Compared with teleosts, sharks are significantly disadvantaged; without a swim bladder, they have to swim from the moment they're born to prevent themselves from sinking to the ocean floor. Teleosts also have a versatile tail fin which they use for manoeuvring and changing depth by altering the angle of the tail's propulsive jet, but it wasn't clear if the shark's asymmetrically shaped tail gave them the same flexibility to plumb the depths. Cheryl Wilga and George Lauder have gathered convincing evidence that the sharks' heterocercal tails produce a single fixed angle jet, no matter which direction the fish swim in (p. 2365), forcing the fish to use its pectoral fins when changing depth.
How shark tails generate thrust has been hotly debated over the last 25 years. Two competing theories suggested that either the tail generates a jet that is angled downwards relative to the fish, or that the tail produces a jet along the line of the fish's body axis to thrust it forward. But if the first theory were correct, the fish would have to counterbalance the downward force from the tail jet to keep it level, and it was thought that the pectoral fins produced the balancing force.
Wilga and Lauder knew that the only way to resolve the problem was to visualise the vortices swirling around sharks' tails with digital particle image velocimetry (DPIV). Wilga explains that DPIV `is like a force plate for water'. By flooding the water with tiny polystyrene spheres and analysing the particles' trajectories as they whirl through a sheet of laser light, Wilga and Lauder could clearly see the direction of the tail's propulsive jet.
After analysing jets produced by both leopard and bamboo sharks, they realised that the results agreed with the first theory! Instead of producing a jet that lay along the axis of the fishes' bodies, both species produced a jet that was angled back and downwards, at an angle of 11°. Puzzled how the sharks prevented themselves from spinning head-over-heals, they analysed the different forces acting on the fish's body, and realised that the shark had angled its body upwards. So the angle of the fish's body produced the downward turning force, disproving a stabilising role for the pectoral fins.
Having figured out how the sharks swim at a constant depth, the team wondered whether the fish manipulate the angle of the tail fin jet while ascending or descending. Wilga remembers that the sharks needed some convincing to swim through the plane of light as they ascended in the water tunnel, but no matter whether the fish was rising or falling the tail fin's jet remained inclined at 11°, so sharks must have found some other way to change depth.
Having spent the last 11 years working on sharks, Wilga knew that they altered the angle of their pectoral fins as they glided up and down through the water. Tying the two results together, Wilga and Lauder realised that the way sharks manoeuvre to change depth is similar to the way a jet aircraft takes off and lands: the heterocercal tail is the shark equivalent of the jet engine, while the pectoral fins are the analogue of the jet wings' tilting flaps.