Coelacanth

Coelacanth (‘hollow spine’ in Greek, coelia meaning hollow and acanthos spine) is the common name for an order of fish that includes the oldest living lineage of jawed fish known to date. The coelacanths, which are closely related to lungfishes, were believed to have been extinct since the end of the Cretaceous period, until a live specimen was found off the east coast of South Africa, off the Chalumna River in 1938. Since then, they have been found in the Comoros, Sulawesi (Indonesia), Kenya, Tanzania, Mozambique, Madagascar, Greater St. Lucia Wetland Park in South Africa, and more recently, sister-species in Sulawesi (Indonesia)[1], strikingly increasing the geographical distribution ascribed to this species. (reference for divergence dated on mitochondrial genome[2]).

Biological characteristics

Coelacanths are lobe-finned fish with the pectoral and anal fins on fleshy stalks supported by bones, and the tail or caudal fin 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, which can only be found on extinct fish. Coelacanths also have a special electroreceptive device called a rostral organ in the front of the skull, which probably helps in prey detection.

Coelacanths first appear in the fossil record in the Middle Devonian, about 410 million years ago.[1] Prehistoric species of coelacanth lived in many bodies of water in Late Paleozoic and Mesozoic times.

The average weight of the living west Indian Ocean coelacanth, Latimeria chalumnae, is 176 pounds (80 kg) and they can reach up to 6.5 feet (2 m) in length. Adult females are slightly larger than males. Scientists believe individual coelacanths may live as long as 80 to 100 years by examining the growth rings in their ear bones called otoliths. Coelacanths live as deep as 700m (2296.5ft) below sea level, but are more usually found at depths of 90 to 200m. Living examples of Latimeria chalumnae have a deep blue colour which probably camouflages them from prey species, however the Indonesian species is brown. Latimeria chalumnae is widely but very sparsely distributed around the rim of the west Indian Ocean, seemingly occurring in small colonies.

Coelacanths are the only living species known to have a functional intracranial joint, which almost completely separates the front and back halves of the skull internally. Flexure at this joint may aid in the consumption of large prey by the use of suction. Coelacanths are also mucilaginous; their scales release mucus and their bodies continually exude oil. This oil is a laxative, and makes the fish almost inedible unless dried and salted. Although most fish have smooth scales, only the lower half of the coelacanth’s scales are smooth. Toothlike spines called denticles covered the upper half of each scale, making it rough and scratchy. The smooth lower half of each scale was protected by the denticles of the two scales that overlapped it. These rough, layered scales provided amrmorlike protection against predators and the rough edges of rocks. The rough scales are used by the natives of Comoros as sandpaper.

Coelacanth eyes are very sensitive, and have a tapetum lucidum. Coelacanths are almost never caught in the daytime or on nights with full moons, due to the sensitivity of their eyes. Coelacanth eyes also have many rods–tiny structures that help animals see in dim light. Together, the rods and tapetum help the fish see better in dark water.

Coelacanths are opportunistic feeders, hunting cuttlefish, squid, snipe eels, small sharks, and other fish found in their deep reef and volcanic slope habitats. Coelacanths are also known to swim head down, backwards and belly up to locate their prey presumably utilizing its rostral gland. Scientists suspect that one reason this fish has been so successful is that they can slow down their metabolisms at any time, sinking into the less-inhabited depths and minimizing their nutritional requirements in a sort of hibernation mode.

The coelacanths which live near Sodwana Bay, South Africa, rest in caves at depths of 90 to 150m during daylight hours, but disperse and swim to depths as shallow as 55m when hunting at night. The depth is not as important as their need for very dim light, and water which has a temperature of 14 to 22C, and they will rise or sink to find these conditions. (Ref: Dr Anthony Ribbink, coelacanth expert.)

Fossil record

Although now represented by only two 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, and past which point no fossils are known. 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. However, some of the extinct species, particularly those of the last known fossil coelacanth, the Cretaceous genus Macropoma, closely resemble the living species. 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. This situation is still under investigation by scientists.

Reproduction

Coelacanths give birth to live young called pups. Their reproductive behaviors are not well known, but it is believed that they are not sexually mature until after 20 years of age. Gestation time is 13 months, females give birth to between 5 and 25 babies, which are capable of surviving on their own immediately after birth. Coelacanths are ovoviviparous.

Discovery

First find in South Africa

The first evidence that western scientists had of a modern, living coelacanth was when Hendrik Goosen discovered a specimen while inspecting the catch of a trawl for unusual marine life in 1938. Goosen brought the fish into East London harbour and telephoned Marjorie Courtenay-Latimer that he had this strange fish for her to look at. Failing to find it in any of her books, she attempted to contact her friend, Professor James Leonard Brierley Smith, but he was away. Unable to preserve the fish, she sent it to a taxidermist. When Smith returned, he immediately recognized it as a coelacanth, known only from fossils. Smith named the fish Latimeria chalumnae in honor of Marjorie Courtenay-Latimer and the waters in which it was found. The fish was referred to as a “living fossil.”

Comoros

A worldwide search was launched for more coelacanths, with a reward of 100 British pounds (a very substantial sum to the average South African fisherman of the time). Fourteen years later, they were found in the Comoros, at first only another single specimen, but later it turned out that the fish was no stranger to local knowledge: the Comorians, in the port of Mutsamudu on the Comorian island of Anjouan, were puzzled that someone would pay big money for what the locals called a gombessa or mame, an inferior (nearly inedible) fish that their fishermen occasionally caught by mistake.

They are now aware of the significance of their endangered species and have a program in place to return any accidentally caught coelacanth to deep water, so that they may survive.

The second specimen, found in 1952 by Comorian fisherman Ahamadi Abdallah, was originally described as a different species, at first ‘Malania hunti’ but later as Malania anjounae (after Daniel François Malan, the South African Prime Minister at the time, who had dispatched an SAAF Dakota to fetch the specimen), but it was later discovered that the lack of the first dorsal fin, which was believed to distinguish it from Latimeria, was due to an injury early in the specimen’s life. Ironically, Malan was a staunch creationist; on seeing the supposed ancestor of all terrestrial life named after him, his reaction was a startled, “Why it’s ugly! Is this where we come from?”

Smith, who died in 1968, wrote his account of the coelacanth story in the book Old Fourlegs, first published in 1956. His book Sea Fishes of the Indian Ocean, illustrated and co-authored by his wife Margaret, remains the standard ichthyological reference for the region.

Second species in Indonesia

In 1997, Arnaz and Mark Erdmann were traveling on their honeymoon in Indonesia and saw a strange fish entering the market at Manado Tua, on the island of Sulawesi. Arnaz recognised it as a gombessa, although it was brown, not blue. The Erdmanns did not realise this was a new species until an expert noticed their pictures on the Internet. DNA testing revealed that this species, called raja laut (“King of the Sea”) by the Indonesians, is not related to the Comorian population. It was given the scientific name Latimeria menadoensis. Recent molecular study estimated the divergence time between two coelacanths in the range of 40–30 Mya.

St. Lucia Marine Protected Area in South Africa

In South Africa, the search continued on and off over the years. One diver, 46-year-old Riaan Bouwer, lost his life exploring for coelacanths in June of 1998.

On the 28th of October 2000, just south of the Mozambique border in Sodwana Bay in the St. Lucia Marine Protected Area, three deep-water divers, Pieter Venter, Peter Timm, and Etienne le Roux, made a dive to 104 metres and unexpectedly spotted a coelacanth.

Calling themselves “SA Coelacanth Expedition 2000”, the group returned, this time with photographic equipment and several additional members. On the 27th of November, after an unsuccessful initial dive the previous day, four members of the group, Pieter Venter, Gilbert Gunn, Christo Serfontein, and Dennis Harding, found three coelacanths. The largest was between 1.5 and 1.8 metres in length; the other two were from 1 to 1.2 metres. The fish swam head-down and appeared to be feeding from the cavern ledges. The group successfully returned with video footage and photographs of the coelacanths.

During the dive, however, Serfontein unexpectedly lost consciousness, and 34-year-old Dennis Harding rose to the surface with him in an uncontrolled ascent. Harding complained of neck pains and died from a cerebral embolism while on the boat. Serfontein recovered after being taken underwater for decompression sickness treatment.

In March–April of 2002, the Jago Submersible and Fricke Dive Team descended into the depths off Sodwana and observed fifteen coelacanths. Tissue samples were collected using a dart probe.