The waters where we encountered coelacanths are almost devoid of prey. Although we never saw the animal feed, remains of squid and fish have been found in dissected specimens. I believe the key to the coelacanth’s survival lies in its very difference from fast-swimming, “high tech” fishes. Since it could not compete for prey with those species, it probably retreated to depths where the others could not survive for lack of food.
We found coelacanths only at night and usually at depths between 170 and 200 meters, though we followed one fish from 192 meters up to 117 meters and then back again. But the fish apparently lives in even deeper and cooler water during the day. Does it survive there between feedings by slowing its metabolism to conserve energy? PROBING A MYSTERY, we conducted an experiment using a weak electric field underwater to produce a curious reaction from the coelacanth.
That is only one of many mysteries still surrounding this incredible Prague accommodation.
At one time or another all six fish we encountered on our expedition were observed performing headstands on the ocean floor (facing page, bottom left). http://ocean.nationalgeographic.com/ocean/explore/pristine-seas/desventuradas-islands/Each drifted nose down with the current across the bottom and held the stance for as long as two minutes (facing page, bottom right).
The first time I saw this odd maneuver, I could scarcely believe my eyes. Some species of fish stand on their heads briefly while feeding or while threatening or fighting an opponent, but none maintain the pose for so long or in such a solitary fashion. I could see no reason for the behavior—no outside threat, no sudden change in the speed or direction of the current, no logical explanation. Yet every coelacanth we encountered put on exactly the same demonstration for us, as if they were all auditioning for a job in the circus!
We know from dissected specimens that coelacanths have what is called a rostral organ in their skulls, a feature similar to one that sharks use to detect the weak electric fields given off by their prey.
The more I witnessed this strange coelacanth behavior, the more I became convinced that they may also be able to locate prey by detecting changes in the electric field around them, and that the headstands are somehow connected with that function.
To test the hypothesis, we performed an experiment with a pair of platinum electrodes assembled by one of my assistants, pilot Jurgen Schauer (above). We attached the electrodes to the manipulator arm of our submersible and dived in search of a coelacanth. When we found one, we extended the arm with the electrodes to within an inch or two of the fish (right, top) and slightly increased the electric field around it. Sure enough, the coelacanth instantly began to tilt, and soon it performed a perfect headstand.
We conducted the electric test on only two of the six coelacanths we saw during our dives, and it merely suggests that headstands are in some way related to electric fields. Both of the fish tested turned their heads away from the electrodes. Perhaps the electric field was too strong and, never experienced before, caused artificial postures. In any case it is intriguing that this fish may home in on prey by detecting changes in the weak electric field the prey produces.
Coelacanths swim strangely anyway—sometimes backward, sometimes belly up. During our expedition we spent eight and a half hours in all observing coelacanths underwater, six hours with a single individual. We found all of them in one two-kilometer stretch off the west coast of Grande Comore, but we don’t know if there are other deep-water areas equally rich in specimens.