It was the curious observation of a bubbling mudskipper burrow that led Atsushi Ishimatsu and his colleagues into some very muddy research. Analysing the gas that they had seen coming out of the burrow, Ishimatsu's team were surprised to find that there was a high concentration of oxygen inside the bubbles, despite the fact that the water in mudskipper burrows is notoriously low in oxygen. So how was the oxygen getting there? The answer most likely lay with male mudskippers, which look after fertilised eggs in their burrows,waiting for them to hatch.

As research sites go, the mudskippers' home on soft and watery mudflats is a challenge for intrepid scientists trying to learn more about them,especially when many mudflats across the world are endangered by human activity and disappearing. In the team's study site the mudskipper's breeding season also coincided with the rainy season, with torrential rain making an already muddy research site even muddier. After many years of fine-tuning, the team perfected a technique of recording the oxygen levels inside burrows out on the mudflats and measuring the movements of the male mudskippers, to find out how they care for their brood(p. 3946).

To make measurements from burrows over several days, the team made a small dome from the top of a plastic bottle, inserting an endoscope to observe the eggs inside the burrow's egg chamber and an oxygen electrode to measure oxygen levels. They also inserted electrodes under the egg chamber to measure the movements of the male mudskipper, and pressure sensors to measure the coming and going of the tides. They found that oxygen levels decreased during high tide when the burrows were submerged. However, during low tide, when the burrows were uncovered, oxygen levels increased, and the male mudskippers repeatedly entered their burrows. This suggested to the team that the males were adding gulps of air to the egg chamber, increasing the oxygen levels inside and ensuring that the embryos had enough oxygen to survive.

Adding hypoxic air to two burrows caused the males to add more gulps of air, changing the oxygen levels back to normal within one hour. Having shown that males maintained oxygen levels in the burrows, the team wanted to know whether they helped with hatching, too. Incubating eggs in air in the lab,they found that the eggs survived in air, but they died if they were left for too long in air. Submerging the eggs in water during a critical 5–6 day period during development triggered hatching in over 80% of the eggs.

Knowing that eggs must be incubated in air but submerged for hatching, the team examined their endoscope records to find that the males played a role in egg hatching too. When water covered the burrow during night-time high tides,they saw the males taking gulps of air from inside the egg chamber, and releasing them to the surface, flooding the chamber with water and inducing hatching. Night-time hatching reduces the risk of the youngsters being snatched by predators. Protected in their burrows from the environmental extremes on the surface of the mudflats, the male mudskippers care for their brood by keeping the oxygen levels up during incubation, and then allowing the burrow to flood when the eggs are ready to hatch.

Ishimatsu, A., Yoshida, Y., Itoki, N., Takeda, T., Lee, H. J. and Graham, J. B. (
). Mudskippers brood their eggs in air but submerge them for hatching.
J. Exp. Biol.