388 ADAPTATIONS TO MEDIA AND SUBSTRATES 



cave fishes are believed to have become such through entering caves as 

 'strays'. On the contrary, much of the evidence suggests that the species 

 which have taken up residence in caves have ordinarily been well pre- 

 pared in advance to get along in lightless surroundings : 



Many fishes which live in rocky crevices, on muddy bottoms, or in 

 silty rivers and estuaries, have greatly reduced eyes. Some are even blind, 

 with a microscopic and sadly imperfect eyeball covered with opaque skin 

 or embedded deep in the tissues of the head. In such dim-light fishes, as 

 in many deep-sea forms, the other sense-organs are especially developed, 

 notably those of the tactual and chemical senses. The animals are thus 

 well fitted to find food where it is scarce as well as invisible. The an- 

 cestry of most cave fishes can be traced to such forms. It appears that the 

 typical cave species is one which has taken naturally to the cave and has 

 welcomed the refuge it offered — not one which has wandered in accident- 

 ally and been unable to get out again. Stray individuals of normal-eyed 

 species are encountered in caves, but many of these belong to groups liv- 

 ing outside whose way of life, and sensory and reproductive equipment, 

 would not seem to make them good recruits for the permanent cave fauna. 



Tiny-eyed, nocturnal, bottom-grubbing catfishes of several families 

 have contributed more cave species than any category of outsiders. No 

 cavernicolous gobies are known, but ichthyologists would not be sur- 

 prised to discover one at any moment, for many of the 'sleepers' live on 

 muddy bottoms or in crevices, and have degenerate or obsolete eyes (Fig. 

 133b). One intertidal species, Typhlogobius calijorniensis, shares its 

 rocky hideaway with a blind species of shrimp — a pair of the blind fishes 

 and a pair of shrimps inhabiting each burrow. When adult, the fish is 

 quite dependent for food upon the activities of the shrimp — almost a 

 case of the blind leading the blind! 



Some especially interesting contributions to the cave fauna have been 

 made by the family Brotulids. The brotulids are essentially a deep-sea 

 group. Some species (a couple of them, blind) have secondarily come 

 to the surface to live on reefs. Still others have made the doubly remark- 

 able transition to fresh water and the cave habitat — Stygkola and 

 Lucifuga in Cuba, and another (Typhlias) recently discovered in one of 

 the caves of Yucatan. In the brotulids, the amblyopsids, various families 

 of catfishes, and still others, we see clear indications that what has been 

 called *pre-adaptation' to relative lightlessness can lead to the easy 

 adoption of the cave habitat. And probably such pre-adaptation is prac- 

 tically indispensable, if the invasion of the cave is to be successful. 



