Blind cave fish show a remarkable ability to find their way around, despite their lack of eyes, as Rupert Collins explains.
The Astyanax mexicanus species comprises an eyed epigean (surface) form and at least 29 different pink, eyeless, hypogean (cave) forms, many of which are believed to have independent evolutionary origins over the last 10,000 years.
How the eyes were lost remains unclear; either as a result of random mutations in seemingly redundant eye-making genes, there being specific selective advantage in not having eyes — or that other cave-orientated adaptations have forced the indirect loss of them. Studies have indicated the latter idea as the best explanation so far.
Blind cave fish navigate, feed and reproduce with enhanced senses of smell, taste and feel. They do not shoal like surface cousins and do not feed in the same way either — by not feeding from the water column. They substrate feed at a different angle too, at 45 rather than 90°.
The cave form has also been shown to out-compete surface fish for food in complete darkness.
Keys adaptations for cave life are larger and increased neuromast (lateral line) cells around the head which sense pressure differences in water movement.
The lateral line organ senses pressure changes caused by underwater objects and the fish builds up and remembers a complicated spatial map.
These fishes have even been shown to respond to and ‘re-map’ a changing environment with an increased swimming rate.
This article was first published in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.