A somatosensory fovea in the fingertips of the Surinam toad

The Surinam toad (Pipa pipa) is a fully aquatic, tongueless pipid frog that captures prey using rapid suction feeding coordinated by the forelimbs. The digit tips bear distinctive lobed structures whose function has been unknown. Here we characterize the morphology, mechanical sensitivity, peripheral innervation, central representation, and behavioral function of these digit tip lobes using scanning electron microscopy, von Frey threshold measurements, in vivo electrophysiology of the brachial nerve and optic tectum, and high-speed video analysis (1000 fps). SEM reveals a progressive quadrification yielding 128 terminal lobules per frog, bearing dome-shaped papillae at 142 ± 18/mm²-nearly fourfold the density on adjacent digit shaft skin. Distal lobule tips have the lowest mechanical thresholds on the forelimb (median 0.07 mN). Brachial nerve recordings reveal disproportionately represented, low-threshold, predominantly rapidly adapting mechanoreceptive afferents with small receptive fields confined to the lobules, some also responsive to water current stimulation. Tectal mapping shows the digit tips are somatotopically represented and disproportionately magnified, occupying 34% of the forelimb representation despite comprising only 8% of forelimb surface area (magnification factor: 4.3×). High-speed behavioral recordings under visible light, infrared, and cobalt-blocked lateral line conditions demonstrate that frogs initiate suction feeding before prey fish contact the digits, indicating detection of hydrodynamic disturbances at short range (~ 5 mm). These results establish the digit tip lobes of Pipa pipa as somatosensory foveae-specialized mechanoreceptive organs analogous to the tactile fovea of the star-nosed mole.