Mudskippers are fish that have, to a remarkable degree, decided that water is optional. Mudskippers live in the intertidal mudflats and mangrove estuaries of Africa, South Asia, and the Indo-Pacific, and they spend the majority of their time out of water, moving across mud and climbing mangrove roots using their pectoral fins as crutches in a gait that is genuinely unlike anything else a fish does.
The fins are muscular enough to support the body weight and flexible enough to function as limbs, and mudskippers move across mud with a speed and agility that surprises anyone watching for the first time. They can also climb, using their pelvic fins as a suction cup to grip vertical surfaces, ascending mangrove roots to bask in the sun or escape aquatic predators.
Breathing out of water is solved through two mechanisms. The skin is highly vascularized and absorbs oxygen directly from the air, provided it stays moist, which is why mudskippers roll in mud and carry water in their enlarged gill chambers. The gill chambers function as a portable water supply, keeping the gills wet enough to extract some oxygen even when the fish is on land. A mudskipper that dries out dies, so the mud-rolling is not optional.
How Mudskippers Build Their Burrows
The eyes are on top of the head, mobile and retractable into moist sockets, giving the mudskipper a wide field of view above the mud surface. On land, the eyes are the primary sensory organ. In water, where the eyes are less useful, the lateral line system takes over. The fish switches between sensory modes depending on which medium it is currently occupying.
Male mudskippers excavate burrows in the mud for breeding, and they do something inside those burrows that required careful study to understand. The burrow has a sealed air chamber at the end where the eggs are deposited. The male maintains the oxygen level in this chamber by gulping air at the surface and carrying it down into the burrow in his mouth, releasing it into the sealed chamber. He is, in effect, running a ventilation system for his eggs using his own body as the pump.
The eggs need oxygen. The burrow floods at high tide. The male solves both problems by manually aerating a sealed underground chamber, making dozens of trips to the surface and back during each tidal cycle. It is one of the more labor-intensive forms of parental care documented in any fish.
Watching mudskippers on a tropical mudflat is one of those wildlife experiences that recalibrates your sense of what a fish is. They chase each other across the mud, display to rivals by raising their dorsal fins, court females with elaborate jumping displays, and defend territories with a persistence that looks, from a certain angle, remarkably familiar. They are fish doing things that fish are not supposed to do, in a habitat that fish are not supposed to occupy, and they have been doing it for long enough that it no longer seems strange to them at all.