Salty Eels

Most third-graders know that panda bears aren't really bears, and starfish aren't really fish. Add freshwater eels to the list of creatures naturalists have misnamed. Japanese researchers report in tomorrow's Nature that at least some freshwater eels never leave the ocean.

Conventional wisdom holds that freshwater eels are born at sea, then slither upstream to freshwater rivers and ponds where they spend much of their lives before returning to the ocean to spawn. Scientists had thought that all freshwater eels make the trek upstream, but ecologist Katsumi Tsukamoto and his colleagues at the Ocean Research Institute at the University of Tokyo, Japan, wondered just how much time these eels really spend in freshwater.

To find out, the researchers examined a chemical passport, of sorts, written in the bony structure of the eels' ears. As eels grow and mature, these structures, called otoliths, become thicker as mineral layers are incrementally added. Otoliths consist mostly of calcium carbonate, but they also contain trace minerals from the environment. By reading the mineral components of each layer, the scientists constructed a record of the eel's environment at various life stages.

Tsukamoto's group collected 49 supposedly freshwater eels--19 from rivers in Germany and Japan, 18 from the North Sea and 12 from the East China Sea--and analyzed their otolith calcium/strontium ratios. Because strontium is more abundant in the ocean than in fresh water, a high strontium concentration signals time spent in the ocean. As expected, eels caught in fresh water had high strontium levels in the otolith layer correlating to early life, but layers grown after adolescence contained little strontium. Surprisingly, the eels collected from the ocean had high levels of strontium in every layer of their otoliths, indicating that, rather than visiting the ocean just to spawn, the eels had never left.

The finding debunks the notion that freshwater eels are obligated to spend time in fresh water. "This is going to turn a lot of heads," says ecologist Andy Wood at the North Carolina Aquarium in Fort Fisher. But Wood warns the results need confirmation in a larger sample size before they'll gain wide acceptance.