When the sardine fishery in the northern Benguela upwelling system, located off the southwest coast of Africa, collapsed because of over fishing in the 1960s the local ecosystem shifted dramatically to one dominated by jellyfish and the bearded goby. Surviving in the hypoxic and sulphide-rich environment – created by high-productivity and decay processes in the nutrient-rich upwelling – the bearded goby population has boomed despite becoming the primary prey species for seabirds, mammals and fish. Intrigued by the bearded goby's success, Anne Utne-Palm from the University of Bergen and colleagues from Norway, Namibia, USA, South Africa, Sweden and Saudi Arabia used a multidisciplinary approach to determine the role of bearded gobies in linking and transferring energy from the harsh conditions of the ocean floor to the food web in the water column.
To determine where gobies are located in the water column, the team performed acoustic surveys and trawl sampling. In addition, they conducted behavioural experiments to assess the fish's habitat preference and to test its aversion to jellyfish. They found that the gobies spend the day resting or hiding within the mud on the seabed and spend the night in the water column with the jellyfish that their predators avoid.
Next, the team performed physiological experiments to uncover how the gobies are able to tolerate the hypoxic and sulphide-rich environment in the sediments and found that the fish were able to maintain aerobic metabolism under extremely hypoxic conditions. They also found that hydrogen sulphide, which is a respiratory poison, suppressed the gobies' oxygen consumption at moderate and high levels. The team suggest that bearded gobies are able to tolerate the high sulphide conditions by being exceptionally anoxia tolerant. This is probably achieved by metabolic depression and having a high capacity to produce ATP in the absence of oxygen.
Finally, the team compared the stable isotope composition of goby muscle with that of jellyfish, krill and sediments, and examined the gobies' stomach contents to determine their diet. They found that although jellyfish made up a substantial portion of the gobies' diet, the fish also feed on organisms living on or within the hypoxic sediments during the day. The gobies ascend to the oxygen-rich water column at night when predators are scarce, to digest and probably feed on jellyfish.
This study shows that bearded gobies play a major role in the food web of the Benguela ecosystem. They transfer ‘dead-end’ resources such as jellyfish and organisms within the hypoxic and sulphide-rich mud back into the food chain for consumption by higher predators. The gobies also possess unique physiological and behavioural adaptations that allow them to flourish in inhospitable low oxygen and high sulphide environments where other species do not thrive. As climate change is expected to increase the frequency of sulphide eruptions and hypoxic water, bearded gobies will probably continue to be an important stabilizing force for the Benguela ecosystem.
