Coelacanth

Coelacanths are an ancient lineage of lobe-finned fish belonging to the class Actinistia. As members of the Sarcopterygii, they share a closer evolutionary relationship with lungfish and tetrapods—the group that includes amphibians, reptiles, birds, and mammals—than with ray-finned fish. Today, only one living genus, *Latimeria*, survives, containing two described species.

The name "coelacanth" comes from the Permian genus *Coelacanthus*, the first scientifically named coelacanth genus, designated in 1839. This became the type genus for the order Coelacanthiformes as more species were identified. Coelacanths appear in both freshwater and marine fossils dating back to the Devonian period, over 410 million years ago, and were long believed to have gone extinct in the Late Cretaceous, roughly 66 million years ago.

The first living species, *Latimeria chalumnae*—the West Indian Ocean coelacanth—was described from specimens caught off the coast of South Africa starting in 1938. These fish are now also known to live around the Comoro Islands near eastern Africa. A second species, *Latimeria menadoensis* (the Indonesian coelacanth), was discovered in the late 1990s in the seas of eastern Indonesia, from Manado to Papua.

The coelacanth (specifically the living genus *Latimeria*) is often called a "living fossil" in popular science, as it was thought to be the sole surviving member of a group otherwise known only from fossils—a biological relict—having maintained a similar body plan for about 400 million years. However, studies of fossil coelacanths reveal that their body shapes and ecological roles were far more diverse than previously believed, and often differed greatly from *Latimeria*.

**Etymology** The word "coelacanth" comes from the Modern Latin *Cœlacanthus*, meaning "hollow spine," derived from the Ancient Greek *koilos* ("hollow") and *akantha* ("spine"). This refers to the hollow fin rays in the tail of the first fossil specimen, described and named by Louis Agassiz in 1839 for the genus *Coelacanthus*. The genus name *Latimeria* honors Marjorie Courtenay-Latimer, who discovered the first living specimen.

**Discovery** The earliest coelacanth fossils were found in the 19th century. They were thought to have died out at the end of the Cretaceous. Because they are more closely related to tetrapods than to ray-finned fish, coelacanths were seen as a transitional form between fish and land vertebrates. On December 22, 1938, the first *Latimeria* specimen was caught off the east coast of South Africa, near the Chalumna River (now Tyolomnqa). Museum curator Marjorie Courtenay-Latimer spotted the fish among a local fisherman's catch. She contacted ichthyologist J. L. B. Smith at Rhodes University, sending him drawings, and he confirmed its importance with a famous cable: "Most Important Preserve Skeleton and Gills = Fish Described." Its reappearance over 60 million years after its supposed extinction makes the coelacanth the best-known example of a Lazarus taxon—a lineage that vanishes from the fossil record only to reappear much later. Since 1938, West Indian Ocean coelacanths have been found in the Comoros, Kenya, Tanzania, Mozambique, Madagascar, iSimangaliso Wetland Park, and off the south coast of KwaZulu-Natal in South Africa. A Comoro Islands specimen was discovered in December 1952. Between 1938 and 1975, 84 specimens were caught and recorded. The second living species, the Indonesian coelacanth, was first recognized in Manado, North Sulawesi, by Mark V. Erdmann and his wife Arnaz Mehta at a local fish market in September 1997, though they could only photograph the first specimen before it was sold. After confirming its uniqueness, Erdmann returned to Sulawesi in November 1997 to interview fishermen and search for more. A second specimen was caught by a fisherman in July 1998 and handed to Erdmann. The species was described in 1999 by Pouyaud et al., based on Erdmann's 1998 specimen, and deposited at a facility of the Indonesian Institute of Sciences (LIPI).

**Distribution** During the Paleozoic and Mesozoic eras, coelacanths were found worldwide, with fossils on every continent except Antarctica. The two living *Latimeria* species—the West Indian Ocean coelacanth and the Indonesian coelacanth—are limited to the southern and eastern coasts of Africa and northern Indonesia, respectively.

**Description** Coelacanths belong to Sarcopterygii, the lobe-finned fishes, which also includes lungfish and tetrapods, all of which have lobed fins rather than rayed fins. Externally, several features set coelacanths apart from other lobe-finned fish: they have eight fins—two dorsal, two pectoral, two pelvic, one anal, and one caudal fin. The tail is nearly symmetrical and split by a terminal tuft of fin rays forming its caudal lobe, a structure called a trilobate fin (three-lobed) or a diphycercal tail. A secondary tail extends past the primary one, separating the upper and lower halves. The pectoral fins are lobed and relatively long. All coelacanths share several traits: a well-developed intracranial joint in the skull (which may improve bite force), a rostral organ at the front of the head (unique to coelacanths and used for electroreception), a pair of external nostrils, a single bone called the lachrymojugal beneath the eyes, an upright jaw suspension, and a triangular palate on the underside of the skull. Each half of the skull and lower jaw has two separate jointed points of articulation.