Shark Week continues here on The Quantum Biologist. Today: The world’s smallest shark, and how to camouflage with the sky.
And here it is, the dwarf lanternshark. Found only in the deepest waters off the coast of Venezuela, it measures an incredible six inches… so, not exactly Jaws. With almost 400 species of shark in the world, you have to imagine that most of them are not very scary… sharks tend to be small and skulking and often very lazy. But the elasmobranchs, the cartilaginous fishes that include sharks, are also wonderfully diverse, including the 60-foot whale shark, a gentle plankton-eater that is the largest fish in the world, and the diminuitive dwarf lanternshark, and the whip-tailed thresher shark, and the ray-like angelshark. And what makes the dwarf lanternshark more interesting than its shrimpy size is another item of shark diversity: it glows in the dark.
As a child, I often wondered why more birds weren’t blue. Wouldn’t that make them more difficult to spot in the sky? Of course, I failed to realize that birds, no matter the color, tend to cast a silhouette against the backlit heavens. Fish, however, have solved this problem.
It’s well and good if you live close to the surface to simply change your color. Think of a penguin swimming. That tuxedo is actually a form of camouflage called countershading: viewed from above, they look dark as the depths, and viewed from the depths they look bright as the surface. But deepwater fish in the benthic zones have to compete with animals with much sharper eyesight. So they use bioluminescence, just on their bellies: a blue that glows the same color, and with the same intensity, as the sky above. Not countershading, but counterillumination. Lanternsharks do it, as do many squid, hatchetfish (which add to the deception by essentially being living mirrors), lanternfish, and many others. The stargazing barrel-eye spookfish can’t always tell the difference between the sky and the underside of potential prey. As the daylight dies, the lanternshark’s belly matches the dusk. At sunrise, it wears the dawn as a cummerbund. And that, my friends, is how you vanish into the sky. It’s not enough to paint yourself its color – you have to glow with its brightness.