Living Fossils: The Coelacanth

It may have been discovered in 1938, but it's still hard to find a fish that people get more excited about than the Coelacanth. It's nowhere near the flashiest fish around, but to anyone who's ever spent much time around fish, it's an immediately striking one. It's got a wide variety of features that distinguish it from other fish, but the biggest one? It's a member of an otherwise nearly extinct clade of fish known as the lobe-finned fish that had their heyday on Earth before the dinosaurs even arrived. On top of that, lobe-finned fish are our ancestors, making coelacanths our great-great-great-ad nauseum uncles.

The Indonesian coelacanth, Latimeria menadoensis, the second species discovered. [Image source]

Coelacanths are deep dwelling fish (~200 meters) that grow to around the size of a human being. (~5 feet). They're slow moving, except for the occasional burst of speed to catch prey or escape danger. They have extremely photosensitive eyes- they only hunt at night in deep water, and will remain in their caves during the day or even the full moon to avoid damaging their eyes. They also possess a strange sensory organ called the rostrom in their nose to help detect the electromagnetic fields of their prey. Their curious three lobed tail is possessed by no other animal in existence.

Each coelacanth is easily recognized as an individual. While the two species are slightly different colors, both have light colored scales scattered among the dark scales. The large, armor-like scales are arranged in unique patterns on every fish, so scientists diving or piloting subs in the deep can easily learn to recognize individuals.

Their most unusual feature, however, is their fins. Instead of the branching bones found in the fins of ray-finned fish, which compose most fish species alive today, they have a single thick limb running down the center of each fin. Essentially, it's the limb that evolved into legs on land animals. The fins have a joint similar to human shoulder or hip joints, allowing 180 degree rotation. They move their fins in a manner remarkably similar to the way that lizards walk- moving a front fin at the same time as they move the back fin on the other side. This makes the clunky, awkward looking fish into one of the most graceful swimmers in the ocean. They can swim forwards or backwards with ease, can turn on a dime, and are often found doing headstands. They can even swim upside down!

Coelacanths and lungfish, the only other surviving lobe-finned fishes on the planet, are actually more closely related to land animals, including us, than they are to other fish. While they're not our direct ancestors, they're reasonably closerelatives to them. They're still quite a distance away on the family tree, of course- they lack backbones, instead having primitive notochords filled with fluid.

[Image source]

These majestic living fossils were rare when we first discovered them, and have since gotten even rarer. The first species discovered, Latimeria chalumnae, has a population that might now be as small as 500 individuals or less. Many of them were caught for scientific research, but the greatest threat to them is as bycatch- Comoros fisherman often catch them when seeking oilfish, who are a similar size and have similar hunting habits. The Indonesian coelacanth, Latimeria menadoensis, is in better condition, with a population measuring possibly as high as 10,000, but still remains quite vulnerable. A black market trade in them has also started- Chinese medicine has added them to the list of endangered species that provide supposed miracle cures- in this case, coelacanth notochord fluid is claimed to grant immortality. Active protection of coelacanths is necessary if we are to preserve these wondrous remnants of ancient time.

If you're interested in reading more about the coelacanth, I highly recommend reading Samantha Weinberg's excellent A Fish Caught in Time: The Search for the Coelacanth.

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Bibliography:

A Fish Caught in Time: The Search for the Coelacanth, by Samantha Weinberg

http://vertebrates.si.edu/fishes/coelacanth/coelacanth_wider.html

https://en.wikipedia.org/wiki/Coelacanth

https://en.wikipedia.org/wiki/Sarcopterygii