You’ll know about coelacanth, a lobe-finned fish that is closely related to the ancestor of tetrapods (four-legged vertebrates). The first pre-amphibian that crawled ashore might have looked much like this fish. Lobe-fins originated about 400 million years ago, and until fairly recently all but the lungfish were thought to have gone extinct about 80 million years ago. No fossils were found between then and the present, but then a live specimen was dredged up off South Africa in 1938, astounding biologists (read the whole story at the link!):
On December 23rd, 1938, the Nerine entered port after a stint trawling off the mouth of the nearby Chalumna River. The dockman called Marjorie [Courtenay, curator of a Museum in South Africa], who was busy mounting a reptile collection, but felt she ought at least go down to the docks to wish the crew of the Nerine a merry Christmas. She took a taxi, delivered her greetings, and was about to leave when, according to her account, she noticed a blue fin protruding beneath a pile of rays and sharks on the deck. Pushing the overlaying fish aside revealed, as she would later write, “the most beautiful fish I had ever seen, five feet long, and a pale mauve blue with iridescent silver markings.” Marjorie had no idea what the fish was, but knew it must go back to the museum at once. At first the taxi driver refused to have the reeking, five-foot fish in his cab, but after a heated discussion, he drove Marjorie and her specimen back to the museum.
It’s called a “Lazarus species,” because it came back from the dead. (Of course, there’s no guarantee that modern coelacanths would be reproductively compatible with their fossil relatives.) The next specimen wasn’t found until 1952, but now they’re caught by fishermen (accidentally) fairly regularly. Two species are recognized, with widely divergent habitats: Latimeria chalumnae, living in a few localities off East Africa, and the congener L menadoensis, known from two specimens off Indonesia, the second captured alive (but dying in a few hours). Their status as different species was first based on differences in skin color (a terrible way to diagnose two species as different), but their status is fully confirmed from differences later found in their mitochondrial DNA, which put the divergence between L. chalumnae and L. menadoensis at around 40 million years ago. (I’d bet there are other populations in between these areas!)
Here’s the African species photographed underwater:
The behavior of both species was largely unknown, but we now know a lot more about L. chalumnae from a new paper in Marine Biology by Fricke et al., based on 23 years of study around the Comoro Islands, including many hours of underwater observation by unmanned submersibles. I’ll just hit the high points:
- The population in the study area was stable about about 300-400 individuals.
- (This is known from earlier studies): the embryos aren’t expelled into the sea until three years after zygotes are formed: the longest period of embryonic development of any vertebrate.
- The submersibles watched 115 individuals over the period and were able to recognize individuals. Several were marked by the submersible with acoustic tags, allowing them to be tracked. One fish in 1987 was seen again in 2008, confirming that some can live at least 21 years as adults. But it takes them years to become adults, so the life span is undoubtedly much longer.
- The rate of natural mortality is estimated at about 0.04, or 4% of individuals dying per year. The authors note that there are no known predators on adults except fishermen.
- Based on this low rate of natural mortality, the authors estimate that the mean adult life expectancy is about 23 years, but that some individuals can live over 100 years. This is a long time, but in line with longevities of some other deep-water fish. Females are estimated to give birth only about 7 times during their lives, producing about 140 offspring.
- There is a strong size dimorphism, with males being larger (range 1.6-1.8 meters) than females (range 1.2-1.3 m). How do they know this? Because they measured live individuals with lasers, like this one (figure from Fricke et al.):
- Coelocanths hunt in the depths at night (they’re piscivores, or fish eaters), often at 500 m, and rest during the day (sometimes in small groups) in underwater volcanic caves at lesser depths (180-206 meters). It’s dark 500 m down, and I wonder how these fish hunt. Some individuals use the same “home caves” over and over again.
- Curiously, the authors did not observe a single subadult or juvenile coelacanth during the entire study. (One has been seen elsewhere). The authors hypothesize that the young stay down deep to avoid cannibalism by adults.
- The good news: the population off the Comoros is not declining, due mainly to the use of motorized fishing canoes that take fisherman further offshore, away from where the coelacanths live. The bad news is that coelacanth populations in Tanzania are far more endangered, since they’ve started using deep-water gill nets. More than 80 coelacanths have been killed in this way since 2003. The IUCN status of the African species is “critically endangered” and the Indonesian species as “vulnerable.”
Here’s a wonderful video of L. chalumnae photographed at depth. Note its curious “head-standing” behavior toward the end, which I think is a mystery to biologists.
This one was taken in a submarine canyon; you can see its distinctive lobe fins very well:
Fricke, H., Hans Fricke, K. Hissmann, R. Froese, J. Schauer, R. Plante and S. Fricke. 2011. The population biology of the living coelacanth studied over 21 years. Marine Biology 158: 1511-1522, DOI: 10.1007/s00227-011-1667-x.