Melinda Massie
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