Punting: An Unusual Mode of Locomotion in the Little Skate, Leucoraja erinacea (Chondrichthyes: Rajidae)

We investigated the use of the pelvic fins for locomotion along the bottom in the little skate, Leucoraja erinacea by video recording locomotor behavior of skates both in the field and in captivity and by examining various anatomical preparations of their pelvic fins. An external notch or concavity in the lateral margin of the pelvic fins partially separates each fin into anterior and posterior lobes. The skeletal elements and musculature of the anterior lobe are highly modified and comprise a functionally distinct appendage (the crus) that possesses three flexible joints. Locomotion of skates along the bottom is almost always due to the exclusive activity of the crura of the pelvic fins pushing off the substrate synchronously to generate thrust. The skate then glides through the water a short distance as the crura are repositioned for the next thrust phase. This type of thrust and glide locomotion is called punting. We conclude that punting is a significant form of locomotion suited to the benthic lifestyle of skates.

Fig. 1.

Ventral views of pelvic region of Leucoraja erinacea. (A) Mature female, showing the crura of pelvic fins. External separation of crura and posterior lobes (Pl) of pelvic fins occurs at level of the notch (white arrows) in lateral margin of each fin. C = cloaca; (B) cleared-and-stained, neonate specimen. The most cranial of the four cartilaginous radials forming the skeletal support for the crus (Cr) is highly modified and consists of a proximal element (Pe), middle element (Me) and distal element (De). Location of the joints between these elements are indicated by white asterisks. The posterior lobe of the pelvic fin is supported by more typical cartilaginous radials (R), which articulate with the basipterygium (B). Pb = puboischiadic bar

Fig. 2.

(A) Dorsal view of right half of pelvic girdle and first radial of Leucoraja erinacea showing movement possible at the joint between the puboischiadic bar and the first radial (dotted outlines) in the horizontal plane. Abbreviations: Pb = puboischiadic bar. (B–D) Cranial view of left half of puboischiadic bar of pelvic girdle and first radial showing primary movements possible at the different joints in the crus; (B) between compound radial and puboischiadic bar, (C) between compound radial and middle skeletal element, and (D) between middle and distal skeletal elements

Fig. 3.

Ventral view of a captive skate pressed up against the side of a glass tank showing the cranial edge of the three segments of the crura as they help to support the skate and also demonstrate range of motion at the different joints. In this stance, the proximal segment (Ps) is adducted, the middle segment (Ms) is flexed and the distal segment (Ds) is extended. Position of the three segments of the crura represents the position of these segments during part of the thrust phase of a punting cycle

Fig. 4.

Schematics of a lateral view of a punting cycle showing details of the movements of the different segments of the crura. A–F are higher magnifications of boxed area in drawing of skate at top of figure. The black bar under each figure represents the substrate. Proximal segment = dark gray; middle segment = medium gray; distal segment = light gray. Thrust phase = A–C; glide phase = D–E; beginning of next thrust phase = F. (See video sequence used for this drawing: captive video sequence; file 4)

Received: 02 Aug 2002

Accepted: 31 Jan 2003