Jellyfish are returning to prominence in the world's oceans, thanks to a combination of overfishing, climate change and even "dead zones". A case in point is in the Benguela Current, an upwelling in the South Atlantic off the coast of Namibia and South Africa. Sardines thrived here, feeding on the rich blooms of plankton fertilized by nutrients carried along by rising deep ocean waters. The sardines, in turn, were consumed by everything from seabirds and sea lions to predatory fish like the mackerel.

Overfishing brought that to an end by the 1970s and jellyfish—as in many ocean ecosystems around the globe from the Yellow to the Black seas—came to dominate. That might seem an ecological dead end because few modern marine critters are equipped to cope with a jelly's stinging defenses. But that assumption has proved wrong in the Benguela, at least, thanks to a 13-centimeter-long fish known as the bearded goby (Sufflogobius bibarbatus).

"Using stable isotope analyses we were able to determine what the fish were consuming and, to our surprise, about a third of the goby diet comes from jellyfish," says fisheries biologist Victoria Braithwaite of The Pennsylvania State University, part of an international collaboration studying the current. "It shows for the first time that jellyfish tissue—once assumed to be a dead end for resources in the food web—can be recycled back into the ecosystem by the gobies."

And that is not the end of these gobies' tricks, as detailed in the July 16 issue of Science. Not only can they hide from mackerel amongst the jellies' stinger-covered tentacles when they rise from the seafloor to feed and digest at night, they can also survive for hours within the ocean floor's low- or no-oxygen muds, which are pervaded by poisonous hydrogen sulfide gas—an inhospitable place lacking any other vertebrate species. "The fish appear to reduce their respiration rate, but they don't entirely switch everything off because they still show a classic escape response if they are touched, even when under anoxic conditions. So they remain alert," Braithwaite says. "Just how they do this is unknown."

Not only that, the gobies' hearts are undamaged by a lack of oxygen and exposure to poisonous hydrogen sulfide gas—unlike other local fish, such as hake. "What we found is a fish with an amazing tolerance to anoxia and hydrogen sulfide that literally holds its breath on the seafloor by day—where it eats lots of carbon-rich mud," says biologist Mark Gibbons of the University of the Western Cape in Bellville, South Africa, another participant in the study. At night, it migrates into shallower, oxygen-rich waters where "it digests its daytime meal and refreshes its blood."

That is also when the fish hides among jellyfish, giving it a prime opportunity to prey on them, although it remains unclear when exactly the gobies are eating the jellyfish (they could be scavenging as well). Nor is it clear when the gobies first evolved these special traits. "The anoxic conditions have been present in this area for hundreds of years, but the large numbers of jellyfish are a recent occurrence, so their tolerance of their stinging tentacles is presumably a more recent adaptation, as is their use of jellyfish as a food source," Braithwaite says.

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