46 Sharks: The Animal Answer Guide
hicles, or AUVs. The mantas are still better, able to turn around quickly
in about half a body width, while the best AUV needs to move forward 1.3
body widths to complete a moving turn. The research involved in develop-
ing these AUVs has led to several discoveries about manta rays, including
what actually moves a manta through the water. Analysis of videos shows
that rippling waves of muscle contractions pass from the front to the back
of a manta’s pectoral fins and from the base of the fin to its tip as the fins
move up and down. (Recall that most rays move via such undulations pass-
ing back along their pectorals, the pectorals remaining relatively horizon-
tal.) The front-to-back waves of the manta fins actually provide four times
more forward propulsion than the up-and-down flapping. “Mantabots” so
far have managed to duplicate only the flapping parts of the movement.
Chimaeras swim primarily by flapping their pectoral fins up and down.
For forward movement to occur, the angle of attack of the fins would have
to change as the fin moves; otherwise, a ratfish would just bounce up and
down in the water. Available studies suggest just such a wave passes along
the pectoral fin with each flap, producing forward thrust. Movement is
made possible by muscles that extend farther out on the fin than in sharks
such as the Spiny Dogfish that don’t flap their pectorals. The ratfish down-
stroke is stronger than the upstroke, perhaps to provide some lift and keep
the negatively buoyant ratfish from sinking. Unlike aquatic birds (such as
penguins) and fishes (coral reef wrasses, kelp bed surfperch) that also flap
their pectorals when swimming, the ratfish moves the tip of its pectoral
fin in a vertical oval with each stroke rather than a figure eight. Chimaeras
once again do things differently from other animals, and much remains to
be learned about their swimming.
Some sharks and skates don’t just swim: they also walk along the bot-
tom. Little Skates (Leucoraja erinacea) move across the sea floor in a curious
manner called “punting,” named after the way that people move a small
boat across shallows by pushing against the bottom with a pole. The front
part of a Little Skate’s pelvic fin has leglike parts with flexible joints. A
Little Skate pushes off the bottom with both “legs” at the same time and
then glides on its outstretched pectorals. The skates can turn by putting
down one leg and pivoting on it (see http://faculty.une.edu/com/dkoester/
punting). The Epaulette Shark, Hemiscyllium ocellatum, also walks along the
bottom, pushing off with the front edges of its paired fins. It pushes with
its left pelvic and right pectoral simultaneously, then its right pelvic and left
pectoral, bending its body and moving very much like a giant salamander.
The cartilages in its paired fins have jointlike gaps that allow the fins to ro-
tate, making them more effective walking sticks.
Many high-quality videos of swimming elasmobranchs can be found by
searching the ARKive website, at http://www.arkive.org.
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