Evidence of force exchanges during the six-legged walking of the bottom-dwelling fish, Chelidonichthys lucerna

Locomotion in terrestrial vertebrates is supposed to be derived from preadaptation in bottom-dwelling fish. A few fish species have been assumed to walk on the substratum, on the basis of coordinated movements of their paired fins. However, the validity of this assumption has remained uncertain, because of a lack of evidence that their fin rays actually exert a force on the substratum. Here, we provide the first conclusive evidence that a benthic teleost fish, the gurnard, Chelidonichthys lucerna (Triglidae), exerts forces on the substratum during its temporary bottom-dwelling hexapod locomotion. This demonstration was achieved by the use of a photoelastic gel technique combined with a force calibration device. The movement patterns of the three first pairs of rays of the pectoral fins were analysed in relation to the forces exerted on the substratum, by measuring deformations of the photoelastic gel substratum produced by the rays. The rays were shown to produce a force pattern that confirmed the existence of a hexapod locomotion in a vertebrate that was consistent with body propulsion and voluntary substratum walking.