Behavioural and physiological adaptations of the bearded goby, a key fish species of the extreme environment of the northern Benguela upwelling

Nutrient-rich, upwelling marine areas with high productivity often produce sediments dominated by organic-rich mud. Here, intense decay processes create hypoxic conditions with high concentrations of hydrogen sulphide and methane in the muddy surface layers. Such environments are inhospitable to most forms of life and those organisms that can survive in these areas tend to be specialists that cope with anoxic or hypoxic conditions, e. g. sulphide-oxidising bacteria and chemo-lithotrophic bacteria. Surprisingly, during recent acoustic and survey work in the northern Benguela region off the coast of Namibia, it was observed that the bearded goby Sufflogobius bibarbatus spends much of the day on the seabed interacting with the hypoxic and sulphidic mud, making a diel vertical migration (DVM) to spend the night in more oxygenated, but jellyfish-rich waters. We describe a series of experiments that demonstrate physiological and behavioural adaptations that enable the gobies to cope with hypoxia, anoxia and exposure to sulphide for prolonged periods of time. We also observed that the fish burrow directly into the muddy substrate when threatened and that, unlike another fish species common to this area, the horse mackerel Trachurus capensis, the gobies tolerate the presence of jellyfish.