Invertebrates
Sea Butterfly
Thecosomata · also called sea butterflies, pteropods
A sea butterfly drifts through open ocean water, flapping a pair of wing-like lobes where a crawling snail would normally have a single flat foot, casting a mucus web out ahead of it to catch drifting plankton. Its thin, glassy shell has worked well for a very long time, but that same thinness now makes it one of the clearest living gauges of how quickly the ocean’s chemistry is changing.
Overview
The sea butterfly, order Thecosomata, is a small, shelled swimming sea snail found drifting throughout the world’s oceans, especially abundant in cold and polar waters. It swims using wing-like parapodia and feeds through a temporary mucus web that captures phytoplankton.
A shell built for a less acidic ocean
Because a sea butterfly’s aragonite shell is unusually vulnerable to dissolving in more acidic water, researchers monitor the species closely as a real-time indicator of ocean acidification, a slow chemical shift with the potential to affect shelled plankton at the base of the marine food web.
Status
Not Evaluated formally, though sea butterflies remain common and widespread, with ongoing scientific attention focused on their sensitivity to changing ocean chemistry.
Sea Butterfly: common questions
How does a sea butterfly swim if it is a type of snail?
A sea butterfly's foot, which in most snails is a broad, flat structure used for crawling, has instead evolved into a pair of wing-like lobes called parapodia, and the animal swims by flapping these lobes rhythmically through the water, a swimming method distinctive enough to give the group both its common name and its overall butterfly-like appearance.
Why are scientists concerned about sea butterflies and ocean acidification?
A sea butterfly's shell is made of aragonite, a form of calcium carbonate that is particularly vulnerable to dissolving as ocean water becomes more acidic due to absorbing rising atmospheric carbon dioxide, making sea butterflies one of the more closely monitored indicator species for tracking the biological effects of ocean acidification in real time.
Is a sea butterfly the same animal as a sea angel?
No, despite both being small pelagic swimming snails often found in similar cold-water habitats, a sea butterfly retains a thin shell and feeds using a mucus web to capture plankton, while a sea angel has lost its shell entirely and is instead an active predator that specifically hunts certain sea butterfly species, extracting them from their shells to feed.
How does a sea butterfly actually catch its food?
A sea butterfly secretes a temporary mucus web, sometimes considerably larger than its own body, to capture phytoplankton and other fine organic particles drifting nearby, and periodically retracts this entire web back toward its body in order to consume both the structure itself and whatever food has become trapped within it.