Meet the bearded goby, a six-inch-long fish that lives in toxic mud, eats jellyfish, lasts for hours without oxygen, and has saved a coastal African ecosystem from a nightmare fate.
Over the last several decades, as other fish populations off the coast of the Namibia collapsed, jellyfish and bacteria populations exploded — a condition widely considered to be ecological an dead end, incapable of supporting rich webs of life.
But amidst this turmoil, the goby has thrived. It circulates nutrients that would otherwise be lost, feeds animals who lost their historic prey, and provides that rare thing: a happy, or at least not-so-bad, ending to an environmental disaster story.
The goby “has the ability to consume what were considered dead-end resources and convert them into bite-sized chunks for higher trophic levels,” said Mark Gibbons, a University of the Western Cape biologist. “Gobies have become anything but a dead-end resource. The gobies are now sustaining the rest of the ecosystem.”
Half a century ago, the bearded goby was just one of many species living in what’s known as the Benguela Large Marine Ecosystem, about 7,000 square miles of continental shelf off the coast of southwest Africa.
The region supported a prosperous commercial fishing industry, but overfishing depleted the northern Benguela’s keystone species, the sardine. By eating plankton and being eaten by larger fishes, sardines had provided a direct conduit between the bottom and top of the Benguela’s food chain. Now that link was gone.
Adding to the upheaval, naturally occurring upwellings of deep, cold water in the Benguela deliver nutrient loads that feed enormous plankton blooms, which feed oxygen-gobbling, dead zone-creating bacteria and eventually fall to the ocean floor, forming a toxic sludge. Methane gathers in the mud, belching out in fish-killing gas eruptions. Without sardines to eat the extra plankton, the effects of this natural feature became more pronounced.
Such radical stresses produced what ecologists call a regime shift. The web of life didn’t simply adjust a bit, but took a whole new form, one that didn’t require a rich assortment of fishes to circulate energy and nutrients. In this lowest-common-denominator system, there were only a few opportunist fish species, bacteria and, at the top of the food chain, giant jellyfish.
Giant jellies have no natural predators, and aren’t even eaten by humans. In the systems they dominate, nutrients and energy go from plankton to jelly, with little between. “The massive increase in jellyfish biomass after the collapse has been regarded as a trophic dead end,” wrote Gibbons and colleagues in a study published July 15 in Science. The same has happened in China’s Bohai Sea, the Sea of Japan, and the northwest Mediterranean. But unlike those ecosystems, the northern Benguela has the bearded goby.
In recent years, fishermen and researchers noticed more bearded gobies than before. Gobies were showing up in the bellies of seals, penguins and the remaining large fish, such as horse mackerel and hake. But nobody quite knew what they were doing, so Gibbons, along with University of Bergen biologists Anne Utne-Palm and Anne Salvanes, decided to find out.
They measured oxygen content and chemical composition throughout the northern Benguela’s waters and across its floors. They used radar to track the movements of goby populations, and conducted a series of aquarium experiments on individual fishes. What they found is a fish extraordinarily well-suited to its new environment.
During the day, gobies live on and in the Benguela’s toxic sea sludge. They do fine without oxygen: After spending hours in aquariums filled with oxygen-free water, gobies are still alert. Given the choice between toxic mud and sand, they picked the sludge.
The gobies feed on the mud, scooping it up and waiting until evening, when they swim into the higher-oxygen water column, to digest it. While in the water column, they prefer to stay among giant jellyfish, whose stinging tentacles discourage predators from following. And the gobies have developed a taste for the jellies: The researchers’ autopsies found that jellyfish can form up to 60 percent of a bearded goby’s diet.
These adaptations are likely rooted in the gobies’ evolution in the Benguela, where they dealt with toxic mud and low-oxygen waters, albeit in lower quantities than now, for millions of years. “This ‘pre-conditioning’ allowed them to capitalize on changes to the system,” said Gibbons.
For many, the Benguela’s current state is still far from ideal. Philippe Cury, a fisheries biologist at France’s Institute of Research for Development, called it a “ghost ecosystem” for fisheries. “So tell your kid, ‘Eat your gobies with your jellyfish!’” he said. But without the goby to feed other species — and, critically, to keep nutrients in circulation during particularly extreme years, when other fish can’t survive — the situation would be far worse.
“There would be less hake, less seabirds, seals and cetaceans and all those other organisms that feed on gobies,” said Gibbons. “That would be a desert.”
Whether other jellyfish-dominated systems will prove to have their own versions of the bearded goby remains to be seen. But at least the northern Benguela has avoided total catastrophe.
“Fortunately for the Benguela, they had the goby,” said Utne-Palm. “It’s a lucky end to something that could have been more of a disaster.”
Images: 1) Benguela goby./Hege Vestheim. 2) Benguela goby and jellyfish./Kim Andreassen.
Citation: “Trophic Structure and Community Stability in an Overfished Ecosystem,” By Anne C. Utne-Palm, Anne G.V.Salvanes, Bronwen Currie, Stein Kaartvedt, Göran E. Nilsson, Victoria A. Braithwaite, Jonathan A.W. Stecyk, Matthias Hundt, Meganvander Bank, Bradley Flynn, Guro K. Sandvik, Thor A. Klevjer, Andrew K. Sweetman Volker Brüchert, Karin Pittman, Kathleen R. Peard, Ida G. Lunde, Rønnaug A.U. Strandabø, Mark J. Gibbons. Science, Vol. 329 No. 5989, July 16, 2010.
Source: Wired Science