Published online 17 September 1998 | Nature | doi:10.1038/news980917-8

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The tadpole’s tail

The tadpole’s tail fin is remarkably fragile - just a double layer of skin with no skeletal support. Yet despite being about as easy to tear as wet tissue paper, the tail fin is stiff enough to provide thrust when the tadpole is swimming.

In the latest issue of the Journal of Experimental Biology, Penny Doherty and colleagues from Dalhousie University in Nova Scotia, Canada, have managed the remarkable technical feat of measuring the mechanical properties of the flimsy tail tissue, and show how this delicate structure maintains its shape during normal swimming. They suggest that the combination of delicacy and ‘viscoelastic’ properties of the fin help tadpoles escape the grasp of predators.

The tadpole fin is a very simple structure. Unlike fish fins, which are supported by bony or cartilaginous rays, tadpole fins are just a double layer of skin, just over one-third of a millimetre thick, extending over the central muscular core of the tail. The skin consists of a layer of loose connective tissue covered on both sides by a dermis layer.

Despite its simplicity, the tail has to resist deformation to provide a thrust surface during the undulating swimming motion. This strength seems to come from the basement layer of the skin, which is an array of crossed collagen fibres, very like the body wall structure of many worm-like and aquatic animals. But the fibres run at an angle of almost exactly 45 degrees, which, explain the researchers, gives it equal support in both the horizontal and vertical direction. The parallel arrangement of fibres may also allow the tissue to act as a hydrostat, so that fluid pressure within the skin could also add to the stiffness.

To examine the mechanical properties in greater detail, and to try to find out how such a delicate structure maintains its thrust surface during swimming, the researchers measured how well the tail responds to stress and strain forces, vibrations, and stress relaxation tests. They designed a delicate rubber clamp to hold the tail without tearing and measured how much the tissue stretched and recovered after being pulled.

When stretched and released repeatedly at 3 cycles per second, the frequency of the preferred swimming speed the tissue retained its stiffness well when the deformation was small, showing that the cartilage arrangement and fluid pressure allows the fin to cope with the undulating swimming movements.

The researchers found that the tissue was surprisingly ‘viscoelastic’, despite being so fragile. Viscoelastic materials show a balance between the properties of elastic solids, which store energy and spring back to their initial position after deformation, and viscous liquids, which show irrecoverable flow and lose energy through internal friction as they are deformed. This means that the tissue recovers well and retains its structure after small deformations, but tears or stretches irrecoverably after large deformations. The tail seems to be designed so that these swimming movements are at the limits of what it can tolerate, and is easily damaged by tensile forces.

But there is an advantage to being as delicate as possible too. If the tadpole’s tail fin is caught by a predator, it tears easily, allowing the tadpole to escape. A more elastic tail, or one that was more stiff, might not allow the tadpole to escape. The tadpole can easily tolerate losing as much as a quarter of the tail without impairing swimming. When the risk of predation is high, the tadpoles may even grow bigger tails - not to improve their swimming speeds, according to the researchers, but to give them a better chance of leaving no more than a mouthful of fin behind.