Coelacanth

• 1. Introduction

  ›

  A cable sent in 1938 carried a message of only a few words: "Most Important Preserve Skeleton and Gills = Fish Described." It concerned the coelacanth, an ancient group of lobe-finned fishes that fossils had placed firmly in the distant past. Scientists had good reason to think no living member remained. Then one surfaced.

  This documentary asks who recognised that fish, and how. It asks why a creature written off as extinct turned up in two seas far apart, one off Africa and one off Indonesia. It also asks whether the famous label "living fossil" survives a closer look at the bones, the habits and the fragile future of the animals that remain.

  195187577

• 2. The Catch at Chalumna

  ›

  On the 22nd of December 1938, a local fisherman landed a catch off the Chalumna River, on the east coast of South Africa. The river is now called the Tyolomnqa. Among the fish, a museum curator named Marjorie Courtenay-Latimer discovered the first living coelacanth specimen.

  She contacted J. L. B. Smith, an ichthyologist at Rhodes University, and sent him drawings of the fish. Smith answered with the cable that later became famous, confirming that the find mattered enormously.

  The specimen arrived more than 60 million years after the group's supposed extinction. That gap makes the coelacanth the best-known example of a Lazarus taxon. Biologists use that term for a lineage that seems to vanish from the fossil record and then reappears much later.

  The curator's part in the story is preserved in the animal's scientific name, because the genus Latimeria commemorates her as the person who found the first specimen.

  191852575

• 3. A Second Species

  ›

  In December 1952, a coelacanth from the Comoro Islands, off the east coast of Africa, joined the scientific record. Between 1938 and 1975-84 specimens were caught and recorded. Since 1938, this West Indian Ocean coelacanth, Latimeria chalumnae, has also been found in Kenya, Tanzania, Mozambique and Madagascar. It has turned up in iSimangaliso Wetland Park too, and off the South Coast of KwaZulu-Natal in South Africa.

  In September 1997, Mark V. Erdmann and his wife Arnaz Mehta were at a local fish market in Manado, North Sulawesi, Indonesia. They found a coelacanth there, but they managed only a few photographs before the specimen was sold. After confirming that the find was unique, Erdmann returned to Sulawesi in November 1997 to interview fishermen and search for more.

  In July 1998, a fisherman caught a second specimen and handed it to Erdmann. In 1999, Pouyaud and colleagues described the new species from that 1998 specimen. It was deposited at a facility of the Indonesian Institute of Sciences, known as LIPI. The Indonesian coelacanth, Latimeria menadoensis, lives in the seas of Eastern Indonesia, from Manado to Papua.

  The two living species are survivors of a much wider story. In the Paleozoic and Mesozoic eras, coelacanths lived worldwide, and their remains turn up on every continent except Antarctica.

  2726242825

• 4. Deep Time

  ›

  The oldest known coelacanth fossils are roughly 420 to 410 million years old, from the Pragian stage of the early Devonian. Eoactinistia, from Australia, is known only from a fragmentary jaw. Euporosteus yunnanensis, from China, is known from a partial skull and is regarded as the earliest anatomically modern coelacanth. Some authors suggest that Styloichthys, a slightly older fish, may also be an early coelacanth. Based on fossils, the split between coelacanths, lungfish and four-limbed land vertebrates is thought to have happened in the Silurian.

  In 1839, Louis Agassiz described a fossil from the Permian under the genus name Coelacanthus. The name comes from Greek words meaning hollow and spine, a reference to the hollow rays in that fossil's tail fin. As more species were discovered, Coelacanthus became the type genus of the order Coelacanthiformes.

  Over 100 fossil species have been described, yet coelacanths were never a diverse group compared with many other fishes. Their peak came in the Early Triassic, 252 to 247 million years ago, during a burst of diversification between the Late Permian and Middle Triassic.

  Size changed over time. Paleozoic coelacanths were generally small, while Mesozoic forms were larger. Two Jurassic and Cretaceous genera, Trachymetopon and Mawsonia, likely reached or exceeded 5 metres in length. That places them among the largest bony fishes of all time. Both belonged to the Mawsoniidae, a family that lived in brackish water, fresh water and the sea. Their cousins, the Latimeriidae, were marine and include the modern coelacanths.

  Megalocoelacanthus dobiei is the most recent known fossil latimeriid, found in marine strata of the eastern and central United States. Among the mawsoniids, Axelrodichthys megadromos lived in freshwater deposits in France, and an unidentified marine mawsoniid from Morocco dates to the late Maastrichtian. One small bone fragment from the European Paleocene is the only plausible later record, but the method used to identify it is of doubtful reliability. That fragment aside, the record falls silent around 66 million years ago.

  339104042

• 5. Built Differently

  ›

  Eight fins set a coelacanth apart from other lobe-finned fishes. It has two dorsal fins, two pectoral fins, two pelvic fins, one anal fin and one caudal fin. The tail is almost evenly proportioned and is split by a terminal tuft of fin rays. That arrangement is called a trilobate, or three-lobed, tail. The pectoral fins are lobed and relatively long, and the front dorsal fin is generally sail-like.

  The skull holds some of the group's most distinctive features. A pronounced joint in the skull may have strengthened the bite. At the front of the head sits the rostral organ, found only in coelacanths, which gives them electroreception. Coelacanths also lack the maxilla, the main tooth-bearing bone of the upper jaw in other fish and in land vertebrates.

  The scales are generally circular and overlapping, and they carry enamel-capped ornament in the form of denticles, tubercles or ridges. Inside the body, a coelacanth keeps an oil-filled notochord. This hollow, pressurized tube is replaced by a vertebral column early in embryonic development in most other vertebrates.

  The lung tells a different story across time. Most fossil coelacanths had a lung wrapped in a heavily calcified casing, and it was probably large enough to work. In living Latimeria, the lung is small, ringed only by scattered calcified plates, and appears vestigial.

  5455333230

• 6. The Living Fossil Label

  ›

  Popular science often calls Latimeria a living fossil. The reason is that it was long treated as the sole survivor of a group otherwise known only from fossils, with a body plan dating back about 400 million years.

  Recent studies challenge the idea. Some researchers now describe the belief that coelacanth shape barely changed as a view not based on data. Fossils show that the group was most morphologically diverse in the Devonian and Carboniferous periods. Mesozoic species, by contrast, look fairly similar to one another.

  Rebellatrix, from the Early Triassic, had a large forked tail. That shape suggests fast swimming like a tuna, and the fish was probably a cruising predator.

  The Bear Gulch Limestone, from the Lower Carboniferous, preserves five coelacanth species that lived at the same time. A study of them found signs that each followed a different way of life, which is called niche partitioning. Allenypterus had a tail with short rays that stretched far forward along the body. That design points to very slow, undulating movement, and the fish probably lived in weedy, sheltered places. Its small teeth and small gape suggest it ate small, slow, soft-bodied prey.

  Another Bear Gulch species, nicknamed "long body" in the study, is now recognised as the genus Caridosuctor. Stomach contents show it ate fish and crustaceans. It was probably a slow cruising predator and an environmental generalist.

  What Latimeria itself does with its fins is known from watching the living animal, and it is stranger than any fossil can show.

  5213575860

• 7. Drift Hunters

  ›

  After dark, living Latimeria hover in a state of near motionlessness, drifting through deep reef and volcanic slope habitats. They are nocturnal hunters of fish and other prey, including cuttlefish, squid, snipe eels and small sharks. Species of Latimeria take whatever is available.

  To find prey, they have been seen swimming head down, backwards and belly up, and even doing headstands. Their many fins let them point the body in almost any direction. For travel, they mostly ride up-currents and down-currents. The paired fins stabilize the body in the water, but on the ocean floor they play no part in movement. For a quick start, Latimeria push off with the tail.

  The rostral organ is thought to help at the last moment. It is suggested to detect where prey is when it comes close to the mouth, just before the final strike, rather than to spot prey from far away.

  Suction feeding appears to be ancient. Researchers suggest that living and extinct coelacanths used it, and that the trait had evolved by the end of the Devonian. Stomach contents of the Early Cretaceous mawsoniid Axelrodichthys, from Brazil, show that it ate fish. Its teeth were too small to break up its meals, so it swallowed them whole.

  The paired fins of Latimeria are very mobile and can move in ways unusual for fish. One example is a figure-of-eight pattern, used for precise movements.

  5354606559

• 8. Slow Births, Fragile Future

  ›

  In 1975, the American Museum of Natural History dissected its first coelacanth and found a pregnant female carrying five embryos. That finding revealed that living coelacanths are ovoviviparous. The female keeps the fertilized eggs inside her body, the embryos hatch there, and the young are born alive, which is rare among fish.

  The eggs are large, with only a thin membrane for protection. Females are typically larger than males. Pups arrive in groups of between five and 25, and they can survive on their own right after birth. A juvenile resembles an adult, but it has an external yolk sac hanging below the pelvic fins, larger eyes for its size and a steeper downward slope to the body. Coelacanths are believed not to reach sexual maturity until after the age of 20.

  Gestation was long thought to last 13 to 15 months. Research carried out in 2021 now suggests a gestation of up to five years. That is 1.5 years longer than the deep-sea frilled shark, which previously held the record.

  People have little reason to hunt the animal. The flesh holds large amounts of oil, urea and wax esters, which give it an unpleasant flavor and make it hard to digest and capable of causing diarrhea. Where coelacanths are more common, local fishermen avoid them for that reason. Their only real value is to museums and private collectors.

  The IUCN currently classifies Latimeria chalumnae as critically endangered and Latimeria menadoensis as Vulnerable. Their remote deep-sea habitat makes the true status hard to judge. The major threat is bycatch, especially from commercial deep-sea trawling. A female that may carry her young for up to five years cannot quickly replace what trawlers take.

  662636869