Rivers aren't always the permanent features that some of us take them for. In arid countries, river beds can be dry for years, and when a river dries up its occupants have little choice but to up-sticks and find water or burrow and aestivate to survive. According to Angela Horner from the University of Cincinnati, African lungfish have gone for the second option. But before the soupy river dries hard, the fish are left floundering in the mud. Which made Horner and her advisor Bruce Jayne wonder how fish swim through mud(p. 1612).

First Horner needed to find a see-through mud: easier said than done. Initially she tried cellulose, but it behaved more like a gel than mud. Eventually, after hours of Googling, Horner hit upon EZ®Mud DP, a transparent gunge used in the petroleum industry to stabilise oil drills; and best of all it was non-toxic. Horner could make transparent mud analogues with viscosities ranging from 10 to 1000 cSt. Next she fitted electrodes down the left and right sides of eight fish to record the electrical activity in the white muscle as they swam through the fake mud. The team were ready to test out the fish's swimming technique, but would they swim in the experimental tank?

According to Horner, African lungfish are notoriously uncooperative, quite large and equipped with a vicious set of teeth, not the easiest of creatures to work with. But having carefully transported the wired-up fish to the swim tank, Horner and Jayne were pleasantly surprised when they began swimming. Horner filled the tank first with water and then with EZ®Mud DP at 10, 100 and 1000 cSt viscosities, before filming the fish and recording their muscle electrical activity.

When Horner and Jayne analysed the fish's swimming technique, they noticed that the fish only used their tails when swimming slowly through thinner fluids. However, as the viscosity increased the fish began wriggling their torsos too until they made swimming waves with the whole of their bodies, just like fast swimming eels. And when the fish were transferred to the thicker`muds', they were no longer able to swim steadily at the lower speeds they had attained in thinner fluids. The team was initially surprised, but eventually realised that like real mud, EZ®Mud DP is a non-Newtonian fluid, which thins under force resulting in unsteady slow speeds. `Sometimes they spin their wheels a bit' says Horner, describing how the fish sometimes swim hard until they suddenly burst free to swim through the thick mud.

The electrical recordings also showed how the fish use their muscles to power swimming. The team saw that the wave of electrical activity that moves down the fish's body was faster than the wave of physical movement. And as the viscosity increased, the electrical activity wave became even faster. Horner explains that by activating the muscle before it contracts, the fish prestiffen the body section when swimming through the thicker muds.

Horner says that understanding how African lungfish swim through mud is another piece in the puzzle of how fish swim. And while modern African lungfish are significantly different from our earliest tetrapod ancestors,they could help us eventually to understand how the first tetrapods dragged themselves out of the primordial sea.

Horner, A. M. and Jayne, B. C. (
). The effects of viscosity on the axial motor pattern and kinematics of the African lungfish (Protopterus annectens) during lateral undulatory swimming.
J. Exp. Biol.