The Pacific leaping blenny is a marine fish in all aspects...except for the tiny detail that it spends all its time living on land. It could help us understand how the first animals colonized the land billions of years ago.
For reasons that one can only hope make perfect sense to the leaping blenny, these fish spend their entire adult lives leaping about on the rocky shores of Micronesia. The creature still needs to breathe through its gills, which means it can't entirely abandon the water. And yet it's still trying to do pretty much exactly that. Dr. Terry Ord of the University of New South Wales explains:
"This remarkable little fish seems to have made a highly successful transition across the water-land interface, although it is still needs to stay moist to enable it to breathe through its gills and skin. Our study showed that life on land for a marine fish is heavily dependent on tide and temperature fluctuations, so much so that almost all activity is restricted to a brief period at mid-tide, the timing of which changes daily. During our field study on Guam we never saw one voluntary return to water. Indeed, they spend much of their time actively avoiding submersion by incoming waves, even when we tried to capture them for study.
"I can tell you they are very hard to catch and are extremely agile on land. They move quickly over complex rocky surfaces using a unique tail-twisting behaviour combined with expanded pectoral and tail fins that let them cling to almost any firm surface. To reach higher ground in a hurry, they can also twist their bodies and flick their tails to leap many times their own body length."
Ord also suggests that the leaping blenny might well provide a modern analogue for how our earliest terrestrial ancestors first emerged from the ocean:
"The Pacific leaping blenny offers a unique opportunity to discover in a living animal how a water-land transition has taken place. We know that our ancient ancestors evolved originally from lobe-finned fish but, today, all such fish are fully aquatic. Within the blenny family, however, are species that are either highly terrestrial, amphibious or entirely aquatic. Remarkably, representatives of all these types can be found on or around Guam, making it a unique evolutionary laboratory."