Animals

Coelacanths

Coelacanths, members of the order Coelacanthiformes, are one of two living types of lobe-finned fish.

Lungfish are the only other lobe-finned fish that are alive today.

Until 1938, coelacanths were believed to have been extinct for more than 65 million years.

Coelacanths live in deep water.

They are carnivorous.

Coelacanth - Latimeria chalumnae, photo by Alberto Fernandez Fernandez

Coelacanths are active mostly at night (are nocturnal.) Their retinas have a reflective layer that allows them to see in deep, dark waters. This layer is similar to the reflective layer that allows cats to see in the dark.

Living Fossil

The earliest coelacanth fossils come from the Devonian period, which lasted from about 416 million to about 360 million years ago.

Fossils of many different species of coelacanth have been found. The most recent fossils came from the Cretaceous period, which lasted from about 146 million to about 66 million years ago.

Because fossils that were more recent had never been found, scientists used to think that Coelacanths became extinct at the end of the Cretaceous period.

However, in 1938, a coelacanth was caught by fishermen who were fishing in the mouth of the Chalumna River off the east coast of South Africa. The captain of the vessel, Hendrik Goosen, brought the fish to Marjorie Courtenay-Latimer, who was the curator of a South African natural history museum.

Fishermen often brought her unusual fish they found, in case she could use them in her collections.

She showed the fish to ichthyologist James Leonard Brierley Smith, and they determined that this fish was a coelacanth.

Smith published news of the finding and named the newly discovered species Latimeria chalumnae.

The discovery of a "living fossil" created a great deal of publicity.

In fact, fishermen from Madagascar had been catching coelacanths, for many years - possibly for thousands of years.

Since then, more Latimeria chalumnae have been identified.

In 1997, Mark and Arnaz Erdmann found what they believed was a coelacanth belonging to a species other than Latimeria chalumnae at a fish market in North Sulawesi, Indonesia.

The Erdmanns did not purchase the fish, but in 1998, they were able to obtain another coelacanth from this newly discovered species, which they donated to the Indonesian Institute of Science.

This new species was given the name Latimeria menadoensis.

As far as we know, Latimeria chalumnae and Latimeria menadoensis are the only living species of Coelacanth.

Many more coelacanths have been found since then.

Scientist now believe that the reason we have found no coelacanth fossils that are less than 66 million years old is that after that time, coelacanths did not live in places where fossils form easily.

The coelacanths that have been found since 1938 lived in caves and overhangs in marine reefs off recently formed volcanic islands. Fossils do not form easily in these environments.

Physical Characteristics

On average, coelacanths weigh from about 150 to about 175 pounds. They may be more than 6 feet long.

Latimeria chalumnae tends to be blue with white flecks and Latimeria menadoensis tends to be brown with white flecks.

Like all lobe-finned fish, coelacanths have fleshy lobes, with bone supports, at the bases of their pelvic and pectoral fins.

A coelacanth's anal fin is also supported by a lobe.

Coelacanths have two dorsal fins (fins on their backs).

The caudal fin (tail fin) of a coelacanth is divided into three sections.

The spines that support the fins of a coelacanth are hollow.

The name "coelacanth" is Greek for "hollow spine."

All bony fish have fins that are supported by spines. These spines are also known as fin rays or lepidotrichia.

Lobe-finned fish are a type of bony fish.

Although a coelacanth is a vertebrate, its spinal column is not fully developed.

It retains its notochord throughout its life.

The middle section of its tail fin contains an extension of the notochord.

A notochord is a flexible rod that provides the body with support.

All chordates have notochords for at least some part of their lives.

A vertebrate is a type of chordate.

A coelacanth has an organ known as a rostral organ at the front of its skull. The rostral organ is probably used to detect electrical impulses in the water. Coelacanths might use their rostral organs to help them find prey.

The swim bladder of a coelacanth is filled with fat that is less dense than water. This allows it to stay afloat.

Respiration

Coelacanths breathe through very small gills, which are not very efficient.

They spend their time in cold water, which has more oxygen than warm water.

Coelacanths move very slowly. This helps them to save energy so that they don't need much oxygen.

Reproduction

Coelacanths are ovoviviparous. This means that the young grow inside the body of the female and are nourished by an egg yolk. The young are born live.

Research

We have not been able to keep coelacanths alive for very long after they have been caught.

This may be because coelacanths cannot adjust to the change in pressure they experience when they are taken out of deep water.

Coelacanths may also suffocate when they are brought closer to the surface, where the water is warmer and contains less dissolved oxygen. Warm water also increases a coelacanth's metabolism, which increases its oxygen requirements.

Much of what we know about coelacanths comes from studying dead specimens.

For example, we know that coelacanths are carnivorous because we've found dead coelacanths with fish in their stomachs.

Scientists learned that coelacanths are ovoviviparous when they found unborn coelacanths with yolk sacks inside a dead female's abdomen.

Did Humans Evolve from Coelacanths?

Humans did not evolve from coelacanths.

Human beings are tetrapods - four limbed vertebrates or the descendants of four-limbed vertebrates.

The lobe-finned fish - coelacanths and lungfish - and the tetrapods had a common ancestor.

Therefore, coelacanths and humans had a common ancestor.

Coelacanths and humans belong to the same clade - the sarcopterygians.

A clade is a group of organisms that share a common ancestor. No organisms outside the clade are descended from the clade's common ancestor.

Although, as far as we know, there are only two species of coelacanth alive today, in the past, there have been many more species of coelacanth.

The coelacanths that are alive today are different from the coelacanths that lived during the Cretaceous period. As time has passed, coelacanths have evolved to adapt to changes in their environment.