Morphological differences in neuromasts of the blind cave fish Astyanax hubbsi and the sighted river fish Astyanax mexicanus.

Vital staining and scanning electron microscopy were used to study the morphology of superficial neuromasts in the blind cave fish, Astyanax hubbsi, and its sighted congener, Astyanax mexicanus. In blind Astyanax the neuromasts are 80 X 50 microns in size and possess cupulae with an oval basal shape. The length of the cupula is correlated to the location of its neuromast. Head neuromasts were found to have the longest cupulae (up to 300 microns), and caudal neuromasts have the smallest. Cupulae of all lengths have been observed to be easily bent by water movements. The neuromasts are directionally sensitive in that the bending of their cupulae perpendicular to the longer axis of the cupulae provides maximal excitation. A comparison of superficial neuromasts in the blind A. hubbsi to those in sighted A. mexicanus revealed several structural differences. The neuromasts in the blind fish are twice as large. Their cupulae, in particular, are much longer and seem to have supporting attachments at their edges. The greater length of the cupulae in blind cave fish may be of particular importance for the functioning of the lateral line organ, since longer cupulae protrude beyond the boundary layer in faster water currents and thus can increase the sensitivity of the neuromast. The specific morphology of the neuromasts in the blind cave fish appears to reflect an evolutionary adaptation which can serve to improve the functioning of the lateral line system and thereby compensate for the lack of eyes.