Cheap trick. Eel larvae save energy by filling up with mucus.

How to Gain Weight on the Cheap

For fish larvae, life boils down to one thing: Eat or be eaten. And so most change into their adult shape as fast as they can. An intriguing exception are eels, ladyfish, bonefish, and tarpon, which spend several months, possibly years, floating as large, transparent larvae. Now scientists have figured out how these larvae bulk up while storing enough energy to metamorphose into juvenile fish The larvae, called leptocephali, have puzzled biologists for more than 100 years. Eels from both North America and Europe swim to the Sargasso Sea, a region in the Atlantic just east of Bermuda, to spawn. Their larvae, in a feat that would put Lassie to shame, somehow navigate back to their parents' continent to develop into adults. Now a new study, described in the 25 August Journal of Experimental Biology, is helping to clear up the equally baffling mystery of how the larvae grow.

To find out how leptocephali turn their food into energy, marine biologists Renee Bishop and Joseph Torres of the University of South Florida in St. Petersburg captured the delicate larvae. They carefully measured how much oxygen the larvae consume and other yardsticks of energy usage. Although larger organisms usually require more energy to support their numerous cells, the researchers found that the size and energy consumption did not increase in larger leptocephali.

The scientists propose that instead of adding new cells, the larvae pad themselves with mucuslike carbohydrates called glycosaminoglycans, deposited outside cells. The mucus is "a way to grow really fast, really cheaply," Bishop says; the larvae can grow up to a half-meter long without dramatically increasing their food requirements. Bishop says the glycosaminoglycans also serve as a low-cost skeleton for the larvae, supporting two layers of muscles.

Leptocephali "are very strange larvae, and this fits in that picture," says marine biologist Edward Pfeiler of Arizona State University in Tempe. He says the new findings may also help scientists sort out the continuing controversy over how the animals can subsist on a meager diet.