Variation in cephalic neuromasts surface and cave-dwelling fishes of the family Amblyopsidae (Teleostei: Percopsiformes)
Cave adaptation has led to unique sensory specializations to compensate for the lack of visual cues in aphotic subterranean habitats. As the role of vision is reduced or disappears, other sensory modalities become hypertrophied allowing cave-adapted organisms to successfully detect and interact with their surrounding environment. The array of aquatic subterranean habitats, from fast-flowing streams and waterfalls, to quiet phreatic pools, presents a diverse palette to examine what possible sensory solutions have evolved against a backdrop of complete darkness. Mechanosensation is enhanced in many subterranean animals to such an extent that a longer appendage is recognized as a prominent troglomorphic adaptation in many metazoans. Fishes, however, not only interact with the environment using their fins, but also with specialized sensory organs to detect hydrodynamic events. We hypothesize that subterranean adaptation drives the hypertrophy of the mechanosensory lateral line, but that other environmental forces dictate the specific neuromast phenotype. To this end, we studied differences in the cephalic lateral line of the fishes in the North American family Amblyopsidae, which includes surface, cave-facultative, and cave-obligate species. Primarily surface-dwelling species, Chologaster cornuta and Forbesichthys agassizii, possessed receded neuromasts throughout most of the head, with a few on papillae located in front of the nostrils and on ventral grooves on each side of the mouth. The cavefishes Amyblopsis spelaea and Typhlichthys subterraneous possessed papillate superficial neuromasts all over the head. We propose that the change from the surface to the cave environment has led to papillate neuromasts in this group, which are likely shaped to detect the hydrodynamic characteristics of the boundary layer created by the swimming fish. Moving sensory organs from the surface of the body out into the boundary layer could increase sensitivity to high frequency stimuli created by prey, predators, and conspecifics.