nditions. 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.
Gibbons adds that millions of years of adaptation to low-oxygen waters allowed the goby "to capitalize on the changes to the system—both increased hypoxia and [hydrogen sulfide] as well as the jellyfish." And that could help them cope with or even thrive under another big challenge: climate change, Gibbons says. "If climate change leads to increased winds and upwelling as some predict, then that means more unused phytoplankton production, more sedimentation and greater anoxia": perfect conditions for the bearded goby.
Regardless, the goby's superpowers are good news for rest of the region's food web, which has been able to continue to survive by switching from a diet largely of sardines to one of gobies. It also holds out hope for other altered ecosystems now dominated by jellyfish. "I suspect that in the fullness of time these other systems may indeed reveal species that feed on jellyfish," Gibbons says. "Most jellyfish-eating fish are generalist feeders after all, including this species."
And it's likely that the gobies have a few more tricks up their fins: Their relatives in other waters build and defend nests. "To grow and develop, eggs need oxygen, so the males and their nests are unlikely to be on the hypoxic mud," Braithwaite notes. And "how do the young larvae develop their physiological and behavioral adaptations? Are they innate or do they require the fish to learn or be exposed to hypoxic conditions to develop appropriate adaptations?"
Either way, jellyfish beware.