Pteropod

About the pteropod

In the weird and wonderful menagerie of the twilight zone, the pteropod stands out for its delicate grace. Fluttering through the water column on translucent “wings,” the more abundant shelled variety (Thecosomata group) are known as “sea butterflies,” while the shell-less adult Gymnosomata are known as “sea angels.” Sea butterflies build their fragile shells out of calcium carbonate, collectively sequestering 500 teragrams (500 billion kilograms) of carbon as biomass. Sadly, this carbon-storing ability also makes the pteropod vulnerable to ocean acidification, whereby increasingly acidic seawater eats away at pteropods’ shells.

With hundreds of species recorded in all the world’s oceans, pteropods are incredibly diverse, particularly in tropical and subtropical regions. They are most abundant at the ocean surface, but some varieties, like Clio recurve and Peracle thrive in the twilight zone—or deeper.

Wherever they hang out, pteropods join their zooplankton brethren in a nightly journey to the ocean surface to feed. They’re “mucus feeders,” meaning that they spread a sticky mucous net around their bodies to snag passing particles and plankton. They wrap up this net like a taco and stuff it into a slit-like mouth between their “wings”, which are actually a modified “foot” seen in other gastropods like whelks or periwinkles. Most pteropods enjoy a nice salad of plankton like diatoms—or, after the seasonal algae bloom is over, an entrée of zooplankton like foraminifera or copepods. Surprisingly, since plankton are scarce in the twilight zone, some deep sea-dwelling sea angel species are extremely picky feeders, exclusively targeting their relatives, the sea butterflies.