A fish out of water is commonly considered a bad thing. But for some intertidal fish, moving over land from one body of water to another is not only good, it's essential for life. Intertidal fish can find themselves submerged in water during high tide but exposed to air or toxic water during low tide. Often, these fish have physical, physiological and behavioral adaptations that allow them to safely spend variable periods of time on land, while seeking out more pristine pools. The well-known mudskipper moves around on land by hopping on its belly using its flexible muscular body and tail. The perhaps less-well recognized mummichog (Fundulus heteroclitus), a small slender intertidal fish, spends less time outside water than the mudskipper and has fewer amphibious adaptions. Instead of skipping on its belly, the mummichog launches across land on its side like a spring-loaded rocket. It bends its head over its body toward its tail then pushes off the ground – boing! – and propels itself backwards.
Though the mummichog's fish-rocket behavior has been previously documented, Noah Bressman, at Cornell University, USA, and other North American colleagues were curious how these fish found the water that they were seeking. Had they memorized the lay of the land, as some researchers suggested, or did they use light cues, as insects have been shown to do?
The authors brought mummichogs from the wild to the lab and used high-speed video to track the fish as they moved around out of water. They placed the fish on a table with seawater on one side and buckets on the other three sides under two experimental conditions: light on one side of the table or with the room in darkness. The researchers then placed crinkled foil over the seawater in the lit room to see if the fish would navigate toward the reflected light, even in the absence of an aqueous cue.
Although many of the fish-rockets never made it off the table, ones that did preferentially moved toward the seawater and foil, under light conditions. However, under dark conditions, fish did not show a preference to move in any particular direction. As the fish had not previously encountered the lay of the lab, it appeared they were using visual cues, the reflection of light off water and foil, to navigate.
To support this finding, the authors also discovered that mummichogs would prop themselves into an upright position, lying on their bellies, in between jumps and when they were not wriggling, to reorient themselves to achieve their desired trajectory. The discovery that mummichogs prop themselves up was a first for this species. However, propping has been noted in slightly different forms in other intertidal fish and the authors suggest this behavior is crucial to enable amphibious fish to survey their surroundings.
Bressman and colleagues hypothesize that mummichog eyes are likely not well adapted to see clearly out of water. They may not be able to focus images, although it appears clear from this study that they are able to catch reflections of light and use those cues to move towards water. And the authors suggest that even though mummichogs are not as well adapted to land as belly-skipping mudskippers, the propping-before-rocketing behavior is an indication that propping to perceive is a necessary navigation adaptation of amphibious fish.