The Latest Pisces of an Evolutionary Puzzle
A fascinating fish that time hasn't forgotten
When Pieter C.R. Venter slipped beneath the waves off the northeastern coast of South Africa last October, he didn’t expect to find biological gold. Venter, a patent attorney on vacation from Pretoria, was taking the final dive in a series leading to his deep-water scuba certification.
The midafternoon sun shone brightly at the ocean surface, but little of that light filtered down to where Venter, his dive mate, and their instructor were exploring a cave-pocked canyon in the St. Lucia Marine Reserve. Near the end of the dive, at a depth of about 340 feet, Venter noticed a glowing eye in a nearby cave reflecting the beam of his underwater light. He swam over to investigate.
“We expected to see sea bass and grouper on the dive, so it took me about 6 seconds to realize what I was looking at,” he says. “It was a coelacanth,” the mysterious, oddly ancient-looking fish that many people deem a living fossil. Venter remembered learning about the fish when he was in school
Beneath an overhang, Venter saw three steely blue, lobe-finned fish–the largest was about 6 feet long, and the smallest measured about 3.5 feet.
“I was ecstatic, and I thought at first that I might have been hallucinating,” he says. Breathing a highly compressed nitrogen-containing mixture of gases for extended periods sometimes convinces divers that they’re seeing weird things, but Venter admits that a 6-foot-long coelacanth would have been an elaborate hallucination.
He waved to his diving partner to come to see the fish, but because their underwater time was running out, the companion couldn’t confirm the find before they began their ascent. Venter immediately began planning a follow-up expedition but almost as quickly had doubts about whether he had really seen a coelacanth.
A month later, Venter returned to the scene, this time with three companions and two video cameras. The images they captured proved beyond a doubt that the coelacanth, once thought extinct for millions of years, still swims in the waters off South Africa.
Fossil coelacanths
Much of the intense scientific scrutiny of the coelacanth (pronounced “see-luh-kanth”) and the public’s fascination with it stems from the fish’s possible evolutionary significance. Fossil coelacanths, which were first discovered in 1839, had an arrangement of bones in the fleshy portion of their fins that was similar to that in the limbs of modern land animals. For almost a century, paleontologists thought early coelacanths were close ancestors of the first vertebrates to walk on land.
Therefore, it caused quite a stir when the first living coelacanth was hauled from the deep by a trawler based in East London, South Africa, in late December 1938. This figure from the ancient fossil record was suddenly inhabiting the 20th century.
The coelacanth’s stumpy, lobed fins; its large, rough scales; and the bony plates on its head all gave the white-blotched fish a distinctively primitive appearance. A museum curator in East London tried to retain the fish’s internal organs for further study, but she couldn’t find enough chemical preservative. The hot, humid conditions in the port town took their toll, and it was up to the local taxidermist to save what he could of the find.
Another specimen
Scientists immediately began looking for another specimen. They believed that the detailed anatomy and biology of the living coelacanth would shed light on its ancestors, says Peter L. Forey, a paleontologist at the Natural History Museum in London. Not another coelacanth was found, however, until 1952, when a fisherman in the Comoro Islands northwest of Madagascar hauled one up from a depth of about 550 feet.
Since that time, scientists have documented catches of nearly 200 coelacanths worldwide and sightings of more than 100 others. Most have come from the Comoro Islands, but people have hooked or trawled a half-dozen or so further south, in the Mozambique Channel, which lies between Madagascar and the African mainland. In the late 1990s, two coelacanths were caught in Indonesia (SN: 9/26/98, p. 196), and two others were sighted there.
Most of the nearly 200 coelacanths taken in the Comoro Islands have been hauled up as by-catch. In other words, local anglers accidentally hooked the fish–known in the Comoros as Gombessa–when they were fishing for something else. Because coelacanth meat doesn’t taste good and can even upset a person’s stomach, the fishers had no reason to keep the fish before scientists came looking for specimens.
“We’d have known about coelacanths long before now if they weren’t so inedible,” says Forey. “They taste so bad, they get thrown right back.”
A long record
According to the fossil record, coelacanths first appeared about 380 million years ago. Although the rocks in which the fossils lie suggest that most ancient coelacanths inhabited shallow marine environments, there were freshwater species as well, Forey says. Scientists have identified more than 80 species in the fossil record and are still discovering new ones.
In the June 2000 Journal of Vertebrate Paleontology, Forey and three coauthors describe recently found fossils of one of
the earliest coelacanths, which probably lived in the brackish water of a river delta in what is now northeastern Latvia.
The fossil record indicates that coelacanths persisted for some 305 million years. That’s an impressive run, especially considering that they survived a mass extinction 245 million years ago that wiped out about 95 percent of all species in the seas (SN: 2/1/97, p. 74). In the wake of those extinctions, however, coelacanths reached their peak diversity–16 species living at once, Foley notes.
Ancient coelacanths came in many different shapes and sizes, ranging from just a few inches to around 6 feet in length. Today’s large, plump species are 5 to 6 feet long at maturity. They can’t accurately be called living fossils because they’re different from any fossil yet found. Forey says the living fish are, however, remarkably similar to the most recent of the fossil coelacanths.
Why the coelacanth suddenly disappeared from the fossil record 75 million years ago remains a mystery. It never was a commonly occurring fish, so its rarity probably contributed to its vanishing act, Forey says. Before the discovery of live coelacanths, there was no way to tell whether the fish really became extinct or simply moved into environments that wouldn’t naturally preserve fish remains. Today’s coelacanth habitats aren’t conducive to fossilization, he notes.
The largest known population of living coelacanths is found around the Comoro Islands, says Hans Fricke, a coelacanth specialist at the Max Planck Institute for Behavioral Physiology in Seewiesen, Germany. These volcanic islands popped up from the floor of the Mozambique Channel within the past 5 million years, and their slopes are too steep to hold sediments that could rapidly cover a dead fish and thus begin the fossilization process.
Fricke has led several expeditions to the Comoros since 1987 and has used submersibles and ultrasonic tracking devices to study the coelacanths there. Each fish has a unique pattern of white splotches on its side, which enables scientists to identify individuals. Fricke and his colleagues have spotted only 109 coelacanths during their research, but the scientists estimate that between 400 and 600 of the fish live in the waters of the Comoros. Repeated sightings of several recognizable individuals around Grand Comore, the largest of the Comoro Islands, indicate that the fish live in the same area for extended periods.
In the upcoming months, Fricke plans to study the South African coelacanths that Venter discovered 1,500 miles from the Comoros population and look for genetic differences between the two.
For the most part, coelacanths have a lethargic lifestyle. By day, they stay in deep-water caves, which shelter them from the strong currents that sweep past the Comoros. At night, Fricke says, the coelacanths range into even deeper waters, where both radio and direct observations indicate that they usually drift with the current to prey on other fish. Surveys show that very few fish populate the depths where the coelacanths forage, but researchers say that several adaptations make coelacanths an efficient predator that can get by on occasional nibbles.
First, the coelacanth has an extremely slow metabolism. Calculations show that it needs only 20 to 30 grams of food per day to fuel its lazy lifestyle, Fricke says. The fish also has a long spiral valve in its intestine, which slows the passage of food and enables it to be more completely digested.
Second, the coelacanth uses the same sort of electrosensitive cells as sharks and some other fish do. These animals detect weak electric fields emitted by their prey, says William E. Bemis, an ichthyologist at the University of Massachusetts in Amherst who has studied the coelacanth.
He says the coelacanth is unique among vertebrates because its electrosensitive cells are concentrated in a single organ, the so-called rostral organ in its head.
Another unique feature of the coelacanth is its intercranial joint, which enables the fish’s upper jaw to flex upward about 10 degrees when the lower jaw drops about 30 degrees. This increases the suction created when the coelacanth suddenly opens its mouth to gulp its prey. A large muscle in the fish’s head–as big as the muscles in a human forearm–flexes this joint and helps the coelacanth hold its prey, Bemis says.
Finally, the coelacanth can see extremely well in dim light thanks to its tapetum, a crystal layer behind its retina. Light reflecting off the tapetum gives the retina a second chance to capture photons that weren’t detected during their first pass through.
Comoros expedition
While on an expedition in the Comoro Islands last year, Fricke and his colleagues collected samples of DNA from 15 coelacanths. Their analysis will reveal the amount of genetic variation found in the Comoros population and serve as a baseline for comparison with DNA from coelacanths in other areas.
Fricke plans to take his submersible to South Africa next March or April to collect DNA from the coelacanths that Venter discovered last October. During this 2-to-3-week expedition, Fricke’s group will also examine whether the coelacanths in the St. Lucia Marine Preserve form a viable population. Before that, Venter plans to do his own series of dives to study the coelacanths he saw there.
Fricke says he’d love to discover genetic differences between the Comoro and South African populations, but he doesn’t expect to. The coelacanth expert predicts that the handful of fish outside the Comoros are simply strays. The fish’s lifestyle makes it prone to being swept away from home by sudden currents that often surge southwestward past the Comoros at the depths where the coelacanth forages.
Fricke is even more convinced that the Indonesian coelacanths found in recent years, like those off South Africa, are also strays. From tracing the currents that buffet the area where the fish were found, Fricke estimates that the undiscovered homeland of the Indonesian coelacanths may be near isolated islands in the Philippines or possibly further out in the South Pacific.
“I don’t believe we’ve found the mother population of coelacanths,” Fricke says.
Uncertain future for an ancient survivor
Now that the coelacanth has reappeared among the living, what are its prospects for survival? The short answer: Uncertain.
Despite detailed study of dozens of specimens, scientists still know little about this mysterious Lazarus of the deep. Pieter C.R. Venter’s sightings in South Africa last year were the first made by a free-swimming diver at the coelacanth’s natural depth.
Coelacanths brought to the surface don’t survive very long. The fish’s gills are small, so it can’t absorb enough oxygen from the relatively warm water near the surface. Also, its slow metabolism doesn’t allow it to easily recover from the stress of fighting at the end of a line or in a net.
The survival of coelacanths as a species depends in large part on their reproduction rate. But Phillip C. Heemstra of the J.L.B. Smith Institute of Ichthyology in Grahamstown, South Africa, says that the fish’s fecundity is one of its biggest mysteries.
Each female has only one functional ovary, which produces eggs the size of small grapefruit. In fact, coelacanths produce the largest fish eggs known–each about 4 inches in diameter. The eggs hatch inside the female, so she bears live young. Scientists don’t know when the fish reach their sexual maturity, how often they reproduce, or even how they mate–the male fish has no obvious intromittent organ.
The average size of a litter of coelacanth pups isn’t known. Of the nearly 200 documented catches, only 2 have been pregnant females. Although one carried 5 pups, the other carried 26. Based on these two pregnancies, scientists estimate that the pups are 14 to 16 inches long at birth. The smallest coelacanth ever caught was a pup about 17 inches long.
Regardless of how many coelacanths are born in a litter, nobody knows where they go immediately thereafter to mature, Heemstra says. Except for the one pup caught, all of the other specimens either brought to the surface or observed at depth have been at least 3 feet long.
Some researchers have used rings on the fish’s scales to estimate that the coelacanth reaches maturity at age 12 and has a 20-year life span. But Heemstra says that analysis of an otolith–an ear stone–from one fish suggests that the fish may live to be at least 60.
Coelacanths could live even longer. Recent observations suggest that the fish may have an exceedingly slow growth rate.
Fricke says that last November he recognized the blotch pattern on a 43-inch coelacanth that he had first seen from his submersible in January 1987. Although it was probably not a mature coelacanth–the largest specimens are typically more than 50 percent longer–he says the fish he identified had grown little, if at all, during the 14 years between sightings.
