Fish without eyes
It is sometimes quite shocking what some people think the work of an animal biologist consists of. One day, when visitors to our house took a look at my aquarium and admired the fish in it, my ears had to hear the following comment: “Very interesting, but doesn’t it hurt them when you remove their eyes?” “What do you think?”, came my reply, “they are blind fish. They never have any eyes! They are Mexican cave fish, Astyanax mexicanus.” Stupid visitors (I did not say that), but they obviously did not know that some species of fish are naturally eyeless.
Fishes living exclusively in dark caves or in the deepest parts of the oceans have no use for eyes. For them to have eyes would be like having webbed fingers for us: a burden. They would have to spend energy to maintain a useless structure and, moreover, run the risk that this superfluous structure gets injured or infected. They are better off without eyes. However, they need to compensate the lack of one sense (photoreception) with another. In deep sea fishes elongated fin rays or long barbels with sensitive touch receptors on them may take over the role of the eyes (rather similar to why a blind person uses a tapping-stick) or the sense of smell may be more keenly developed, or there are special sense organs like electroreceptors.
My own fingerlong Mexican cave fish would elegantly evade obstacles, never bump against anything including the tank’s glass walls or each other, but would react sensitively to human foot steps and other forms of vibration. In no time they would detect a mosquito larva or a wriggling worm, dropped into the water, frequently “playing” tug-o-war with such elongated pieces of food. Like other fish, the blind, pinkish Mexican cave fish have a lateral line organ with which they can detect water displacements and currents. But their lateral line system is better developed than that of most other fish species and is a key feature of their ability to survive and prosper without being able to see anything.
The reflections of the fish’s own bow wave from obstacles in the vicinity of the fish’s body are readily perceived and processed in the fish’s tiny brain, which on the basis of the information received would then send appropriate commands to the swimming muscles of the body and its fins. By swimming around and registering the “echoes” of their own movements, these fish can obtain a pretty good idea of the objects they are surrounded by and habitually begin to memorize them. A prey organism emitting its own “waves” by wriggling or struggling and disturbances of the usually calm waters dues to a rock fall or an earthquake perhaps, are immediately sensed and acted upon accordingly: approach in the first instance, but total and utter panic in the latter.
Thus, my advice to the visitors, who thought I had been cruel to my pet fish “if you want to be nice to my fish, please walk softly when you approach the aquarium and don’t step heavily on the ground.” This advice of mine just reinforced their view that scientists must be strange people.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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