The coward of the coral reef

The broad barred goby is a coward. A sweeping statement you may say, but even though the goby is poisonous to its predators, the cowardly fish still spends almost all its time cowering in an inaccessible coral home 20-30 cm in diameter. A new study, to be presented to the Society for Experimental biology on Wednesday 3rd April, reveals how the fish has adapted to extremely low oxygen concentrations (hypoxia) in order to avoid venturing out from its coral den.

Dr. Göran Nilsson, from the University of Oslo, and colleagues found that the water between the branches of the Acropora genus coral, the home of choice for the goby in question, had very low concentrations of oxygen during calm nights. “This is one of the first times areas of hypoxia have been looked at in coral reefs, levels can even drop as low as 2% ” said Dr. Nilsson. “I guess no-one expected to find hypoxia in paradise!”

How can the cowardly fish cope with such low levels? Using closed respirometry, measuring how the fish breathed in a tank that had no fresh oxygenated water flowing into it, the team realised the hardy fish could survive very low levels of oxygen before having to alter its metabolism to cope.

But the goby’s commitment to cowardice doesn’t end there. “About once a month, the reef experiences a very low tide,” Dr. Nilsson explained. “During this time, areas of coral can be out of water for up to four hours at a time.”

Even during this exposure the goby refuses to leave the safety of the coral shelter. Dr. Nilsson discovered that the goby still managed 60-70% of its normal oxygen consumption under these conditions, and hence could survive exposure to air for at least 4 hours. “However, the fish does develop an oxygen debt over this time, a bit like when we do physical exercise, which it must redress when the water returns. In effect, being out of water is the equivalent of the fish running a marathon.”

The study is one of the first to find hypoxia adaptations in a coral fish with a bony skeleton. As Dr. Nilsson stated, “We known there must be more out there, we just have to find them. We hope to measure oxygen levels over the reef, which will allow us to identify the areas where hypoxia occurs.” It seems that, in time, the cowardly goby could help to improve our knowledge of coral reefs.