Sea butterflies, also known as pteropods, are tiny free-swimming marine snails that inhabit the open ocean. They typically measure less than one centimeter but are among the ocean’s most abundant animals in certain regions. Their name is inspired by their delicate, transparent appearance and effortless movement. These creatures are a foundational part of the marine food web and play an outsized role in the health of the global ocean.
Biological Identity
Sea butterflies are classified as pelagic gastropods, meaning they live exclusively in the water column. They belong to the order Pteropoda and the suborder Thecosomata, which translates to “case-body.” Their most distinctive feature is the pair of wing-like lobes, called parapodia, which evolved from the muscular foot seen in other snail species.
These flexible parapodia propel the animal through the water with a rhythmic flapping motion. The Thecosomata are distinct from their shell-less relatives, the Gymnosomata, commonly called “sea angels.” The sea butterfly’s delicate shell is made from aragonite, a crystalline form of calcium carbonate. This translucent and fragile shell protects the soft-bodied snail within.
Life in the Water Column
Sea butterflies are considered holoplankton, meaning they spend their entire life cycle suspended in the water column. They are found in every major ocean, from the tropics to the polar regions, though species diversity is higher in warmer waters. The highest concentrations of these animals often occur in the upper 200 meters of the water column.
They actively swim by beating their parapodia in a figure-of-eight pattern, a rare technique that mimics the flight of tiny insects to generate lift. This locomotion allows them to perform diurnal vertical migration. During the day, they descend to deeper waters to avoid predators, and then ascend toward the surface at night to feed.
Their diet consists primarily of phytoplankton and other small plankton, which they capture using a unique feeding strategy. The sea butterfly secretes a large, spherical mucous web, often several times the size of its body. This sticky net passively traps microscopic prey, and the animal then retracts the web to ingest the collected particles.
Ecological Significance
These small gastropods occupy an outsized position in the marine food web as a foundational energy source for many larger animals. They are a primary component of the diet for commercially important species, including pink salmon, mackerel, and herring. In regions like the Gulf of Alaska, pteropods can constitute up to 60 percent of a juvenile pink salmon’s diet.
Pteropods are also a significant food source for large marine predators like baleen whales, seabirds, and other zooplankton. Their abundance and high fat content make them a caloric necessity for many organisms, earning them the nickname “the potato chips of the sea.” A decline in sea butterfly populations would cause negative effects throughout the ecosystem, impacting fisheries and marine mammals.
Their aragonite shell makes them highly susceptible to ocean acidification, a process where the ocean absorbs excess atmospheric carbon dioxide (CO2). When CO2 dissolves in seawater, it forms carbonic acid, which lowers the water’s pH and reduces the availability of carbonate ions. Aragonite is particularly vulnerable because it is 1.5 times more soluble than the calcite used by many other shelled organisms.
This vulnerability means sea butterflies are viewed as an early warning indicator for the effects of climate change on the ocean. Increased acidity is already causing the shells of pteropods off the U.S. West Coast to dissolve and become pitted. When a sea butterfly dies, its aragonite shell sinks, contributing to the oceanic carbon cycle, but this process is threatened as increasing acidity causes the shells to dissolve before they can reach the seafloor.