The Coelacanth: A Living Fossil & Tetrapod Ancestor

The Coelacanth

The Coelacanth: A Living Fossil & Tetrapod Ancestor

By Jonathan Lowrie

Living Fossil.  The term living fossil, is a science jargon term that conveys a meaning other than what it says; contradictory term in itself, but one most fitting for  the animals it describes. The living fossils of today can be anything from the shark, alligator to the lung-fish, horseshoe crab or the coelacanth.    A living fossil, in this case, may be defined as an organism once believed to be extinct, but later discovered to be living.  Another connotation to the term is an organism that has not significantly changed in its evolutionary history, as is the case with the shark, and horseshoe crab. Only a select few organisms fit these criteria.  

To find an ancient living fossil believed to be extinct is quite a discovery. The most unusual of the living fossils is a fish called a coelacanth  (pronounced “seel-uh-kanth”), a member of the subclass Crossopterygii from the Paleozoic era.  It was originally found in fossil form and was described back in the 1800's.  Fossil dating found the samples to be around seventy million years old.  In 1938, an African fisher trawled a live coelacanth up from deep water, starting the scientific frenzy over the living fossil.  This fish, when found, was so unusual that numerous questions were asked by both the scinetific and lay community.  It is believed, from its appearance, that the coelacanth may be the direct ancestor to the tetrapods, the first fish that  came to land.    Paleobiologists are just beginning to uncover the facts (Thomson, Keith, Stewert.  1969).

The twenty-second day of December was a special one in biological history.  This  fish, glamorous in reputation but not appearance, made its modern debut just a few days before Christmas in the year 1938.  The coelacanth was first captured in an estuary of the Chalumna River, in Cape Province, South Africa.  The captain of the Nerine , Hendrik Goosen, was on a routine fishing trip when he caught by trawl the first coelacanth in approximately seventy meters of water.   Marine biologists realized four years later that the fish was not native to South Africa, but to the deep waters off the coast of the Comores Islands, between Madagascar and the African mainland.  This is where all subsequent specimens have been caught ( Smith, J.B.L. 1949).  The first specimen caught off South Africa was the only one to be caught outside the Comores in the sixty-two year history of the fish, much to the dismay of the many who tried for quick fame and fortune during those years. 

The search for the coelacanth began almost immediately after J.B.L. Smith realized what Ms. Majorie Courtney-Latimer, a local museum curator in South Africa,  had reported.  Smith began to work on its origin soon after he saw the remarkable sketches of the wondrous fish (Smith, J.B.L. 1956).  Because of the vast lack of facilities ate the museum, Ms. Courtney-Latimer had to dry-mount the first coelacanth specimen rather than preserve it.  Most of the other specimens at her local museum were not four feet long, and close to one hundred thirty pounds. and were small enough to preserve in jars with formalin.  Such a large fish was just too much for the facilities to handle. Without a container to keep the specimen in, there was just no way to preserve the fish.   Furthermore, after going through the museum and to the local pharmacy and found there was no formalin to be found.  This was catastrophic for further anatomical studies of the coelacanth, as with the loss of internal structures, in-depth scientific review could not be completed..   She fretted over the loss of the visceral organs, but  she had no way to preserve the living fossil. But she had made thorough sketches and notes.  Through correspondence about the sketches, Smith was convinced they were on to something, and he came to Cape Town a few weeks after the fish was caught.  Although the fish was mounted on a plaque, Smith was able to confirm it as the coelacanth of so long ago.  This meant they had the first living specimen of a fish believed extinct (Smith, J.B.L. 1956).  Smith put up a reward for any living specimen caught, and posted his notices all over the region of coastal Africa, Madagascar, and the islands surrounding the area.    Notifications were distributed well, especially in academic settings, of the discovery and reward.   It was not until a four years later that he heard from a long-time companion and ship captain that a local fisher in the Comores Islands might have caught another coelacanth, but by then, the race for knowledge of the coelacanth was on in other ways.  Smith’s personal problems and the French authorities governing the Comores balked his use of further specimens.  Smith had to rely largely on a black market specimen and notes of others to continue his research of the fish that so intrigued him (Interview.  Dec. 1989).  

  As many live specimens began to appear, it was established that the mysterious lobed-fin fishes were from the islands of the Comores.  They were indeed something special.  Eventually, in the 1970’s, the first frozen coelacanth was delivered to Peabody Museum of Yale University.  This specimen was the first one presented with unpreserved tissue, allowing for blood tests and reproductive studies.  This ability to conduct blood studies led to some of the most intriguing questions concerning this fish (New York Times.  1989). 

The coelacanth, or Latimeria chalumnae (Smith 1939), was named after M. Courtney-Latimer, the discoverer, and the Chalumna River, near where it was first caught.  Professor J.L.B. Smith was the first biologist (familiar with fishes) to study and describe the fish, and he is also the man who named the coelacanth (Thomson, Keith, Stewert.  1991). The most recent taxonomic status of the coelacanth puts it in the order Coelacanthiniformes and the family Latimeridae, all of class Osteichtyes, phylum Chordata, and kingdom Animalia.  The taxonomic placement of the coelacanth is secure, but the level of placement has been changed over the course of years (Patterson, C. et al.  1977).    Recently caught specimens have been a shade of blue with white and pink splotches over the body (Smith, J.B.L. 1949), but all the preserved specimens were a dark purple and brown color, suggesting a colour change after death. This has been a helpful fact for paleontologist, as they know what organisms from the Paleozoic era looked like (Lagios, M. 1979). Until recently the coelacanth never seen alive in its habitat, but modern undersea submersibles allowed the live video recording of a coelacanth.

 The coelacanth is very unlike any other fish alive today. The coelacanth is not a small fish; it can reach up to one hundred eighty  centimeters and may weigh as much as ninety five kilograms. It is related to other lobed fish, the lung-fishes of Africa and Australia, which also have the fleshy appendages like the coelacanth. However, coelacanths also have a hollow region in their lobed fin and are the only deep-water fish to exhibit these traits.  The lobed structure is what helped Smith identify the first specimen as the fabled coelacanth of so long ago, as they are likened to the early appendages of the tetrapods.  Biologists are not yet sure as to the actual function of the lobes in the coelacanth (McCosker, John & Lagios, Michael.  1979). It may be a vestigial remnant of the ancestral form, or there may be some obscure use that we cannot yet fathom. As far as biologists know, the fish does not rest on the substrate or used the lobes for movement.   It was once believed to be that the fish rested on the benthic substrate, but live video and sightings prove otherwise. All video shows the coelacanth as a float and wait predator. While the fish swims, the lobes do move, but not in a fashion to help its propulsion.  It is the presence of these lobes that lead biologists to hypothesize about the evolutionary tree of vertebrates and land animals.  By no means is it suggested that the coelacanth is “missing link,” or the link to the land. While this course of evolution is only theorized,  some relationships between tetrapods, especially juvenile amphibians, do exist  (McCosker, John E.  1979). The theory states that the design of the coelacanth and the air-breathing ability of its taxonomic cousins show a reasonable course for evolution to follow as a natural course.

The coelacanth also shares many traits with its chondrichthyes cousin, the shark (Compagno, Leonard.  1979). It has a primitive type of swim bladder that is filled with oil (Ichthyology:An Introduction to Fishes.  1990).  Instead of a pheisotomus  or pheisoclistus swim bladder, the coelacanth has an oil filled organ.  This oil emersion system of buoyancy control allows the coelacanth to remain neutrally buoyant at depths of sixty to three hundred and fifty meters.  Because very little swimming energy need be expended to move, the fish is believed to hover mostly above the ocean floor, lurking for prey.  The oil in the swim bladder is more buoyant than the water surrounding the coelacanth, providing the positive lift the fish needs (Lombardini, J. & Pang, Peter.  1979).  This is almost identical to the oil suspension used by sharks.  In fact, the coelacanth is the only bony fish to exhibit this method of buoyancy. This dual relation with both the primitive shark and the lung-fish is unique among fish.

 During blood study tests completed by German biologists, a genetic relationship was discovered between the coelacanth and tetrapods.  The scientists analyzed blood proteins isolated from living coelacanth tissue and compared them with other species.  The work conducted in this study showed that many of the amino acid sequences matched those found in tadpoles almost exactly (Forey, Peter.  1991).  This fits with the basic evolutionary model of fish to amphibians, as tadpoles are amphibians.  Further analysis, however, showed that the isozymes and enzymes do not match the adult frog, but are close in some genetic areas (Fisher, Suzanne, E.  & Whitt, Gregory. 1979).  The juvenile frog has matching proteins, while fewer match with the adult frog. In addition, the genetic composition of the coelacanth matched higher vertebrates when in the embryological state of early development.  This is a puzzling area for biologists.  The course of development of the frog causes a metamorphosis that no longer reflects the similarities to the coelacanth . Alone, these genetic studies do not provide conclusive evidence of the tetrapod relationship.  With the blood studies, the movement of the coelacanth matches that of terrestrial animals.  The lobes of the fish move synchronously, as is the case in land vertebrates.  When the coelacanth swims, its fleshy lobes move like it was walking.  It has been noted that it is an amusing experience to see a slow moving fish try to run in place.  Some sort of behavioral and musculature similarity must be present between the coelacanth and tetrapods.  This type of movement does not occur in other fish, even the lungfishes.      This leads one to believe that this fish has some sort of genetic and evolutionary connection with higher animals (McCosker, John E.  1979).  With positive DNA matches and the “walking” lobes, the coelacanth could potentially be the tetrapod ancestor biologists have been looking for. 

 Of course, doubters exist.  These groups feel the coelacanth does not deserve the honor of being the tetrapod ancestor .  They  explain how enzymes and DNA tests cannot be totally accurate.  Using examples of human genetics testing as their foundation, the biologists say this is not an accurate way to compare the genetics of tetrapods to that of a coelacanth.  Their studies suggest that the relationship exists in almost all fish and sharks, and it not unique to the coelacanth.  Of course, the proven idea of ontology recapitulates phylogeny, and the test involving tadpoles and embryological tetrapods should be correct (Patterson, C.  1977).  With the extensive research now being conducted, the questions of the coelacanth may be answered in the future.  The 1990’s will be innovative years, as new biochemical techniques and lab equipment have been developed to elucidate the mystery.  

This phylogeny of the blood line of vertebrates is obviously a debated topic.  As the coelacanth is closely related to the lungfishes, the only other possible tetrapod ancestor,  it is safe to say we all share a common ancestor.  The lungfish exhibit the characters to survive in land.  They have the ability to breathe air and prevent desiccation for short durations.  The coelacanth has evidence of appendages for movement on the land.  Both of these relationships help illustrate how this fish could possibly have led to the terrestrial animals of today.    With the coelacanth’s relationship to  sharks in the class Chondrichtyes, the primitive origin is still present.  But it shows that they both shared a common ancestor, and how the coelacanth could have evolved (McCosker, John, & Lagios, Michael.  1979).  The “living fossils,” sharks and the coelacanth, have many of the primitive characters of their era.  The shark has completed its evolution, as has the coelacanth.  This connection also holds a key to the evolution of bony fish.  As the relationship between bony fish and the cartilaginous fish can be shown by the sharks,   the relationship between tetrapods and the coelacanth can be shown by the tadpole studies. Yet, this is only a small part of the uniqueness of the coelacanth.

The coelacanth is just as elusive today as it was when we first learned of it.  Only a total of around two hundred individuals have been caught since the  first one of the coast of South Africa.  The fish has remained elusive to even our best scientific technology and apparatus.  Every one of the specimens, save for the first, was caught using the traditional line and hook method used by  the Comorian fishers, not a scientist (Thomson, Keith, Stewert.  1991).  The traditional method of the native fishers is to take a hemp line about three hundred meters in length baited with dead fish on a single large hook.  The line is then dragged along behind the skiff in the late hours of night  No modern drag line or net fishing methods have been successful.  Many expensive research expeditions have been tried and failed.  Such rarity makes the coelacanth unlike other studied fish.  Although there are many species of fish we know little about,  the coelacanth is the only fish we need to know more about and are not even provided the opportunity.  We know  little about certain facts, like geographical distribution.   Based on that we can extrapolate on their history based on their neighbors and close kin, the coelacanth is believed to be located primarily in the Comores only (National Geographic.  Dec. 1988).  With such a local distribution many questions arise.  Is the small distribution because of the environment, or are there more habitats that we have yet to discover?  If the answer to the habitat questions can be discovered, determining the survival technique the coelacanth has used for the last million years may be possible.  These survival techniques could also help explain other oddities in evolution or the relationships between the various classes and families of organisms.  

 The Comores are a small set of four islands.  Geologically, they are new, being formed from volcanic activity. This could be explained by a lack of competition.  A new island system, has very little to offer pelagic fishes, unless prey moves in.  The only fish in the area are small schooling fish and the oil fish.  Neither fish poses a threat or competition to the coelacanth.  The chain is composed of four islands, only two of which have the coelacanth have been captured.  The two islands where the coelacanth has been  found are Grand Comorore and Anjouan.  Both of these islands have the steepest and deepest slopes  offshore (Thomson, Keith, Stewert.   1991).  This data does not provide any conclusive evidence of the requirements of the coelacanth  to the deep shelves off the islands,  though,  since the fishers of the Comores, only fish off of the deeper islands  to catch a similar fish, Ruvettus pretiosus, the oil fish.  However, the catches of the coelacanth suggest that the two fish share the same habitat, as they are caught the same way and at the same time.  Most of the coelacanths captured were caught in a depth of two hundred to two hundred fifty meters of water.  This very definitely suggests a rather deep dwelling fish.  This evidence also matches the geological topology of the two islands. 

 Some physiological aspects of  deep sea existence can be shown by the oil filled swim bladder of the coelacanth, designed to remain neutral at deep depths.  Another adaptation is the eye.  It is designed for low light and deeper water.  All coelacanth specimens that have been captured have a ruptured lens, resulting from  pressure differences upon capture on the delicate crystalline structure of the lens.  Yet, French studies on photoreceptors in the coelacanth eye determined that the wavelength of light absorbed by the coelacanth does match that of the depth they are thought to come from (Thomson, Keith, Stewert.  1991).   As such, some believe that the coelacanth follows a vertical migration pattern.  Because the fish has only been caught at night, it may be following a pattern of feeding further up in the water column and living in deeper waters (Neill. 1990).  Depths below three hundred meters are beyond the Comoran fishers ability, and the data cannot predict if the coelacanth exists any deeper   The coelacanth is believed to eat smaller predatory fish.  These other fish are part of the deep-sea scattering layer.  These fish migrate towards top water in search of plankton and copepods.  The coelacanth may follow these smaller fish in their nightly migration.  This is also supported by the evidence that coelacanths have only been caught at night   Furthermore, the coelacanth prefers rather cool water.  It has been found only in fourteen to seventeen degrees centigrade, thus putting another restriction on the depth it can live.  Water of that temperature would be at depths close to three hundred meters.  The coelacanths caught at around one hundred seventy meters may have been in upwelling currents, bringing the coelacanth closer to the surface.  In  live video, the coelacanth never left the narrow temperature margin (McCosker, John, & Lagios, Michael.  1979   This very elusive fish has a mysterious habitat, and yet obviously a very successful one.    

Of the currents that run through the Comores, now believed to be the modern popular coelacanth hideaway, one flows from the southern Comores to South Africa.  The other runs from the northern islands out into the Indian Ocean.  Obviously, the stray original, was traveling this southern current.  Whether it was for migration,  food, mating, or if it lost its way, is not clear.  These currents may also have swept a population of coelacanths out into the Indian Ocean, but no specimens have been found in random catches in this area.   If the Comores are special, it has to be for one of a few select reasons.  The geological age of the young chain, the lack of other big fish, and its relative isolation from other areas may be possible explanations.  Using a geological match up of the Comores to other areas that meet the requirements of a new island, similar depth, similar temperature, and a lack of competition, no other known habitat on planet Earth exists for the coelacanth (Thomson, Keith Stewert.  1991). 

Still, the geological history of the Comores and the coelacanths presence are puzzling.  As mentioned, the Comores are a new island system.  The coelacanth is an old fish, in fact, older than the island chain.  So how did the only surviving group come to reside in such an environment?  For a brief history of geology, two hundred fifty million years ago, all the continents were one land mass called Pangea.  At this time, there were no Atlantic or Indian Oceans.  The Atlantic Ocean opened up in the Mesozoic, where a fossil relative of the coelacanth is found.  During this time, the South Atlantic Ocean was being formed.  Movement of Antarctica away from Africa and South America formed the chunk of land known as Madagascar.  About eighty five million years ago, India was a free island and Australia was connected to Antarctica.  At forty eight million years ago, India had only slightly moved, forming the Indian Ocean.  Both of these histories were in the span of time when the coelacanth was swimming the seas.  It is not known where the coelacanth existed  before the geological upheaval.  Fossil records of Latimeria's ancestors, show a cosmopolitan distribution over the ancient seas.  The islands of the Comores are at most 6 million years old.  Grand Comoros, the island off of which the most coelacanths have been caught, is the youngest at 130,000 years.  This certainly suggests, that whatever habitat the coelacanth prefers, it must be relatively new in terms of the fish’s history (Patterson, C.  1977).

 It appears now that the coelacanth has survived for the multitude of years, by migrating to new habitat where competition has not yet established itself.  It may be that the Comores are the last such safe haven on Earth.  If this is so, our conservation of this relic fish is imperative.  The coelacanth has found a niche free of predators, save for man, abundant with food and correct in physical makeup.  For the last few million years  the coelacanth has made the Comores its safe home for continued survival. .  It is strongly believed that  Latimeria  is the only species of coelacanth living today  Other fossil relatives of Latimeria have been found all over the world.  Their success has not been as good. Some say that jeweled pendants in the Mediterranean Sea region were found that dated back hundreds of years. But on close observation, these could represent many fishes as they are a conglomeration of parts  (Thomson, Keith, Stewert.  1991).  It is not suspected that Latimeria  ever lived in the Mediterranean Sea.  Most likely, Latimeria ‘s relatives ranged the oceans until a group branched off on an island chain.  As this group specialized, its ancestors died out, while it migrated to new habitats until it encountered the Comores.  Most of the changes would have been behavioral.  Even with modern science, the coelacanth has remained relatively free of  influence, except for the native traditions of the Comores’ fishers. 

The history and excitement of the first catch is a marvel of science; an actual living fossil, alive and free in the world.  After fifty years, the elusive Latimeria  has still kept itself shrouded in mystery. We understand its importance in evolutionary lines,  but still cannot prove a direct link. .  It has the genetics to be an ancestral form for all tetrapods,  but does not fit  perfectly into the  scheme of things already established.  We understand its ability to survive and adapt to new habitats, but we don’t know how.  The coelacanth is still a dinosaur to scientists, and will likely remain so, as the following quote illustrates:

I met a miner.  He handed me a lump of coal with a 1909 sovereign embeded within.  I have a trilobite fossil preserved in a footprint of a sandal.  There is a room, in the basement of the museum of Natural History, which they keep locked, which contains a Tyrannosaurus with a wrist watch, and a Neanderthal skull with three gold fillings. 

     What are you going to do about it?

                                                                                                                                      -anonymous

This quote sums up the coelacanth accurately.  It answers some of our hardest questions, but only poses more challenging ones.  If we can accept the coelacanth, then much of what we consider factual is not as accurate anymore - and we cannot deny the evidence of the coelacanth.  It is then our choice to embrace the knowledge given by such an old and wish living fossil.                      

Sources Cited

Compagno, Leonard. (1979).    Coelacanths: Shark Relatives or Bony Fishes  San Francisco:  California Academy of Sciences.

Fisher, Suzanne, E.  & Whitt, Gregory. (1979).    Evolution of The Creatine Kinase Isozyme System In Primitive Vertebrates.   San Francisco:  California Academy of Sciences.

Forey, Peter. (1991).  Blood lines of the coelacanth.  Nature, vol. 351, issue no. 6325.  pp. 347-349.

Lagios, Michael.  (1979).  The coelacanth and the chondrichtyes as a sister group.  San Francisco:  California Academy of Sciences.

Lombardini, J. B. & Pang, Peter, K. (1979).  Amino Acids and Taurine in Intracellular Osmoregulation in Marine Animals.  San Francisco:  California Academy of Sciences.    

McCosker, John, E, & Lagios, Michael.  (1979).  The Biology and Physiology of the Living Coelacanth.  San Francisco:  California Academy of Sciences.

McCosker, John, E.  (1979).  Inferred natural history of the living coelacanth.  San Francisco:  California Academy of Sciences.

Moyle, Peter B.  (1988).  Fishes: An Introduction to Ichthyology. Edgewood Cliffs, New Jersey:  Prentice-Hall Inc.

Neill, W.E., 1990.  Induced vertical migration in copepods as a defence against invertebrates predation.  Nature 354: 524-526.

Patterson, C. (1977).  The Contribution of Paleontology to Teleostean Phylogeny.  New York:  Plenum Press.

Patterson, C., and Rosen, D. E. (1977).  Review of ichthyodectiform and other mezoic teleost fishes and the theory and practice of classifying fossils.  Bull.  Amer. Mus. Nat. Hist.  155:81-172.

Smith, J.L.B.  1949.  The sea fishes of Southern Africa.  Capetown: Central News Agency.

Smith, J.L.B.  1956.  The search beneath the sea.  New York: H. Holt & Co.

Thomson, Keith, Stewart. (1991).  Living Fossil: The Story of the Coelacanth. New York:  W.W. Norton & Co.

Thomson, Keith, Stewart. (1969).  The biology of lobed fined fishes. Biol. rev 44:91-154.