Coelacanth: A Glimpse into the Deep Past

The coelacanth (seel-a-canth) is one of the world's most famous "living fossils." This incredible fish was once presumed to have gone extinct alongside the dinosaurs about 65 million years ago (mya). The world was stunned in 1938 when a live coelacanth was unexpectedly caught in a fishing trawl, revealing that the lineage had survived. Today, two living species are known, with the earliest fossils of coelacanths dating back an astonishing 420 mya to the Devonian period.

Anatomy: A Legacy of Lobe-Finned Fish

Coelacanths are one of the last remaining examples of lobe-finned fish, a group distinct from the more common ray-finned fish.

• Limb-like Fins: Their most notable feature is their fleshy, paired fins, which are supported by internal bones and move in a synchronized pattern much like the limbs of a four-legged animal.
• Unique Head: They possess a special electro sensory organ in their snout called the "rostral organ," which is gel-filled and helps them "see" in the dark by detecting low-frequency electrical signals. Additionally, they have a rare "hinge" in the skull that allows the front part of the head to swing up and enlarge the mouth's opening—a feature common in Devonian fish but absent in all other living vertebrates.
• Other Primitive Features: The coelacanth also has a hollow, fluid-filled notochord (a primitive spinal feature), incompletely formed backbones, enamel teeth, and an oil-filled gas bladder.

Species and Distribution

For many years after 1938, the only known living species, Latimeria chalumnae, was thought to be confined to the western Indian Ocean, primarily around the Comoros Islands.

However, a second population was discovered in 1997 off the coast of Sulawesi, Indonesia, nearly 6,000 miles to the east. Genetic analysis confirmed this as a distinct species, Latimeria menadiones, suggesting the two groups have been separated for millions of years.

Coelacanths are currently considered endangered and are protected globally by CITES (Convention on International Trade in Endangered Species).

Evolution and Significance

Coelacanths are classified as primitive bony fish (Osteichthyes). Their closest living relatives are the primitive lungfish of South Africa, Australia, and South America. While their exact evolutionary placement is debated, coelacanths hold a key position in the tree of life, as they are likely closely related to the ancestor of all land vertebrates (mammals, birds, and reptiles).

Recent research on their face and jaw structure has corrected previous misidentifications of muscles and ligaments, leading to a rewrite of the early evolutionary history of cranial muscles in all jawed vertebrates.

Ecology and Reproduction

Coelacanths inhabit the deep, cold, temperate waters of the "twilight zone," typically found between 500–800 feet (152–243 m) deep, often living near steep, rocky volcanic slopes.

• Behaviour: During the day, they are known to cluster in underwater caves, venturing out at night to feed.
• Feeding: They are generally passive drift feeders, moving slowly near the seafloor to hunt cephalopods (squid, cuttlefish, and octopus) and fish. New research indicates that, unlike most other bony fish, coelacanths must actively bite their prey rather than using suction feeding.
• Reproduction: Coelacanths have a rare reproductive mode called ovoviviparity, where eggs are fertilized internally, and the embryos develop inside the mother, feeding off a yolk sac for a long gestation period (thought to be about a year), before she gives birth to fully formed live young ("pups").

By Jorja-Lee Wright