THE BLIND CAVE FISH 



A Classical Problem of Evolution — Many Theories to Account for Its Loss of 



Eyesight — Cave-Darkness Probably Allows Unfavorable 



Variations to Survive^ — Life Habits of the Fish 



OF ALL the strange creatures that 

 live in subterranean waters, none 

 is more familiar than the blind 

 lish.^ Fig. 4 shows one of the 

 most commonly known, and perhaps 

 the most interesting, of the blind fishes, 

 that inhabit the Alammoth Cave of 

 Kentucky. 



This fish (Amblyopsis spclaens) at- 

 tains a length of 5 or 6 inches when 

 adult, and ^'aries in color through several 

 shades of light pink and white. In 

 general shape, it resembles a minnow of 

 equal size. Such a comparison holds 

 true only for general appearance, how- 

 ever, as upon closer insijection a mul- 

 titude of adaptations are to be seen. 



The eyes in most specimens have 

 entirely disappeared, so far as can be 

 seen by external examination, although 

 in many instances dark spots may be 

 detected where the eyes ought to be. 

 The structure of the vestigial eye differs 

 greatly in different individuals, but a 

 microscopic examination will always 

 reveal some traces of the organ. In 

 some specimens, an external eye is 

 found, presumably a throw-back to 

 the original condition. 



It has generally been stated that 

 Amblyopsis brings forth its young 

 alive, but it now appears that such 

 statements are based on ignorance. 

 The eggs issue from the oviduct and 

 lodge within the gill cavities of the 

 female. It is probable that they arc 

 fertilized in that situation, as unfer- 

 tilized eggs are often found there. The 

 young fish are also guarded in the cavi- 

 ties for some time. If the eggs do not 

 hatch, they remain in the gill cavity 

 until they disintegrate. 



The young are blind when halched, 

 and presumably when in an embrN'onic 

 stage as well, and remain so throughout 



the duration of their life, irrespective 

 of their surroundings. A'arious experi- 

 menters have taken very young fish or 

 even eggs, and raised them in bright 

 light, but in every case the environment 

 has proved to have little or nothing to 

 do with any development of the eye. 



Amblyopsis spclaeus is found in vari- 

 ous localities in Kentucky, Indiana, 

 Michigan and Ohio, nearly always in 

 pools in the limestone caves of those 

 States, but is occasionally found in 

 deep wells, and even in open water, 

 where it is possible it may have been 

 brought accidentally. It probably lives 

 to a large extent on small organisms 

 which m.ay be invisible to the eye, but 

 it also eats small arthropods when such 

 are procurable, and in captivity will 

 even eat small pieces of meat. 



DEPENDS ON SENSE OF TOUCH 



Since the fish is entirely devoid of 

 sight, it must necessarily depend upon 

 other senses for securing food, and 

 probably the most important of these 

 is the sense of touch. Tactile papillae, 

 which are akin to the antennae of 

 insects, but are generally like small 

 ridges in the skin, are located profusely 

 on various parts of the body, especially 

 around the mouth and head and on the 

 \'entral fins. If, when swimming close 

 to the bottom, the fish touches some 

 substance with its ventral fins, it will 

 nearly always stop and investigate. It 

 seems that it is unable to detect whether 

 the substance is edible or not by the 

 fins, but the papillae around the head 

 are so developed that they can tell the 

 difference Ijctwecn edible and non- 

 edible substances. 



It has been stated that the "hearing" 

 of this fish is remarkably acute, but 

 here again it appears that this acuteness 



» The most detailed account of cave fish is that of Carl H. Eigcnmann (Cave Vertebrates of 

 of America. Carnegie Institution of Washington Publication No. 104, \9W). 



448 



