Being out of water is a rather different place from being in the water, and so this fish obviously have some evolutionary adaptations that allow it to pull off this stunt. But a new paper asks a different, possibly more subtle: do mangrove rivulus adapt to being in or out of water in the short term?
Mangrove rivulus have an advantage for studying these sorts of short-term physiological changes, as many of them are genetically identical, because they are hermaphrodites - not all that unusual among animals, but that they are self-fertilizing hermaphrodites is a rare and exceptional feature among vertebrates.
Turko and colleagues first did a simple correlative study, allowing the fish to jump out of their tanks as often as they want. Most stayed in the water most of the time, but a few appeared to have what would have been a death wish in most other fish: they were out of the water almost two thirds of the time (64%). The authors saw differences in the gill shape that were correlated with the amount of time fish spent in or out of water.
But because correlation does not mean causation, the authors sensibly went back and did an experiment. They monitored animals for a week, then prevented them all from leaving the water, sacrificed half to check on their gills, and then left the remaining half go back to being free to leave the water if they chose.
The first that were prevented from leaving the water had different gill shapes than those that were allowed to return to the air. This strong suggests that the fishes’ behaviour drove the changes in the gill morphology.
But there is a problem in interpretation here. At the start of the second experiment, the fish were leaving water rather less than in the first correlation study. And there were no correlations between gill shape and the fish’s behaviour after the first week, as there was in the first study. The differences in gill shape emerged only after the week were the fish were forced to stay within water. The researchers suggest that there may be a minimum time the fish have to spend out of water for the gill remodeling effect to occur.
This makes me wonder if there were be a way to do the experiment were fish were forced to stay out of water for set periods of time. Here, the experimenters were at the mercy of the fish voluntarily leaving the water. It may be a little bit trickier, but the results would be much easier to interpret.
Conquest of the land, a la Chubby Checker
Celebrate diversity: The fish that fertilizes itself (on Marmorkrebs)
Turko A, Earley R, Wright P. 2011. Behaviour drives morphology: voluntary emersion patterns shape gill structure in genetically identical mangrove rivulus. Animal Behaviour 82(1): 39-47. DOI: 10.1016/j.anbehav.2011.03.001