"Under the endangered species act, buying or selling an endangered animal requires a permit. The permits are hard to get — even for zoos and aquariums."

"But there's a loophole."

"'If I donate or loan an endangered species to you, I need no permit,' says Kris Vehrs of the Association of Zoos and Aquariums."

"So a barter system has sprung up among zoos and aquariums to trade animals without using money. They even do it with species that aren't endangered. But barter can be complicated."

"For example: The New England Aquarium in Boston was recently in the market for some lookdown fish, and they knew of an aquarium in North Carolina that was willing to trade some."