Coelacanths are fish that predate the dinosaurs and still live in deep waters off the coasts of Africa and Indonesia today. They have unusually shaped bodies and lobbed fins on which they move on one side at a time, looking sort of like they are “walking” underwater. Their most ancient known ancestors lived in the Devonian Period, at least 410 million years ago, long before the first Tyrannosaurs roamed the Earth. It was once thought coelacanths went extinct around the same time the dinosaurs did, about 70 million years ago, as no fossils of them were found after that time. But then surprise, surprise, a living specimen was dredged up from the deep ocean in 1938 off the coast of South Africa! That fish, nicknamed “Old Fourlegs”, was thought to be the direct ancestor of all land animals (today we now know that is partially true but strictly not correct). [Source: Alice Clement, Research Associate in the College of Science and Engineering, Flinders University, The Conversation October 10, 2022]
Coelacanths (pronounced SEE-la-kanths) were named by the the eminent 19th-century naturalist Louis Agassiz. The word "coelacanth" comes from the Greek for "hollow spine" — a reference to the hollow spines that are part of its fin structure. Today there are two living coelacanth species, known as Latimeria, which have basically remained unchanged over the past 100 million years: the West Indian Ocean coelacanth (Latimeria chalumnae), primarily found near the Comoro Islands off the east coast of Africa, and the Indonesian coelacanth (Latimeria menadoensis).
Coelacanths are about a meter and half (five feet) in length and weigh around 65 kilograms (150 pounds). They are dark metallic blue with white spots — marking that may help camouflage the fish in its shadowy, dark habitat — and have a rostral organ in their skulls similar to ones used by sharks to detect weak electric fields given off by their prey. [Source: Susan L. Jewett, Washington Post November 11, 1998]
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
Books: “History of the Coelacanth Fishes; Environmental Biology of Fishes”; “Living Fossil: The Story of the Coelacanth”
The name "coelacanth" originates from the Permian genus Coelacanthus, which was the first scientifically named coelacanth. Once widespread in the sea and in fresh water, coelacanths have changed relatively little in 300 million years. Modern coelacanths have many characteristics that set them apart from other living fishes.
Susan L. Jewett wrote in the Washington Post: Their most distinctive feature is their lobed fins. , one of the primary characteristics that places them in the subclass of fishes whose only extant members are Latimeria and their closest relatives, lungfish that occur in freshwater in Australia, Africa and South America. Both are very primitive, occupying a bottom branch on the "family tree" that led to higher vertebrates.
The coelacanth’s fins move in a synchronized pattern typical of tetrapods, four-footed vertebrates — the right pectoral fin or forelimb is coordinated with and moves in the same direction as the left pelvic fin or rear limb. Most experts believe, however, that coelacanths are less closely related to tetrapods than are lungfish, which have less pronounced lobed fins but share other features with higher vertebrates, notably a primitive lung that allows them to remain out of water for long periods.
Coelacanths hand, might best be described as an offshoot from the main evolutionary stem that led to higher vertebrates — amphibians, reptiles, birds and mammals. Although placed in typical fin locations for most fishes, the lobed fins are structurally different. They consist of a fleshy base, scale-covered and lobe-like, supported internally by bones from which the fin rays radiate. In contrast, typical fish fin rays are supported by bone at the base but emerge from the main body of the fish rather than from a limb-like extension. The only normal-looking fin on a coelacanth is the first dorsal fin.
Coelacanths share characteristics with both major groups of living fishes: the cartilaginous such as sharks, rays and chimaeras with soft, flexible cartilage skeletons; and the bony fishes. Unlike most vertebrates, however, they lack vertebrae. Instead, they have a tough, hollow, fluid-filled tube known as the notochord, which runs from the skull to the tip of the tail and whose outline is clearly visible on the rear portion of the body.
In 1836, the naturalist Louis Agassiz first described a fossil fish he called Coelacanthus. Since that time more than 125 species of coelacanth have turned up in fossils, dating from about 410 million to 66 million years ago. Since 1938, when the first living coelacanth was caught, many other individual coelacanths have been observed. [Source: Peter Tyson, Ancient Creature homepage, 2003]
Coelacanth fossils have been found in sediments that have been dated back to 180 million years before the age of the dinosaurs.. The tail fins of the coelacanths found off the Comoros Islands looks almost exactly like the tail fins of 140 million-year-old coelacanth fossils. lungfish.
David Attenborough wrote in his book 1979 "Life on Earth": "Many species of coelacanth have been found as fossils. They are not large — thirty centimeters or so in length. Some specimens have been preserved in miraculous detail with every scale and fin-ray present. A juvenile was uncovered in the rocks of Illinois with traces of its yolk sack beneath its belly, plain to see. They are most abundant in deposits about 400 million years old, but thereafter they become scarcer and none has been found in rocks younger than 70 million years. Since they were flourishing during the period when the land was invaded and since they certainly possessed limb-like fins, it seemed likely that they were the creatures from which the first land vertebrates were descended. Their fossils were therefore studied with great care to try and determine exactly how they moved and how they breathed.”
It is estimated that the present coelacanths have survived for 30 million generations. The key to their success, scientists, theorize, is they have adapted to a niche in a somewhat hostile environment and therefore have not been threatened by more aggressive competitors.The German biologist and coelacanth expert Hans Fricke wrote: "I believe the key of the coelacanth's survival lies in its very difference from fast swimming, 'high tech' fishes. Since it could not compete for prey with those species, it probably retreated to depths where the others could not survive for lack of food."
A 66 million-year-old fossilized lung from a previously unknown species of the coelacanth family of fishes, was found in Morocco in the early 2020s. What this fish so remarkable was that
it was as large as a great white shark. Based on the size of the lung, the fish was estimated to be between 3.65 and 5.52 meters (12-18 feet) in length — substantially larger than modern-day coelacanths, which reach a maximum length of two meters (6.6 feet). [Source: Harry Baker, Live Science, February 20, 2021
According to Live Science: The fossilized lung was part of a large slab, uncovered in phosphate beds in Oued Zem in Morocco, which contained several other bones belonging to pterosaurs. The bones confirm that the coelacanth dates back to the end of the Cretaceous period 66 million years ago, just before the dinosaurs became extinct. Research was published in the journal Cretaceous Research.
During the Cretaceous period (145 million to 66 million years ago), there were two known coelacanth families: 1) the Latimeriidae, which survived to the present with the genus Latimeria, and the Mawsoniidae, which went extinct at the end of the Cretaceous. Enrico de Lazaro wrote in Sci News: The newly-discovered specimen belongs to the latter family, and is the last record of coelacanths before their pseudo-extinction 66 million years ago. “The thin bony plates were arranged like a barrel, but with the staves going round instead of from top to bottom,” said Professor David Martill, a paleontologist in the School of the Environment, Geography and Geosciences at the University of Portsmouth. “Only one animal has such a structure and that is the coelacanth — we’d found a lung of this remarkable and bizarre looking fish.” [Source: Enrico de Lazaro, Sci News, February 22, 2021]
The fossil was recovered from the uppermost Cretaceous deposits of Oued Zem. It had been embedded in a block of phosphate, backed with plaster and covered in a coating of lacquer, which had caused the bones to turn brown. It was found next to a pterodactyl which proves the giant coelacanth lived in the Cretaceous period.
On the fish’s size, Professor Martill said. “We only had a single, albeit massive lung so our conclusions required some quite complex calculations...It was astonishing to deduce that this particular fish was enormous — quite a bit longer than the length of a stand-up paddleboard and likely the largest coelacanth ever discovered.”
Coelacanths — Living Fossils
Susan L. Jewett wrote in the Washington Post: The phrase "living fossil" has been applied to several different kinds of plants and animals that developed in ancient geological times and have changed relatively little over a long period of time. Living fossils are important because they help scientists to understand the biology of prehistoric organisms by examining and observing closely related living forms. They also may help them to understand conditions that existed on ancient Earth. Thus, living fossils are, in a sense, "a kind of time machine that allows us to glimpse part of a lost biological world," writes Peter Douglas Ward in his book “On Methuselah's Trail”. [Source: Susan L. Jewett, Washington Post November 11, 1998]
The study of fossils has limitations. Much guesswork is involved, in part because of the fragmentary nature of the fossil record and because researchers are limited to physical evidence. When a living organism that shares anatomical characteristics of fossil organisms is discovered, it provides an opportunity to test hypotheses formulated from the fossil evidence alone. An example is the presumed function of the paired lobed fins in the coelacanth.
When only fossil evidence was available, many believed that the limb-like fins may have been used to "walk" on the sea floor and that coelacanths may have been a "missing link" to higher vertebrates, the tetrapods. Underwater observations of living coelacanths have disproven this hypothesis and shown no evidence that the fins are used to support the fish on the substrate. They also revealed, however, that the paired fins do move in a tetrapod's typically synchronized pattern, providing hints as to its evolutionary position relative to tetrapods. Another example of a testable hypothesis is the function of the coelacanth's intracranial joint. This feature is present in many fossil lungfishes and lobe-fin fishes, but in no other living vertebrates. In Latimeria, it appears to enable widening the mouth during sudden opening.
One must wonder what allows living fossils to survive for eons when so many other creatures vanished during Earth's many mass extinctions. Many hypotheses exist to explain this phenomenon. For example, these living fossils may exist in a relatively competition-free environment or be ecological generalists capable of living in a wide variety of environments. They may inhabit "relict habitats," regions of Earth shielded from most of nature's predators and from more recently evolved and more efficient competitors. Probably no single answer suffices.
Living fossils also provide a test case for measuring a paleontologist's ability to restore fossils. Latimeria was discovered after paleontologists had been reconstructing fossil coelacanths for 100 years. Interpreting openings to the previously unknown "rostral organ" — a structure in the snout that may house part of the coelacanth's electrosensory system — as nasal orifices was incorrect. But the reconstructions otherwise were generally accurate. This test provided what one expert has described as "a powerful endorsement for the science of comparative anatomy and vertebrate paleontology."
As with many questions in science, experts often do not agree on a specific definition of the term "living fossil." Charles Darwin coined the term. He considered living fossils to be "remnants of a once preponderant order" and forms "which like fossils, connect to a certain extant orders at present widely sundered in the natural scale." In this century, George Gaylord Simpson has described a mode of evolution characterized by very slow rate of change. Among these are forms whose evolution seems to have been arrested, forms that showed little or no evolutionary change over vast periods of time, otherwise known as living fossils. Keith Thomson, author of the book Living Fossil, The Story of the Coelacanth, defines the term as a "living representative of an ancient group of organisms that is extinct — it may for a long time have been thought to be extinct — but isn't. Usually this means, in addition, that the living representative is rare or at least uncommon and has a restricted geographical range.” Also "it is a member of a group that was formerly widely distributed in time and space, as indicated in the fossil record, and that otherwise became extinct, usually some long time ago. Finally, there is also usually the connotation that the living representative is itself very primitive in comparison with other groups of organisms, even closely related ones." British paleontologist Peter Forey points out, however in his History of the Coelacanth Fishes that Latimeria is appropriately viewed as a sole survivor of a once abundant group but is not a living example of the primitive member of a lineage.
Coelacanth Habitat and Where They Are Found
Coelacanth have been found at depths of 70 to 700 meters (230 to 2300 feet) in the Indian Ocean and have most typically been see at depths of around 400 meters. They live in tropical, saltwater, and marine environments and have been seen in coastal areas and in the open ocean far from land as well as in caves. The fish prefer cool water, ideally between 15̊ and 18̊ C (59̊ and 64̊ F, temperatures at which the hemoglobin in their blood shows maximum efficiency for absorbing oxygen and releasing carbon dioxide.[Source: Animal Diversity Web (ADW) /=\, Washington Post]
Most coelacanth have been found along the coast of East Africa. The largest known group — 300 or so — lives near the Comoros. Small breeding groups have also been found along the African coast from Kenya to South Africa. In 1997 a honeymooning U.S. marine biologist came across another species — Latimera menadoenis — being rolled on a cart at an Indonesian market
During daylight hours, coelacanths observed off of the Comoros gather in caves roughly 100 to 200 meters below the water surface where temperatures range from 16̊ to 22̊C (61̊ to 71.5̊ F). At night, when they hunt, coelacanths travel to depths ranging from 70 to 700 meters, depending on prey abundance and ambient temperature. /=\
Coelacanth have been caught by fishermen off Malindi in southeast Kenya The Tanzanian distinct population segment (DPS) of African coelacanth lives among deep, rocky terraces comprised of sedimentary limestone between 70 and 140 meters (230 to 460 feet) in depth. In this habitat, coelacanths are thought to use submarine cavities and shelves for shelter. They are found in temperatures around 68° F. The Tanzanian DPS is thought to represent a single, isolated population of the species, having diverged approximately 200,000 years ago. The size of the population is likely small, with no connectivity to other populations. It also has late maturity and the longest generation time of any vertebrate. [Source: NOAA]
The Comoros Islands, approximately 300 kilometers north of Madagascar and about 300 kilometers east of mainland of East Africa is home to the largest known coelacanth population. As of the late 1990s about 200 coelacanth had been caught, nearly all of them in waters off the west coast of Grande Comore (Njazidja) the main island in the Comoros Islands. Other have been found at Anjouan (Nzwani), the second largest Comoros Islands, Most coelacanths have been caught by long-line fisherman in search of eating fish. Comorans generally don't eat coelacanth because the meat is too oily. Some islanders used to use its rough, scaly skin as sandpaper.
Although it very difficult to come up with a firm figure, scientists estimate that about 500 coelacanths live in the caves off the Comoros Islands. The caves, which penetrate solidified lava flows on steep underwater embankments and are found to a depth of about 250 meters (820 feet), provide refuge to as many as 14 coelacanths at one time.
Coelacanth Size and Lifespan
Coelacanths are typically around 1.2 to 1.5 meters (four to five feet) in length and weigh around 40 to 70 kilograms (90 to 150 pounds). The largest one observed weighed around 90 kilograms (200 pounds and was two meters (6.5 feet0 long.
Among the African species females are considerably larger in size, and have higher thickness ratios and metabolic rates than males. The average weight of adult females is 82.1 kilograms, and their average length is 170 centimeters, for males the average weight is 37.2 kilograms, and average length is 125 centimeters. The basal metabolic rate for adult females is about 7400 cubic centimeters of oxygen per hour, compared to 4100 for males. [Source: Animal Diversity Web (ADW)]
The average lifespan of coelacanths is estimated to be 48 years of age. Female coleacanths reach maturity between 16 and 19 years, According to Animal Diversity Web: The lifespan of coelacanths is not known for certain. It is certainly at least 22 years, based on repeated observation of the same adult individuals over that time period. Froese and Palomares (2000) determined the expected lifespan of coelacanths to be 48 years using the equation Ap = t0 — ln(1 — p)/K. Fricke et al. (2011) proposed that coelacanths could live longer than 100 years using an alternate method, and pointed out that there are other deep-water fish species with similar lifespans. [Source: Nicholas White, Animal Diversity Web (ADW) /=]
Susan L. Jewett wrote in the Washington Post in 1998: No one is certain how long coelacanths live because experts disagree on the interpretation of growth rings laid down in scales, otoliths (ear stones) and bone. Depending on which approach is taken, the oldest specimen studied lived for either 11 or 22 years. [Source: Susan L. Jewett, Washington Post November 11, 1998]
Coelacanth Physical Characteristics
The coelacanth’s name is Greek for "hollow spine" and comes from the unique hollow fin rays first seen in fossil coelacanths. The fish has several other unique physical features. Most notably are its paired lobe fins that extend away from the body and move in an alternating pattern. The body of the fish appears iridescent dark blue in film or video footage but under natural light the color is light brown with white blotches throughout that have been used for individual identification. They also have thick, armor-like scales and a unique joint at the back of the skull that allows them to open their upper and lower jaws at the same time.[Source: NOAA]
Coelacanth are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (having a body temperature that fluctuates with the surrounding environment) and have bilateral symmetry (both sides of the animal are the same). A crystal layer behind the coelacanth's retina reflects light like a mirror, an advantage the dim waters where they live. The fish’s color and patterns probably serve as camouflage in its shadowy habitat. The fish possesses unusual stubby fins and rostral organ in their skulls similar to ones used by sharks to detect weak electric fields given off by their prey. Coelacanths probably are neutrally buoyant because of the high oil content of their liver and flesh as well as a swim bladder filled with oil rather than gas as in other fishes.
Unlike most vertebrate coelacanth lack vertebra. Susan L. Jewett wrote in the Washington Post: Instead, they have a tough, hollow, fluid-filled tube known as the notochord, which runs from the skull to the tip of the tail and whose outline is clearly visible on the rear portion of the body. [Source: Susan L. Jewett, Washington Post November 11, 1998]
According to Animal Diversity Web: “While they have many characteristics in common with other Osteichthyes (bony fish) such as their bony skeleton and diphycercal tail, some features are usually found in Chondricthyes (sharks and rays) such as their reliance on fat for buoyancy and ovoviviparity. Other coelacanth characteristics appear to be derived specializations (a vestigial lung and intracranial joint). /=\
Coelacanth bodies are covered in blue scales, which turn brown after death, with a white speckling that is unique to each individual. Their seven fins have fleshy lobes and they move their two paired sets of fins (pectoral and pelvic) in a diagonally synchronous manner like a four-limbed terrestrial animal. Unlike all other vertebrates, coelacanths possess an intracranial joint and an associated basicranial muscle. The purpose of this structure is disputed, with some experts arguing that it assists with suction feeding, while others suggest that it increases bite force. Another dissimilarity is their notochord, which in coelacanths is a hollow tube filled with fluid. /=\
Coelacanths have a fatty organ that serves the same purpose as a swim bladder. It was recently discovered that, within this fatty organ, a vestigial lung can be found that is surrounded by small hard plates. It is believed that these plates were involved in lung volume regulation in an ancestral species, but have become rudimentary in extant coelacanths. /=\
1) Rostral Organ is a large, jelly-filled, sensory organ between the eye and snout that perceives weak electrical pulses in the water, probably to help the coelacanth locate prey in the dark depths. Peter Tyson wrote: Evidence for this function first came from studies of the organ's anatomy and its nerves as well as the structure of the brainstem. Later, experiments conducted from a submersible confirmed that coelacanths can detect and respond to electrical fields in the water, strongly implicating the rostral organ for this role. [Source: Peter Tyson, Ancient Creature homepage, 2003]
2). Intracranial Joint was previously known only from fossils of primitive fishes. It allows the coelacanth to open its mouth especially wide in order to swallow fish and other prey. A pair of powerful muscles — the largest muscles in the head — cross the intracranial joint and are thought to allow the coelacanth to powerfully grip prey fishes between its teeth. The intracranial joint and rostral organ are not found in any other living animals.
3) Mouth of the coelacanth, aided by the intracranial joint and other cranial muscles, allows the fish to swallow its prey whole. Its teeth are designed not so much to grab or slice fish but to prevent them from escaping once the coelacanth has sucked them into its gaping maw. Beneath the chin lie large, bony plates.
4) Eyes of the coelacanth are large and attached to thick optic nerves. Each eye has few cones, which register color, but many rods, which detect light. So the coelacanth is virtually color-blind but can see extremely well in the dim light of the great depths where it lives. This ability to see in near darkness is enhanced by a layer behind the retina called the tapetum lucidum. Similar to that found in domestic cats, the tapetum acts like a mirror, intensifying weak light.
5) Brain of the coelacanth is small, occupying only about 1.5 percent of the braincase in mature individuals. Its weight is negligible: in a 200-kilograms coelacanth, the brain weighs only a few grams (less than a tenth of an ounce). Adult human brains typically weigh about a kilogram and a half (three pounds). No known living vertebrate has such a small brain in relation to its body size. However, having a minuscule brain has not gotten in the way of the species' long-term survival.
6) Gills of the coelacanth are similar to those of fish living at depths in which it is commonly found (150 to 243 meters, 500 to 800 feet). The gills have a relatively small surface area compared with the body mass of the fish — the gills of a slow-going rather than an active fish.
7) Scales are hard, rough to the touch and fitted tight together like armor. The roughness is produced by tiny, tooth-like spikes called denticles, which protect the coelacanth from rocks and predators. The scale color patterns of coelacanths resemble the walls of the caves in the Comoros where they spend their daytime hours and may play a role in crypsis (the ability of an organism to conceal itself especially from a predator by having a color, pattern, and shape that allows it to blend into its surrounding environment).
8) Notochord — a tough, fibrous,elastic thick-walled tube, which is partially hollow and filled with fluid — acts like a spinal chord. Many creatures with backbones have a notochord in the embryonic stage of development, which is replaced by vertebrae they are born. The coelacanth has no fully formed vertebrae, and thus the notochord give it support and provided the basis for its structure.
9) Swim Bladder lies beneath the notochord, surrounded in fat. In most fish, the swim bladder is filled with air, but in the coelacanth, it is filled with oil and fat. Swim bladder help fish boost their buoyancy.
Lobed Fins of Coelacanth
Susan L. Jewett wrote in the Washington Post: Coelacanths have many characteristics that set them apart from other living fishes. Perhaps their most immediately noticeable features are the two sets of paired lobed fins, pectorals and pelvics, lying to the side and beneath the belly. Those fins move alternately in a synchronized pattern typical of tetrapods — four-footed vertebrates. The right pectoral fin or forelimb is coordinated with and moves in the same direction as the left pelvic fin or rear limb” or the left pectoral fin and right pelvic fin do the same so it appears sort of like the fish is “walking” underwater.
Most living fish are ray-finned fish (Actinopterygii). The two living species of coelacanths, along with six living species of lungfishes, however belong to a different group — lobe-finned fishes (Sarcopterygii). Sticking out from the body on stalks rather than attaching directly to the body, paired lobe fins are supported by the same basic bones as your arms and legs. Coelacanths move their paired fins one side at a time: the right pectoral fin moves in conjunction with the left pelvic fin, for example. And their movement is extremely dexterous and precise. Coelacanth can scull the water like oars, hoover in one place and rotate 180°.
The coelacanth Its sail-like first dorsal fins provides stability while swimming. An extra tail lobe, unique to coelacanths, can be seen in the fish today and in fossils from hundreds of millions of years ago. The second dorsal and anal fins have stout, muscular lobed bases. The first dorsal fin, however, lacks such a lobe, but it can be raised or lowered like a fan to change its area.
The Tail is flat, muscular and powerful, enabling the coelacanth to thrust forward in short bursts to seize prey or escape predators. The tail is divided into three sections, with a small tail fin in the middle. Coelacanth discoverer Marjorie Courtenay-Latimer called the secondary tail fin a "puppy dog tail." Able to both rotate and flex from side to side, this “epicaudal fin” is thought to help the coelacanth with orientation and balance.
Although they are situated roughly in the same locations of fins of most fish, lobed fins are structurally different. They have of a fleshy base and are scale-covered and lobe-like, supported internally by bones from which the fin rays radiate. In contrast, typical ray-finned fish fins are supported by bone at the base but emerge from the main body of the fish rather than from a limb-like extension. The only ray-style fin on a coelacanth is the first dorsal fin.
Some scientists have speculated that the stubby fins of coelacanths were in fact primitive protolegs and the fish were a kind of missing link between sea and land creature. Fricke frequently saw the fish resting like a tripod on the ocean floor with the fins but never observed them walking with them.
Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated March 2023