Walking in skates (Chondrichthyes, Rajidae): anatomy, behaviour and analogies to tetrapod locomotion

LUIS O. LUCIFORA; ALDO I. VASSALLO

Buenos Aires

Congreso; I Congreso "Osvaldo A. Reig" de Vertebradología Básica y Evolutiva e Historia de la Ciencia; 2002

Pelvic fin walking in skates is common. However, the structure and function of pelvic fins have not been analyzed. We analyzed pelvic fin skeleton and musculature of five skate species and video-recorded walking locomotion by two captive specimens of Psammobatis bergi. Pelvic fins of skates of the genus Psammobatis and Rioraja agassizi are externally divided into an anterior leg-like lobe and a posterior fin- like lobe. This division is absent in Sympterygia bonapartii. Internally, the anterior lobes of all skates examined are supported by a compound radial, a proximal radial and distal radials which resemble a thigh, a calf and a foot, respectively, and three associated radials arising from the pelvic girdle. A highly developed radial condyle on the pelvic girdle enables broad ?limb? movements. The muscular arrangement of the anterior lobes is formed by protractor, retractor, flexor and extensor muscles, clearly departing from the generalized fin muscle arrangement of elasmobranchs. Walking is composed of propulsion and recovery phases. A backward movement of the compound radial in the horizontal plane characterizes the propulsive phase. The proximal radial connects vertically the compound radial with the foot-like distal radials, which are anchored on the bottom. During the recovery phase the foot-like structure is lifted off the bottom and the entire limb-like anterior lobe is moved forwards for starting a new cycle. Walking instead of swimming was commonly used in short distance bottom movements. Walking in skates resembles the ancestral tetrapod sprawling locomotion seen in many salamanders and lizards. Though convergent, walking and pelvic fin anatomy in skates is markedly different to those in hemiscylliid sharks. Ecologically, walking may be advantageous because it seems to be less expensive than swimming by undulating the massive pectoral fins. Walking may also help skates in maintaining themselves undetected by both prey and predators because it minimizes water displacement.