Bearded Gobies to the Rescue

The bearded goby is an ecological superhero. In less than five decades, this six-inch fish managed to revive an entire marine ecosystem—one that had careened to the brink of collapse. Now scientists are beginning to unravel how the bearded goby stabilized the communities around it and capitalized on conditions seemingly too harsh for life.

Off the coast of southwest Africa there lies a region where strong ocean currents periodically churn nutrients from deeper waters producing what's known as the Benguela Upwelling. This upwelling of nutrients promotes the growth of plankton which, in turn, provides a rich supply of food that fuels an entire food web.

Several decades ago, a thriving population of Pacific sardines inhabited these coastal waters and fed on the plankton generated by the Benguela Upwelling. The sardine population was plentiful enough at that time to support commercial fishing—at least for a while. But by the late 1960s, overfishing brought about the collapse of the sardine population and the ecosystem soon began to unravel.

Without the sardines, a surplus of plankton accumulated. The excess plankton died and sank to the sea floor where it decomposed and created an oxygen-poor dead-zone. Large numbers of jellyfish moved into the area to feed on the now plentiful plankton. Since jellyfish are a poor food source—few other organisms eat them—their proliferation in a marine environment usually indicates the ecosystem is fast-failing. But the Benguela ecosystem, though lacking sardines and overrun by jellyfish, had an ecological hero in its ranks: the bearded goby.

The bearded goby—a hardy fish that tolerates low-oxygen conditions and jellyfish-laden waters—held the key to enabling the Benguela ecosystem to cheat a grim ecological fate.

After the collapse of the sardine population, animals that once fed on the sardines turned to the bearded goby. The gobies became the primary food source for predatory seabirds, mammals, and fish. Surprisingly, the bearded goby population thrived despite the intense predation pressure.

A group of scientists led by Anne Utne-Palm from the University of Bergen in Norway sought to uncover the secrets of the bearded goby's success. Utne-Palm's team used radar-like technology to construct an underwater picture of the gobies' habitat. The "acoustic maps" they created revealed that the gobies spend their days on the sea floor and ascend to shallower depths each evening where they join the masses of jellyfish that swim in their own layer of the water column.

Both locations—sea floor and jellyfish layer—provide the gobies with shelter from predators such as horse mackerel and hake. On the sea floor, the gobies enjoy freedom from predators because the can tolerate the low-oxygen conditions that would kill other fish species. At shallower depths, gobies seek refuge amongst the jellyfish. Other fish actively avoid the jellyfish and their toxic stings.

Not only do the gobies take advantage of habitats inhospitable to other organisms, they also consume food that other animals cannot. Utne-Palm's team sampled the gobies' stomach contents and found that gobies feed on jellyfish as well as marine worms that live in the toxic deposits on the sea floor. By taking advantage of these "left-over" habitats and food sources, the gobies not only thrive, they also fuel the food web of an entire ecosystem.

Refs:
Utne-Palm AC, Salvanes AG, Currie B, Kaartvedt S, Nilsson GE, Braithwaite VA, Stecyk JA, Hundt M, van der Bank M, Flynn B, Sandvik GK, Klevjer TA, Sweetman AK, Brüchert V, Pittman K, Peard KR, Lunde IG, Strandabø RA, & Gibbons MJ (2010). Trophic structure and community stability in an overfished ecosystem. Science (New York, N.Y.), 329 (5989), 333-6 PMID: 20647468

Photo © Victoria Braithwaite / Penn State.