The creek behind my uncle’s house here in Western Ohio is flooding; normally a laconic and nameless little tributary with quietly dipping mallards, last night’s thunderstorm and rapidly melting snow has raised the water level almost twelve feet and transformed it into a swollen, churning torrent. As I sit here watching the lawn furniture and Fisher Price playsets rush downstream, I thought it’d be appropriate to talk about animals for whom floods are home.
I’ve written before about the flooded forests of the Amazon basin, the Amazon river dolphin in particular, but it’s worth another visit. The Amazon is sometimes referred to as the River Sea, and the reason why becomes clear when the water level rises 30 feet and covers three times its already substantial area. During the Spring floods, a gondola navigating the trees in the rainforest might come upon a pair of giant otters chasing each other through the water, or glide into a mysterious pool of shimmering gold which, on closer inspection, turns out to be a school of piranhas. Here in the varzea, the underwater forest, the Amazonian manatee does the dead man’s float while grazing on submerged meadows, and the anaconda rolls like water boiling. And if you’re lucky, you may find the dragon of the Amazon: the arapaima.
It is a living fossil. Its shovel-like, scrunched face and lobed tail are testament to the arapaima’s ancient origins, but it’s the size that makes this freshwater fish truly majestic. Before they became overhunted for their meat, arapaima were known to grow almost 15 feet in length, and weigh 440 pounds. What makes this remarkable is that this enormous fish favors shallow, oxygen-starved water; with its primitive lung and sophisticated labyrinth organ in its head, this monster prefers to breathe air, and so stays close to the surface. Its bony, barbed tongue is used by native ribeirinhos (river people) to scrape papaya pits. Its enormous scarlet scales are used as jewelry.
Arapaima rely on floods in order to breed. Though they can be bred in captivity, they prefer to swim to the furthest, shallowest shores of the varzea to make their nests. The sandy meadows where these giants lay their eggs could normally never be reached by a fish, and the arapaima likes it that way: distant and too low in dissolved oxygen to be reached by potential predators. This far into the forest, they do occasionally fall prey to jaguars, but possessing the strength to leap entirely out of the water, it’s a fair fight. When the fry are hatched, they will stay near their father, who will carry them in his mouth when danger is near.
But to see a truly flood-dependent animal, you have to go halfway around the world to the rivers of Australia and the South Pacific. The pig-nosed turtle, a soft-shelled species, has become more suited to the aquatic lifestyle than any other freshwater turtle; their claws have evolved into flippers, and their nose has become a flexible snout that acts as a snorkel. Once hatched, it will never return to land except to lay its own eggs. Such an animal not only welcomes the seasonal floods, it requires them. As you’ll see in the video below, the pig-nosed turtle, which lays its eggs in dry, sandy riverbeds, actually needs water in order to hatch. The baby turtles inside the eggs are dormant, waiting for a sudden drop in barometric pressure that signals a coming storm to awaken them. Falling rain, instead of drowning the babies inside as it would do with the eggs of any other species, actually triggers an enzymatic change in the shell that makes it easier for the hatchlings to crack open.
By tying the hatching to one distinct meteorological event — in this case, the first rainstorm of the flooding season — the pig-nosed turtle has given its offspring the greatest chance at life. As I’ve said before, turtle hatchlings make a quick and easy meal for predators, and most will never survive their first half hour on earth. By synchronizing the hatching, pig-nosed turtles tumble into the river faster than predators can catch them. Of course, the torrential weather doesn’t hurt their odds, either. And because they hatch just as the river is rising, the pig-nosed turtle hatchlings inherit the best and easiest possible world for them. They are born into their golden days, a flooded paradise. When the levy breaks, the swimmers can claim the land as their own empire, if only until the rain stops.