrapods. The first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River (now Tyolomnqa) in 1938.
Coelacanths are a part of the clade Sarcopterygii, or the lobe-finned fishes. Externally, there are several characteristics that distinguish the coelacanth from other lobe-finned fish. They possess a three-lobed caudal fin, also called a trilobate fin or a diphycercal tail. A secondary tail that goes along and extends past the primary tail separates the upper and lower halves of the coelacanth. Cosmoid scales act as thick armor that protects the exterior of the coelacanth.
Structure Spotted Handfish
There also are several internal traits that aid in differentiating coelacanths from other lobe-finned fish. At the back of the skull, the coelacanth possesses a hinge, the intracranial joint, which allows it to open its mouth extremely wide. Coelacanths also retain a notochord, a hollow, pressurized tube which is replaced by the vertebral column early in embryonic development in most other vertebrates.
The heart of the coelacanth is shaped differently than most modern fish; the heart's chambers are arranged in a straight tube. The coelacanth braincase is 98.5% filled with fat; only 1.5% of the braincase contains brain tissue. The cheeks of the coelacanths are unique because the opercular bone is very small and holds a large soft-tissue opercular flap. The coelacanth also contains a unique rostral organ within the ethmoid region of the braincase.