The Lateral Line System in Anuran Tadpoles: Neuromast Morphology, Arrangement, and Innervation

Abstract

Anuran larvae have been classified into four morphological types which reflect intraordinal macroevolution. At present, complete characterizations of the lateral line system are only available for Xenopus laevis (Type I) and Discoglossus pictus (Type III). We analyzed the morphology, arrangement, and innervation of neuromasts related to the anterodorsal and anteroventral lateral line nerves in 10 anuran species representing Types I, II, and IV with the aim of interpreting the existing variation and discussing the evolution of the lateral line in anuran larvae. We found: (1) the presence of two orbital and three mandibular neuromast lines in all anuran larvae studied,(2) the ventral arrangement of mandibular neuromast lines appears to have evolved convergently in Larval Types I and II, and the lateroventral arrangement of mandibular lines of neuromasts appears to have evolved in Larval Types III and IV; (3) interspecific variation in the organization, size, and number of sensory cells per neuromasts within the lines; and (4) the supralabial extension of the angular line in Lepidobatrachus spp. and the tentacular location of the oral neuromasts in X. laevis are concomitant with their particular morphologies. Based on the variation described we find that the lateral line system in anuran larvae seems to have been maintained without significant changes, with the exception of Lepidobatrachus spp. and Xenopus. These unique features added to other of Lepidobatrachus tadpoles are sufficient to propose a new Larval Type (V).