American Oceans

Sea Butterfly

A sea butterfly is an odd sort of creature, as it’s a snail that looks like it flies.

a sea butterfly underwater

In addition to its shell, the sea butterfly has a pair of lobes used for propulsion when it swims.

It’s found in saltwater oceans globally, and while it’s not an endangered species, it faces threats from human activity. 

Read on to learn more about this amazing creature!

What Is the Sea Butterfly?

a sea butterfly swimming underwater

Sea butterflies are sea snails, though the name covers a wide variety of snails, not just one species.

They are gastropods found abundantly in the seas, though most commonly in the North Atlantic ocean.

Because they’re so common, they integral to the food chain. They are sometimes referred to as the potato chip of the sea since they feed so many creatures.

While very common in the Atlantic, sightings of various species have been recorded near the South Pole, in the Pacific Ocean, and south of Australia, so they are not exclusive to the western hemisphere. 

The sea butterfly is part of the clade known as Thecosomata, which is the sea butterfly’s scientific name. Thecosomata is part of the Gastropoda class. Some families of Thecosomata include:

  • Limacinidae
  • Cavoliniidae
  • Clionidae
  • Creseidae
  • Cuvierinidae
  • Praecuvierinidae
  • Peraclididae
  • Cymbuliidae
  • Desmopteridae

They were scientifically categorized in the 19th century, so while they’re relative newcomers in terms of species on the earth, humans have known about them for a few centuries. 

Appearance and Size

up close picture of a sea butterfly

Sea butterflies have evolved two lobes roughly analogous to arms. They use these wing-like lobes for swimming, and their curved appearance makes them look like wings, hence the “sea butterfly” moniker.

Even though they’re snails, they look like butterflies in flight when they swim.

They are mostly translucent with a calcified shell. It’s made of aragonite, a form of calcium carbonate that also plays a role in maintaining the acid levels in the ocean.

Their “wings” evolved from feet, so when they flap these wings, the shell hangs under them as they swim. The shell’s shape can affect how straight a path sea snails can swim.


To say sea butterflies are small is relative— they’re bigger than plankton, for example. But on average, they’re about two centimeters long, so compared to a whale, they’re tiny. 

That’s an average, though. Some Arctic species can grow up to eight centimeters after hatching from an egg about one-tenth of a millimeter wide.

Smaller sea butterflies grow barely longer than a centimeter, but even the largest ones are still pretty small.

As a result, they can often end up as helpless prey to any number of larger fish and other ocean dwellers.


two sea butterflies swimming in the ocean

Like many sea creatures, the sea butterfly feeds mostly on planktonic food. However, where some whales use filter-feeding, the sea butterfly casts a net into the ocean to snare its prey— a novel approach.

These petite mollusks produce a web made from mucous. The web dwarfs their bodies, as it’s up to five cm wide.

Since it’s secreted mucous, the net is detachable, and if the sea butterfly feels threatened or encounters some other issue while casting that net, it can disconnect, abandon it, and swim away. 

Because they eat plankton, sea butterflies are considered herbivores.


a close up of a sea butterfly

As a pelagic animal, the sea butterfly lives exclusively in salt water. Sea butterflies are rarely found near the shores, as they prefer the open ocean.

They also rarely spend time on the ocean floor, though they end up resting there after they die. Part of their affinity for deeper waters relates to their swimming patterns.

Sea butterflies generally follow the water column, hewing to a vertical migration cycle that takes them near the ocean surface and into the depths during daylight hours.

They do this not for the sake of traveling but to follow the food supply. The plankton ride this same column, so a sea butterfly that wants to eat goes where the food goes.

They occur in earth’s oceans from pole to pole, equally at home in tropical or arctic waters.

Different species live in different areas, but in general, sea butterflies are widely distributed between the top and bottom of the globe. 

Life Cycle

The lifespan of a sea butterfly enjoys a proportional relationship to where the snail lives.

Sea butterflies in the cold waters of the Arctic and Antarctic tend to live for about a year. Those in warmer, subarctic waters can live up to two years.

When sea butterflies die, their calcium carbonate shells dissolve, and that disintegrating aragonite partially alkalizes the water around it.

As such, they are responsible for alleviating some of the ocean acidification related to rising carbon dioxide levels. 

Breeding Habits and Offspring

Sea butterflies are hermaphroditic, which means they develop both male and female sexual characteristics.

Notably, they develop their respective reproductive organs at different stages in their life. They are all born as males, and as they grow and reach the time for breeding, they develop female sex organs to carry out the process.

When two sea butterflies mate, two males complete the mating process. A male sea butterfly stores the sperm he receives during mating until he develops his female organs.

When this transformation occurs, the male reproductive organ disappears. The now-female sea butterfly uses the stored sperm to fertilize eggs, which then get released into the ocean.

After floating in the sea in a gelatinous group, the nearly microscopic eggs develop into larvae that start swimming and carry on the genetic line.

Add comment