Innervation of the lateral line system in the blind cavefish Astyanax mexicanus (Characidae) and comparisons with the eyed surface-dwelling form

Abstract

The lateral line system and its innervation were examined in the blind cavefish Astyanax mexicanus (Characidae) and compared with that in the eyed surface-dwelling form of the same species. In both forms, superficial neuromasts occurred on almost the entire surface of the body and caudal fin, the number in the blind form being greater on the head (831) and tail (511) than in the eyed form (531 and 251, respectively), but lower on the trunk (1,305 vs. 2,125). In the blind form, superficial neuromasts were greatly proliferated on the cheek and lower jaw, the mandibular ramus being extensively ramified, including an additional major ramule that was absent in the eyed form. In both forms, superficial neuromasts on the trunk were innervated by a number of dorsal and ventral ramules (roughly corresponding to the number of lateral line scales) derived from the lateral ramus. In both forms, canal neuromasts in the infraorbital and trunk lateral line canals were innervated by nerves without associated elements innervating superficial neuromasts. Accordingly, the lateral line scales each accommodated only a single canal neuromast. This lateral line scale condition was recognized as common in Characidae, being in sharp contrast to the condition in Cyprinidae (both included in Otophysa) in which the lateral line scales each had a vertical row of superficial neuromasts.