At high latitudes, the open ocean is treacherous in winter. The water is frigid and turbulent, food is scarce, and predators are on the prowl. So copepods, tiny herbivorous crustaceans that look like microminiaturized shrimp, head down deep and wait out the winter months in a hibernation-like state called diapause. A new study helps explain how they manage to stay down without constant swimming. Surprisingly, the trick is similar to the one sperm whales use when diving.
Copepods are abundant, calorie-packed morsels that numerous marine creatures depend on for food, so biologists are keenly interested in their survival strategies, especially diapause. During diapause, copepods gather in layers, generally between 500 and 3500 meters deep depending on the species, for 6 months at a stretch. They don't eat. They don't move. Their metabolism slows way down.
Yet after copepods gorge on diatoms at the surface all summer, their bodies can be half fat and presumably pretty buoyant. For decades, researchers have puzzled over how diapausing copepods manage to stay at depth without wasting calories swimming downward. After all, their lives depend on it. "If the animals can't maintain neutral buoyancy, then they burn up their energy really quickly, so they have to either reascend to the surface at the wrong time or they just die," says David Pond, lead author of the new paper, published this month in Limnology and Oceanography.
Pond, a biochemist, and Geraint Tarling, a zooplankton ecologist, both at the British Antarctic Survey in Cambridge, U.K., collected Calanoides acutus copepods from various depths in the Southern Ocean. Analyzing the copepods' fat stores, the duo found that the deeper the animals had been caught, the more fat they tended to have and the richer the fat was in compounds called polyunsaturated wax esters. Moreover, lab tests showed that these wax esters had the peculiar property of changing from an oily liquid to a butterlike solid at temperatures and pressures that occur below 500 meters in the Southern Ocean.
Pond and Tarling think that, as summer ends and Antarctic copepods journey downward to enter diapause, their fat solidifies and becomes denser and thus less buoyant. Once they pass 500 meters, it functionally transforms from balloon to ballast. The descent ends at a depth where the copepod is neutrally buoyant, which depends on the composition of the fat it stored eating diatoms all summer.
The researchers point out that sperm whales are thought to do something quite similar. When they dive, they pump cool blood to a massive fat store in their heads, so the fat solidifies and becomes dense enough that they can descend without much swimming. By adjusting the fat's temperature, they can keep it solid to remain at a particular depth, or liquefy it for an easy ascent.
Both species have a similar problem that they've adapted to in much the same way, Pond says. "[It's] a classic example of convergent evolution."
The new work gives "the whole idea of lipids a new direction," says physiologist Franz-Josef Sartoris of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. "Usually, you think about lipids as positive[ly] buoyant because they are less dense than seawater." Last year, Sartoris authored a paper outlining a very different buoyancy-regulating mechanism in which copepods use ammonium in their body fluids to help them float. Sartoris, Pond, and Tarling all agree that the two mechanisms probably complement each other to let diapausing copepods stay down deep.
Pond's paper tackles an important question with implications for the entire ocean food web, says Mark Baumgartner, a marine ecologist at Woods Hole Oceanographic Institution in Massachusetts. However, having studied copepods that appear to adjust their depth during diapause, he wonders whether their summer diet really predetermines their winter depth as much as Pond and Tarling suggest. He also notes that, as the paper acknowledges, the authors did not directly measure copepods' buoyancy, so it remains to be seen whether the fats really affect buoyancy, as the researchers propose. "The jury's still out," Baumgartner says.