Coelacanth

Coelacanths (, adaptation of Modern Latin Cœlacanthus "hollow spine", from Greek κοῖλ-ος koilos "hollow" + ἄκανθ-α akantha "spine", referring to the hollow caudal fin rays of the first fossil specimen described and named by Agassiz in 1839 ) are members of an order of fish that includes the oldest living lineage of Sarcopterygii (lobe-finned fish + tetrapods) known to date.

Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish that are related to lungfish and other extinct Devonian fish like osteolepiforms, porolepiforms, rhizodonts, and Panderichthys. Coelacanths were thought to have gone extinct in the Late Cretaceous, but were rediscovered in 1938 off the coast of South Africa. Latimeria chalumnae and the Latimeria menadoensis are the only two living coelacanth species and are found along the coastlines of the Indian Ocean. The coelacanth has been nicknamed a “living fossil”, because its fossils were found long before the actual discovery of a live specimen. The coelacanth is thought to have first evolved approximately 400 million years ago.

Discovery
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 the "missing link" between the fish and the tetrapods until the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River (now Tyalomnqa) in 1938. Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local fisher, Captain Hendrick Goosen, on December 23, 1938. A local chemistry professor, JLB Smith, confirmed the fish's importance with a famous cable: "MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED".

The discovery of a species still living, when they were believed to have gone extinct 65 million years ago, makes the coelacanth the most well-known example of a Lazarus taxon, a species that seems to have disappeared from the fossil record only to reappear much later. Since 1938, Latimeria chalumnae have been found in the Comoros, Kenya, Tanzania, Mozambique, Madagascar, and in iSimangaliso Wetland Park, Kwazulu-Natal in South Africa.

The second extant species, L. menadoensis, was described from Manado Sulawesi, Indonesia in 1999 by Pouyaud et al. based on a specimen discovered by Erdmann in 1998 and deposited at the Indonesian Institute of Sciences (LIPI). The first specimen of this species was only photographed at a local market by Arnaz and Mark Erdmann before being bought by a shopper.

The coelacanth has no real commercial value, apart from being coveted by museums and private collectors. As a food fish the coelacanth is almost worthless, as its tissues exude oils that give the flesh a foul flavour. The continued survivability of the coelacanth may be at threat due to commercial deep-sea trawling.

Diagnosis
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. 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 a thick armor that protects the exterior of the coelacanth. There are also 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 gape its mouth extremely wide. Along with the hinge, the coelacanth contains a hollow backbone, which is unique to coelacanths. The heart of the coelacanth is shaped differently than most modern fish and its structure is that of a straight tube. The coelacanth braincase is 98.5% filled with fat; only 1.5% of the braincase actually contains any brain. The cheeks of the coelacanths are unique because the opercular bone is very small and holds a large soft-tissue opercular flap. The coelacanth contains a rostral organ within the ethmoid region of the braincase.

General description
Latimeria chalumnae and L. menadoensis are the only two known living coelacanth species. The word coelacanth literally means, “hollow spine,” because of its unique hollow spine fins. Coelacanths are large, plump, lobe-finned fish that grow up to 1.8 meters. They are nocturnal piscivorous drift-hunters. The body is covered in cosmoid scales that act as armor. Coelacanths have 8 fins - 2 dorsal fins, 2 pectoral fins, 2 pelvic fins, 1 anal fin and 1 caudal fin. The tail is very nearly equally proportioned and is split by a terminal tuft of fin rays that make up the caudal lobe of the tail. The eyes of the coelacanth are very large, while the mouth is very small. The eye is acclimatized to seeing in dark light by having rods that absorb mostly low wavelengths. Coelacanths' vision has evolved to a mainly blue-shifted color vision. Pseudomaxillary folds surround the mouth, which replaces the maxilla, which is absent in coelacanths. There are two nostrils along with four other external openings that appear between the premaxilla and lateral rostral bones. The nasal sacs resemble those of many other fish and do not contain an internal nostril. The rostral organ of the coelacanth is contained within the ethmoid region of the braincase. It has three unguarded openings into the environment. The rostral organ is used as a part of the coelacanths’ laterosensory system. The coelacanths’ auditory reception is mediated by its inner ear. The inner ear of the coelacanth is very similar to that of tetrapods because it is classified as being a basilar papilla.

Locomotion of the coelacanths is unique to their kind. To move around, coelacanths most commonly take advantage of up or downwellings of the current and drift. They use their paired fins to stabilize their movement through the water. While on the bottom of the ocean floor their paired fins are not used for any kind of movement. Coelacanths can create thrust for quick starts by using their caudal fins. Due to the high number of fins, the coelacanth has high maneuverability. Coelacanths can also orient their bodies in any direction in the water. They have been seen doing headstands and swimming belly up. It is thought that their rostral organ helps give the coelacanth electroperception, which aides in their movement around obstacles.

Taxonomy


Subclass Coelacanthimorpha (Actinistia) are sometimes used to designate the group of Sarcopterygian fish that contains the Coelacanthiformes. The following is a classification of known coelacanth genera and families:


 * Class Sarcopterygii (lobe-finned fishes and tetrapods)
 * Subclass Actinistia
 * Order Coelacanthiformes (Coelacanths)
 * Family Miguashaiidae (Devonian)
 * Miguashaia
 * Family Diplocercidae (Devonian and Lower Carboniferous)
 * Diplocercides (Nesides)
 * Family Hadronectoridae
 * Allenypterus
 * Hadronector
 * Polyosteorhynchus
 * Family Rhabdodermatidae (Primarily Carboniferous)
 * Caridosuctor
 * Rhabdoderma
 * Family Laugiidae (Lower Triassic to Lower Cretaceous)
 * Coccoderma
 * Laugia
 * Family Whiteiidae (Triassic)
 * Whiteia
 * Family Coelacanthidae
 * Axelia
 * Coelacanthus
 * Ticinepomis
 * Wimania
 * Family Mawsoniidae (Triassic and Jurassic)
 * Alcoveria
 * Axelrodichthys
 * Chinlea
 * Diplurus
 * Mawsonia
 * Family Latimeriidae Berg, 1940
 * Holophagus
 * Latimeria J. L. B. Smith, 1939
 * Latimeria chalumnae J. L. B. Smith, 1939 (Coelacanth)
 * Latimeria menadoensis Pouyaud, Wirjoatmodjo, Rachmatika, Tjakrawidjaja, Hadiaty & Hadie, 1999
 * Libys
 * Macropoma
 * Macropomoides
 * Megacoelacanthus
 * Undina

Fossil record
According to genetic analysis, the divergence of coelacanths, lungfish, and tetrapods is thought to have occurred 390 million years ago. Coelacanths were thought to have gone extinct 80 million years ago during the Cretaceous-Tertiary extinction. The first recorded coelacanth fossil was found in Australia and was of a coelacanth jaw that dated back 360 million years, named Eoachtinistia foreyi. The most recent species of coelacanth in the fossil record is the Macropoma. Macropoma, sister species to Latimeria chalumnae, is separated by 80 million years. The fossil record of the coelacanth is unique because coelacanth fossils were found 100 years before the first live specimen was identified. In 1938, Courtenay-Latimer rediscovered the first live specimen, L. chalumnae, which was caught off of the coast of East London, South Africa. In 1997, a marine biologist was on a honeymoon and discovered the second live species, Latimeria menadoensis in an Indonesian market. In July 1998, the first Latimeria menadoensis live specimen was caught in Indonesia. Around 80 total species have been described. Before the discovery of a live coelacanth specimen, the coelacanth time range was thought to have spanned from the Middle Devonian to the Upper Cretaceous period. During that time all fossils found, excluding one or two specimens, retained a similar morphology.

Geographical distribution
The current coelacanth range remains primarily around the eastern African coast, although the Latimeria menadoensis was discovered off the coast of Indonesia. Coelacanths have been found in the waters of Kenya, Tanzania, Mozambique, South Africa, Madagascar, Comoros, and Indonesia. Most Latimeria chalumnae specimens that have been caught have been captured around the islands of Grande Comore and Anjouan in the Comoros Archipelago, Indian Ocean. Though there have been cases of L. chalumnae being caught elsewhere, amino acid sequencing has shown there is no big difference between these exceptions and the ones found around Comore and Anjouan. Even though these few can be considered strays, there have been several reports of coelacanths being caught off of the coast of Madagascar. This leads scientists to believe that the endemic range of Latimeria chalumnae coelacanths actually stretches down the eastern coast of Africa from the Comoros Islands, past the western coast of Madagascar, to the South African coastline. The geographical range of the Indonesia coelacanth, Latimeria menadoensis, is believed to be off the coast of Manado Tua Island, Sulawesi, Indonesia in the Celebes Sea. The key components that keep coelacanths in these areas are food and temperature restrictions.

Ecology
During the daytime, coelacanths rest in caves anywhere from 100–500 meters deep while others migrate to deeper waters. By resting in cooler waters (below 120 meters) during the daytime, coelacanths reduce metabolic costs. By drifting towards reefs and feeding at night, they save vital energy. Staying in the caves during the day also saves energy because they do not have to waste energy fighting the currents.

Because coelacanths hide in caves during the daytime, the Anjouan island and Grande Comore provide ideal habitats for them. The steep underwater, eroded volcanic slopes covered in sand also house an obscure system of caves and crevices, allowing the coelacanths a place to stay during the daylight hours. These crevices support a large benthic fish population that can sustain coelacanth populations.

Coelacanths are nocturnal piscivores that feed mainly on benthic fish populations. By floating along the lava cliffs, they presumably feed on whatever fish they come across.

Coelacanths are fairly peaceful creatures when dealing with other coelacanths. They remain calm, even while in a crowded cave. They do avoid body contact, withdrawing immediately if contact is made with another coelacanth. When approached by foreign, potential predators (i.e. a submersible), they react with panic flight reactions. Coelacanths are most likely prey to large deep-water predators. Shark bite marks have been seen on coelacanths and sharks are very common in areas inhabited by coelacanths. Electrophoresis testing of 14 coelacanth enzymes has shown that there is little genetic diversity between coelacanth populations. Among the fish that have been caught, there have been about equal amounts of males and females. Population estimates range from 210 individuals per population, all the way to 500 individuals per population. Because coelacanths have individual color markings, scientists think that coelacanths are able to recognize other coelacanths via electric communication.

Life history
Coelacanths are ovoviviparous, meaning that the mother retains the eggs within her body while the embryos develop within the eggs. The gestation period of is longer than a year in coelacanths. The females are typically larger than the males, and their scales and folds of skin around the cloaca differ. The male coelacanth does not have distinct copulatory organs, just a cloaca. The cloaca has an urogential papilla that is surrounded by erectile caruncles. It is hypothesized that the cloaca turns itself inside out to serve as a copulatory organ. The eggs of the coelacanth are very large and have only a thin layer of membrane protecting them. The embryos hatch within the mother and eventually the mother gives live birth. Young coelacanths look very similar to adult coelacanths. The main differences include an external yolk sac, larger eyes compared to their body size, and their body has more of a downward slope. The yolk sac of the juvenile coelacanth is broad, and comes out from below the pelvic fins. The scales and the fins of the juvenile coelacanth are completely matured. The young coelacanth does lack odontodes, but gains them during maturity.

Conservation
Because little is known about the coelacanth, the conservation status is hard to characterize. According to Fricke et al. (1995), there should be some stress put on the importance of conserving this species. From 1988-1994, Fricke counted some 60 individuals on each dive. In 1995 that number dropped to 40. Even though this could be a result of natural population fluctuation, it could also be a result of overfishing. Coelacanths are usually caught when local fishermen are fishing for oilfish. Fishermen will sometimes snag a coelacanth instead of an oilfish because they usually fish at nighttime when the oilfish (and coelacanths) are feeding. Before scientists became interested in coelacanths, they were usually just thrown back into the water if caught. Now that there is an interest in them, fishermen trade them in to scientists or other officials once they have been caught. Before the 1980s, this was a problem for coelacanth populations. In the 1980s, international aid gave fiberglass boats to the local fishermen and it resulted in fishing out of coelacanth territories into more fish-productive waters. Since then, most of the motors on the boats have broken down so the local fishermen are now back in the coelacanth territory, putting the species at risk.

Different methods to minimize the number of coelacanths caught include moving fishers away from the shore, using different laxatives and malarial salves to reduce the amount of oilfish needed, using coelacanth models to simulate live specimen, and coelacanth protection awareness. In 1987 the Coelacanth Conservation Council was established to help protect and encourage population growth of coelacanths.

In 2002, the South African Coelacanth Conservation and Genome Resource Programme was launched to help further the studies and conservation of the coelacanth. The South African Coelacanth Conservation and Genome Resource Programme focuses on biodiversity conservation, evolutionary biology, capacity building and public understanding. The South African Government committed to spend R10 million on the program.