CAVE FISHES 



387 



While the eyes of most good-eyed bottom fishes look perpetually up- 

 ward, those of one genus, Corydoras, periodically look sharply down- 

 ward. These are tiny South American armored catfishes which are popu- 

 lar as scavengers in home aquaria, and they are commonly believed to be 

 'the only fish that wink'. Since there are no lids, there is no true wink — 

 the eyeball simply rolls downward until the pupil is largely or wholly 

 concealed; and the gray superior conjunctiva, which is thus exposed, does 

 give the appearance, from above, of an upper lid going into action. The 

 utility of this phenomenon is not apparent. It might be suspected that 

 the fish has an upwardly-aimed fovea, and has to turn the eye down to 

 use the fovea for occasional horizontal vision; but serial sections of a 

 Corydoras eyeball have revealed no such feature. 



Fig. 133 — Microscopic, degenerate eyes of blind fishes. After Franz. 



a, a hagfish, Myxine glutinosa (internal parasite), b, a goby, Trypauchen wakt (littoral, 

 crevice-dwelling), c, an amblyopsid, Troglichthys rosce (cave-dwelling). 



Cave Fishes — The cave fishes come close to bowing themselves entirely 

 out of this book, for most of them have 'no eyes worth mentioning' (see 

 Fig. 133c). The origin of the cave habit, and the cause of the disappear- 

 ance of the eyes, are fascinating puzzles however. Cave fishes belong to 

 many different families and represent many independent invasions of the 

 cave habitat. All are members of teleostean families in which normal- 

 eyed fishes occur, though the eyes of these 'outside' relatives are some- 

 times very small. The North American group of cave forms (the family 

 Amblyopsidae) have but one non-cavernicolous representative, and the 

 eyes of this form {Chologaster, in the Dismal Swamp) are much reduced. 

 Only a very few cave species — notably, several catfishes of the genus 

 Rhamdia — have kept their eyes in good condition. One, the Mexican 

 Anoptichthys jordani, was lately shown to contain normal-eyed individ- 

 uals as well as others showing all grades of reduction of the eye, down 

 to obsolescence. Such forms have perhaps not long been in the cave envi- 

 ronment. But, with the possible exception of Anoptichthys, no known 



