The Coelacanth—Fish Frozen in Time
The coelacanth, also known as the dinosaur fish, comes from a lineage that was thought to be extinct with the dinosaurs. This fish has been in the fossil record dating back to 360 million years ago with its peak in the records about 240 million years ago. They were believed to be extinct after disappearing from the record about 80 million years ago. Fossil remains have been found on every continent except Antarctica.
However, in 1938 a living coelacanth was discovered. Just before Christmas one was caught at the mouth of the Chalumna River near East London, South Africa. This fish was originally considered a straggler, but it led to the discovery of a population off of the Comoros Islands.
Ancient coelacanths used to be very small. However, today they reach up to 6 feet long and weigh 150 pounds or more. Furthermore, they may live to be 60 years old. These fish give birth to as many as 26 live pups that develop from eggs in the oviduct. They feed off of a large yolk sac until birth. Juvenile fish have rarely been seen or caught be local fishermen or searching scientists.
The coelacanth has nine fins—paired pectoral and pelvic fins, two dorsal fins and a tri-lobed tail fin. Extra trunk and fin makes up the epicaudal fin, or center section on the tri-lobed tail fin. The paired pelvic and pectoral fins are seen as a pre-adaptation to land motion, although these fish have never been seen walking on the sea floor.
Coelacanths are the only living animals to have a fully functional intercranial joint. This is a division that separates the ear and brain from the nasal organs and eye. It allows the front part of the head to be lifted while the fish is feeding. The fish also has true jaws and a tiny brain encased in a hardened skull.
The vertebral column is not fully developed. Instead it is a cartilaginous tube that is filled with fluid. This provides a firm yet flexible support system for the muscles. It also has hollow fin spines. The outside of this fish is covered with thick scales lined with serrated rows of hardened, pointy denticles. Furthermore, the fish found in the Comoros Islands are steel blue with white spots. Those found near Sulawesi are brown.
A coelacanth heart is chambered and pumps blood. It is very much a prototype to the human heart. The fish has extremely well developed eyes with reflecting cells called tapita to enhance its vision at the dark sea floor. These tapita cells are also found in the eyes of cats. Also well developed in this fish is its sense of proximity to other fish and its surroundings.
While swimming, the coelacanth looks like blimp with positioning fins instead of propellers. The two paired pectoral fins paddle in alternation with the two paired pelvic fins in a similar fashion to human arms and legs. On an expedition to the bottom of the sea to view these fish in 1987, the scientists in the submersible witnessed the coelacanths using the intercranial joint to stand on their heads. However, this behavior had now been attributed to the electromagnetic field that the submersible was leaking out.
The coelacanths are opportunistic feeders. They will eat cuttle fish, squid, snipe eels, small sharks and other fish found in their deep reef and volcanic slope habitat. Scientist who tagged some of these fish found out that they will leave the cave at the same time each day—late in the afternoon—to forage along the coastal incline at night.
An explanation for their excellent sense of timing and coastal navigation skills has still yet to be found.
The entrance to the cave and the walls of the coelacanth home have white encrustations of oyster shells and other materials that were mimicked be the white patterns on the coelacanth body. The fish congregate in these caves located at steep island drop offs and hover near each other without touching.
Scientists had once estimated that the number of live coelacanths was in the hundreds. However, it is difficult to determine the number of coelacanths alive today because new populations are being discovered. Until recently, it was thought that the only living fish were off of the Comoros Islands. However, on August 11, 1991, a female carrying pups was trawled in Maputo, Mozambique. Four have been netted off of Madagascar in 1995, 1997 and 2001.
On July 30, 1998, a population was discovered off of Sulawesi, Indonesia. The locals of Sulawesi call the fish raja laut meaning “king of the sea.” November 27, 2000, divers caught six coelacanths on film near East London, South Africa at Sodwana Bay. Until then, none had been found in South African waters since the first one found in 1938. The numbers of live coelacanths are now speculated to be around 1,000.
In 1930’s, the coelacanth was thought to be a direct descendant of the tetrapods—land living animals (which includes humans). It appears to be a cousin of Eusthenopteron—the fish credited with being the ancestor. The paired pectoral and pelvic fins contain trunk bones and mimic those fins of the Eusthenopteron that later developed into the arms and legs of tetrapod amphibians. The hot debate is whether these are the closest related fish or if the lungfish is the closest related fish to the tetrapods. Genetic evidence points in both directions.
The coelacanth is an excellent reason why more research at the bottom of the sea should be conducted. Had a fisherman not caught one back in 1938, people would still think that this fish was extinct.
“Millions Could See ‘Dinosaur Fish’ Live on Internet if…” http://allafrica.com/stories/200012050366.html
“Coelacanth, Latimeria chalumnae” http://amonline.net.au/fishs/fishfacts/fish/coela.htm