Undulating their cape-like wings to swim gracefully, skates and rays (batoids) are some of the most enchanting inhabitants of coastal waters. But did you know that many also ‘punt’ along the bottom? Laura Macesic from Mount Holyoke College, USA, explains that some skates and electric rays propel themselves at high speed across the sea floor by pushing against the seabed with their small pelvic fins – it almost looks like running – while others supplement their pelvic fin punting action by pushing with their wings. And they do all this without having a single bone in their bodies: the skeletons of skates and rays are built from cartilage. Intrigued by the mobility range of batoids, Macesic and her colleague Adam Summers decided to find out how a punting lifestyle influences the tiny pelvic fin skeletal structure – the propterygium – that skates and rays use to push off with. They decided to investigate the stiffness and other mechanical properties of propterygia from accomplished punters to non-punters to find out whether the fish's lifestyle fine-tunes the structure to match (p. 2003).
Joining Summers at his University of California Irvine laboratory, Macesic measured the flexural stiffness of the left and right propterygia from true punters (the lesser electric ray and the clearnose skate), two punters that supplement their punting action by pushing with their wings (the yellow stingray and the Atlantic stingray) and the non-punting pelagic stingray, Pteroplatytrygon violacea, which lives in open water. She says, ‘Flexural stiffness is an object's ability to resist bending when a load is applied. A flexurally stiff object would be better to push off with – you'd get more force transferred.’ Supporting the bone-like skeletal element at two points and pressing down in between with an indenter, Macesic found that the flexural stiffness of the punting rays' propterygia was the highest (around 126.1 N mm2), with the punting clearnose skate having the highest flexural stiffness for its size. Meanwhile, the propterygia of the supplemented punters had intermediate flexural stiffnesses and the non-punting pelagic stingray had the lowest flexural stiffness (94.9 N mm2).
Curious to find out how the skates and rays achieve such a wide range of flexural stiffness from cartilage-based structures, Macesic began looking at other aspects of the propterygium structure. Taking cross-sections through the propterygia and looking at the distribution of cartilage across the structure, Macesic saw that the clearnose skates oriented the thickest dimension of the elliptical propterygia along the direction of the punt force, protecting it from bending. However, the non-punting pelagic stingrays oriented the thinnest dimension of their propterygia along the punt force direction, reducing the structure's flexural stiffness, making it useless for propulsion along the seabed.
Next, Macesic analysed the mineral composition of the propterygia and found that the supplemented punters' propterygia were stiffer than those of the open-ocean species because they had a higher mineral content. However, when she compared the mineral content of the cartilage propterygia with that of bone, it was only a fraction of that of mammalian spongy bone, even though the cartilage was as stiff as some mammalian bones. The team suspects that that the tiled structure of the surface of the punter's propterygia contributes to their remarkable stiffness.
So punting rays and skates have stiffened their propterygia in response to the small forces they experience as they propel themselves across the seabed, and Macesic adds that the open-ocean P. violacea, which is poorly equipped for punting, ‘has seemingly lost the adaptation for benthic locomotion.’