This resilience could help to restore the food web in a region that, until overfishing led to a collapse of sardine stocks in the 1960s, was one of the most productive ocean areas in the world.
"We show a fish that has come to play a key role in an ecosystem that has been pushed out of balance by overfishing," Anne Christine Utne-Palm of the University of Bergen, Norway told environmentalresearchweb. "This little fish is rebalancing the ecosystem by becoming the key prey and by feeding on dead-end products. This way it aids in sourcing otherwise lost production back into the food chain."
Following the collapse of the sardine stocks, two species of large jellyfish (Aequorea forskalea and Chrysaora fulgida) moved into the region. But jellyfish have few natural predators, which means that without the bearded goby their resources, and those of the organisms they feed on, would largely be lost to the food web.
"Much has been written, notably by Daniel Pauly and colleagues, about 'fishing down the food chain', such that over-fishing of large species of fish tends eventually to favour an ecosystem of jellyfish and microbes," said Utne-Palm. "Here we show that a goby is effectively converting jellyfish into fish biomass, which then fuels higher levels of the food web."
Despite being feasted on by the fish, birds, mammals and penguins that formerly ate sardines, populations of bearded goby in the region are thriving. The fish seems to be using its unique physiology to enable behaviour that helps to avoid predators.
With this in mind, Utne-Palm and colleagues from Namibia's National Marine Information and Research Centre; the University of Oslo; Pennsylvania State University; Namibian Ministry of Fisheries and Marine Resources; University of the Western Cape, South Africa; Norwegian Institute for Water Research; Stockholm University, Sweden; and King Abdullah University of Science and Technology, Saudi Arabia, investigated the biology of the bearded goby during a cross-shelf research cruise off Namibia in April 2008.
Acoustic and trawl surveys revealed that the fish spend their days sheltering in the mud at the bottom of the continental shelf. In this region the Benguela Current causes upwelling of cold, nutrient-rich waters from the depths. This makes the ocean ecosystem very productive but leads to intense decay in the mud on the seafloor, which can create high concentrations of hydrogen sulphide and methane.
"[The fish] possess extreme physiological adaptations – they tolerate hours in anoxia [absence of oxygen] and extreme levels of hydrogen sulphide," said Utne-Palm. "The species has adapted an extraordinary behaviour to lower its risk of predation, hiding in the hypoxic [low-oxygen] to anoxic bottom water during daylight hours and at night swimming among stinging jellyfish in the more oxygenated pelagic to burn-off its oxygen debt and digest benthic food; both habitats are avoided by the fish's predators."
The team found that horse mackerel (T. trachurus capensis), which feed on the goby, are reluctant to travel near jellyfish.
"Gut content analyses show clearly that the fish needs to come to shallow, oxygen-rich waters to digest the food it has captured in the mud," said Utne-Palm.
As well as eating mud-containing creatures, such as polychaete worms and diatoms, the bearded goby also feeds on jellyfish; the team is not yet sure whether it takes jellyfish from the open sea at night or picks up dead jellyfish from the seabed. "Further we would like to know how and where the gobies are reproducing, and how other fish species in this region are adapted to the extreme environmental conditions," said Utne-Palm.
It's likely that climate change will increase the inflow of low-oxgen waters from the tropics and boost coastal upwelling, putting the ecosystem under further stress, although potentially proving favourable for the bearded goby.
The researchers reported their work in Science.