To understand how mudskippers reproduce, scientists need to get dirty

Studying elusive fish that dwell in dark mud chambers is no easy feat. Thus the science of the many species of mudskippers is incomplete—and some of what’s known is a bit odd. Example: Mudskippers of one sort keep their protruding eyes moist by retracting them deep into their sockets and then popping them out again—hence the genus name Boleophthalmus, or “ejected eye.”

When it’s time for these amphibious fish to breed in the tropical intertidal zones where some of them live, the male stages flamboyant courtship displays, flaring his fins and leaping high into the air. If a female’s impressed, she follows the male to a burrow for procreation away from prying eyes. But thanks to an endoscope, excavation tools, and patient research, Atsushi Ishimatsu of Japan’s Nagasaki University and his team have pieced together a vision of how mudskippers reproduce.

(Watch photographer Thomas P. Peschak discuss photographing mudskippers and whale sharks on assignment for National Geographic.)

The male builds a burrow to serve as a nest. One or more shafts lead to a chamber that fills partly with water but has a domed ceiling to hold an air pocket. The female deposits eggs on the ceiling, and the male fertilizes them. Once she departs, he tends the eggs for their few days of gestation. To maintain the oxygen the eggs need, the male will swim to the surface, gulp air, bring it back, and exhale, over and over. Watching video that Ishimatsu made with the endoscope, his colleague Karen L. M. Martin deduced that a male might take “roughly 100 mouthfuls” to create the air bubble.

Then somehow, Martin says, the expectant male “keeps track of tide and time”—and at the right moment, he begins gulping the air in the burrow and blowing it out. Water pours in, triggering the larvae to hatch; they swim up from the burrow and away. The male, Martin says, is “really a very good papa.”