Coelacanths

The term coelacanth refers to fish that are members of the order Coelacanthiformes, renowned for their multiple limb-like lobed fins and extensive fossil history. Prior to the 20th century, they were thought to have been extinct for approximately 70 million years, but the fish catapulted to worldwide fame when several living populations were discovered.

The ancient lineage of coelacanths is part of the order of lobefin fishes and currently represented by two living species, Latimeria chalumnae and Latimeria menadoensi. They display a remarkable combination of morphological features that are unique or rare among all living fish, among which the most obvious are their seven fleshy fins. These include several pairs of muscular lobed pectoral and pelvic fins that are quite dexterous, being able to rotate and serve as multidirectional paddles for greater control in currents.

The shape and placement of the bones in these lobed fins show a clear similarity to amphibians' forelimbs, and at times it was thought that the colloquially named “four-legged fish” might be the direct ancient relative of all land-dwelling vertebrates. However, the molecular and paleonto-logical evidence suggests that Coelacanthiformes are at most a side branch of an earlier ancestor that would later give rise to limbed animals capable of terrestrial domination.

Adult coelacanths can weigh up to 90 kilograms and grow as long as 2 meters, although they are often smaller. Specimens of L. chalumnae have thick scales of a pale mauve to blue color, while L. menadoensi is brown; both have flecks of iridescent white or silver. These markings provide excellent camouflage within their preferred habitat of encrusted rocky marine caverns, where they shelter at depths between 100 and 700 meters during the day, before foraging in shallower zones at night for fish, eels, skate, octopus, squid, and crustaceans. They are relatively sluggish, seen feeding by drifting head down in currents, aligning their gape with crevices in which to find food and swallow it whole.

The fossil record shows coelacanths emerged during the Devonian period, approximately 400 million years ago (mya). The group diversified into both marine and fresh water, becoming most abundant in the Triassic before suffering a slow decline terminating during the Cretaceous period 70 mya, after which no more fossils are known. Thus, the world was shocked when in 1938 a live specimen, dubbed Latimeria chalumnae, was caught off the coast of South Africa. Sixty years later, a new population of coelacanths (Latimeria menadoensis) was discovered north of Sulawesi in the Indonesian archipelago. Genetic analysis indicates that the two populations diverged about 5 mya.

The coelacanths have been an icon of evolution to both scientists and the public, thanks to their renown as a “living fossil,” seemingly surviving extinction and appearing entirely unchanged over time. Both terms are somewhat misapplied. Coelacanths have always been present in the world's oceans. Humans were just unaware or ignorant of their presence—as we surely remain for much marine life. And far from remaining unchanged over time, there are periods of rapid morphological evolution in their history. One of the oldest specimens from this group, for example, has an eel-like morphology (Holopterygius, from 385 mya, found near Cologne, Germany). Other relatives from the same epoch had round, disk-like bodies or a range of disparate morphologies. This indicates the group went through an early period of anatomical experimentation before setding on the distinctive features of present-day coelacanths.

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