Kangal, Turkey--Tucked between brown hills in central Turkey is a natural hot spring where, for a fee, you can become fish food. Dip in a hand or foot, and within seconds small fish will swarm, bump and nibble it. Stand above the pools, and the fish will gather below, waiting. The scaly swimmers--the "Doctor Fish of Kangal"--supposedly have curative powers. But in this unusual case of adaptive ecology, the human visitors may be helping the fish more than themselves.

These fish have acquired a taste for humans largely because they have little choice. The spring is too hot to sustain enough algae and plankton to feed them all. In the past, the fish were able to move between the spring and a creek that runs nearby. But after learning of a story about a local shepherd whose wounded leg healed after being dipped into the spring in 1917, builders walled off the spring from the creek in the 1950s to preserve a captive school. A Turkish family has now constructed a hotel, villas and a playground and markets the resort to psoriasis patients. Some 3,000 people every year pay for the privilege of sitting in the spring and allowing these omnivores to eat their dead skin, a process that may stimulate new skin growth or relax patients and thereby ease stress-triggered psoriasis.