Coelacanth

Coelacanth Temporal range:
specimen of Latimeria chalumnae in the Natural History Museum, Vienna, Austria (length: 170 cm - weight: 60 kg). This specimen was caught on 18 October 1974, next to Salimani/Selimani (Grand Comoro, Comoro Islands) 11°48′40.7″S 43°16′3.3″E.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sarcopterygii
Subclass:	Actinistia
Infraclass:	Coelacanthimorpha
Order:	Coelacanthiformes Berg, 1937
Families
See text.

Coelacanth (/ˈsiːləkænθ/, adaptation of Modern Latin Cœlacanthus "hollow spine", from Greek κοῖλ-ος koilos "hollow" + ἄκανθ-α akantha "spine", referring to the hollow spines of the fins) is the common name for an order of fish that includes the oldest living lineage of Sarcopterygii (lobe-finned fish + tetrapods) known to date.

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. This discovery 65 million years after they were believed to have gone extinct makes them arguably 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 in 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 flavor. The continued survivability of the coelacanth may be at threat due to commercial deep-sea trawling.

Natural history

Fossil of Axelrodichthys araripensis, an extinct coelacanthiform

They first appeared in the fossil record in the Middle Devonian. Prehistoric species of coelacanth lived in many bodies of water in Late Paleozoic and Mesozoic times.

Coelacanths are lobe-finned fish with the pectoral and anal fins on fleshy stalks supported by bones, and the tail or caudal fin diphycercal (divided into three lobes), the middle one of which also includes a continuation of the notochord. Coelacanths have modified cosmoid scales, which are thinner than true cosmoid scales. Coelacanths also have a special electroreceptive device called a rostral organ in the front of the skull, which probably helps in prey detection. The small device also could help the balance of the fish, as electrolocation could be a factor in the way this fish moves.

Description

This fish has some unique traits among vertebrates:

• The kidneys are fused into a single organ, which is located on the floor of the abdomen instead of just under the backbone.
• A trilobate (three-lobed) caudal fin, with upper and lower halves separated by a small secondary tail.
• A tiny heart that is more-or-less a straight tube
• A brain that occupies only 1.5% of the braincase; the rest of the cavity is filled with fat.
• The pectoral, pelvic and anal fins set on muscular lobes.

Fossil record

Undina penicillata

Although now represented by only two known living species, as a group the coelacanths were once very successful with many genera and species that left an abundant fossil record from the Devonian to the end of the Cretaceous period, at which point they apparently suffered a nearly complete extinction. Before the living specimens were discovered, it was believed by some that the coelacanth was a "missing link" between the fish and the tetrapods. It is often claimed that the coelacanth has remained unchanged for millions of years; but, in fact, the living species and even genus are unknown from the fossil record. The most likely reason for the gap is the taxon having become extinct in shallow waters. Deep-water fossils are only rarely lifted to levels where paleontologists can recover them, making most deep-water taxa disappear from the fossil record.

Evolution

The fossils of the coelacanth are believed to be indicative of the order's place as a transitional evolutionary link due to the presence of leg-like structures. Extant specimens of two species of the genus Latimeria have been discovered, allowing study of evolutionary changes within the Coelacanthiforme order.

Taxonomy

Latimeria menadoensis, Tokyo Sea Life Park (Kasai Rinkai Suizokuen), Japan

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:

• Panderichthys, suited to muddy shallows;
• Tiktaalik with limb-like fins that could take it up onto land;
• Early tetrapods in weed-filled swamps, such as:
  • Acanthostega which had feet with eight digits,
  • Ichthyostega with limbs.

Descendants also included pelagic lobe-finned fish such as coelacanth species.

Latimeria chalumnae model in the Oxford University Museum of Natural History, showing the coloration in life

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
Subclass Coelacanthimorpha

• Order COELACANTHIFORMES
  • Family Coelacanthidae (extinct)
    • Axelia (extinct)
    • Coelacanthus (extinct)
    • Ticinepomis (extinct)
    • Wimania (extinct)
  • Family Diplocercidae (extinct)
    • Diplocercides (extinct)
  • Family Hadronectoridae (extinct)
    • Allenypterus (extinct)
    • Hadronector (extinct)
    • Polyosteorhynchus (extinct)
  • Family Mawsoniidae (extinct)
    • Alcoveria (extinct)
    • Axelrodichthys (extinct)
    • Chinlea (extinct)
    • Diplurus (extinct)
    • Mawsonia (extinct)
  • Family Miguashaiidae (extinct)
    • Miguashaia (extinct)
  • Family Latimeriidae
    • Holophagus (extinct)
    • Libys (extinct)
    • Macropoma (extinct)
    • Macropomoides (extinct)
    • Megacoelacanthus (extinct)
    • Latimeria (James Leonard Brierley Smith, 1939)
      • L. chalumnae (Comorese coelacanth) (James Leonard Brierley Smith, 1939)
      • L. menadoensis (Indonesian coelacanth) (Pouyaud, Wirjoatmodjo, Rachmatika, Tjakrawidjaja, et al., 1999)
    • Undina (extinct)
  • Family Laugiidae (extinct)
    • Coccoderma (extinct)
    • Laugia (extinct)
  • Family Rhabdodermatidae (extinct)
    • Caridosuctor (extinct)
    • Rhabdoderma (extinct)
  • Family Whiteiidae (extinct)
    • Whiteia (extinct)

See also

• Lazarus taxon
• Megamouth Shark

External links

• African Coelacanth Ecosystem Programme (ACEP), as the South African Coelacanth Conservation and Genome Resource Programme
• Association for the Preservation of the Coelacanth
• DINOFISH .com Coelacanth: the fish out of time
• Coelacanth at MarineBio.org
• PBS: NOVA - Anatomy of the Coelacanth
• Diving for Coelacanths
• Divergence time estimation of the two coelacanths species based on the whole mitochondrial genome sequences.
• Coelacanth Information
• young Juvenile coelacanth found near Indonesia.