Functional evidence for early origin of tactile acuity in the vertebrate somatosensory system.

Mammals and reptiles possess a sophisticated somatosensory system for precise tactile discrimination via mechanosensory end-organs, such as Meissner and Pacinian corpuscles and others. These structures detect sustained pressure, velocity, and vibrations, thereby facilitating nuanced environmental interactions. It is not known whether the ancestral anamniotic somatosensory system, typically lacking such structures, provides comparable tactile discrimination. Here, we investigate the Schnauzenorgan, a specialized foraging chin appendage in the mormyrid fish, Gnathonemus petersii, and show that it detects touch via functionally distinct myelinated mechanosensory afferents. Although these afferents terminate in the skin as seemingly free nerve endings, they detect sustained pressure, transient touch, velocity, and low- and high-frequency vibrations. Thus, despite lacking typical end-organs, the Schnauzenorgan enables tactile discrimination rivaling that of amniotic extremities. Our findings reveal a previously unrecognized functional complexity in the ancestral piscine somatosensory system, suggesting that the nuanced mechanosensory capacity of amniotes was inherited from anamniote predecessors.