5 Sea Butterfly Facts: They Fly Under the Ocean Like Insects

Have you ever seen a butterfly of the sea gliding through the depths of the ocean? It’s not a real butterfly, but it’s as charming as the real one, leaving marine biologists and ocean enthusiasts in awe of its mesmerizing beauty. Join us as we embark on a journey to uncover the intriguing facts about this species.

Sea butterfly
Scientific name: Thecosomata
Phylum: Mollusca
Class: Gastropoda
Clade: Euopisthobranchia
Order: Pteropoda

1. They are butterflies under the sea

Sea butterflies, also known as Thecosomata, are small swimming sea slugs. They fall under the Pteropoda order, sharing a close kinship with their counterparts, the sea angels. However, in contrast to sea angels, sea butterflies have ingeniously evolved special traits to suit their life in the open waters.

Contrary to sea angels, the majority of these species possess a fragile, calcified shell that is usually light, transparent, and diminutive. They have a size under 1 cm in length. These shells exhibit bilateral symmetry, indicating that they display mirrored left and right sides.

The sea butterfly in the ocean
The sea butterfly looks like a snail on land with wings.

This translucent shell helps them to protect their delicate inner body from predators and from fluctuations in ocean pH levels. Not only does the shell provide essential protection, but it also plays a vital role in maintaining the sea butterfly’s buoyancy and ensuring its very survival underwater.

Although the majority of sea butterflies retain their characteristic curled shells, there have been occasional sightings of individuals exhibiting “straightened-out” shells. Among these butterflies, the Orthoconcha stands out with its unique feature of symmetrical, straight shells, which is believed to be closely linked to their planktonic way of life.

These snails possess a fascinating characteristic: parapodia, uniquely modified flaps acting as miniature wings on their feet. Through gentle flapping movements, these wing-like appendages allow them to swim in the ocean. This gives these snails their name sea butterfly.

What’s more interesting is that they fly in the ocean like the way tiny insects fly in the air, creating a mesmerizing figure-eight pattern during their aquatic journey. This is the first time that a different group of animals can fly in such a way like that – a splendid instance of convergent evolution.

Thanks to their swimming capabilities and their tiny sizes, ocean butterflies can adeptly traverse diverse ocean depths, foraging for sustenance and avoiding potential threats.

2. Habitat

The sea butterfly is a free-roaming creature living in an extensive range of depths, from the surface waters to the mysterious abyss, stretching thousands of meters below. However, they seldom venture near the shore or the ocean floor.

These species are not poisonous or dangerous to humans.

Their habitat spans from the tropical paradises to icy Arctic areas, and the frozen Antarctic expanses. Within tropical regions, sea butterflies exhibit a preference for warmer waters, thriving abundantly in both the North Atlantic Ocean and the Pacific Ocean.

3. Sea butterfly Diet

Throughout the day, sea butterflies embark on their migration into the water column to eat plankton. However, as night descends, instead of joining the deep waters, they opt to linger near the ocean’s surface, where the water retains its warmth. Come dawn, it is only then that they venture into the deeper realms of the ocean.

The diet of these creatures mainly contains small zooplankton and phytoplankton. Unlike the majority of snails that rely on scraping substrate to find their food, these sea snails just wait for food to come to them.

These creatures create a sizable mucous web measuring 5 cm, exceeding their own body size. This web operates akin to a sieve, ensnaring minuscule edibles suspended in the encompassing oceanic waters. Subsequently, the gathered food, intermingled within mucus nets, is consumed by sea butterflies. Nevertheless, these delicate creatures swiftly abandon their net and feeding endeavors if any disturbance interrupts their mealtime.

The sea butterfly is eating

The feeding behavior of the sea butterfly has a significant influence on the marine food chain. These remarkable creatures play a crucial role as primary consumers, maintaining balance by feeding on phytoplankton and controlling the population of these vital producers.

Apart from their passive feeding habits, certain predatory snails engage in active hunting for prey. Despite this variation, all snails, sea butterflies included, possess an impressive capability to distinguish and expel indigestible particles from the food they collect.

The sea butterfly has quite a few predators, including their close cousins sea angels, whales, sea birds, and some fish specie. To defend, they rely on their transparency to be nearly imperceptible to most of their predators and enable perfect camouflage within their marine habitat.

4. Reproduction

The sea butterfly exhibits protandric hermaphroditism, having both male and female reproductive organs. It starts life as a male with a smaller size and then turns into a female.

These creatures release their transparent egg masses in the water column, where the eggs are fertilized. These eggs then undergo development, transforming into planktonic or veliger larvae, ultimately undergoing metamorphosis into mature sea butterflies. The sea butterfly’s lifespan lasts for about 1 – 2 years.

A pair of sea butterfly

As day turns into night, these creatures embark on a vertical migration, triggering a daily transformation in the community structure. Furthermore, shifts in water temperature and nutrient availability with the changing seasons can influence their movements. The sea butterfly tends to trail these ocean currents, seeking the best environments for its development and breeding.

5. Threats

The sea butterfly, a fragile creature adorned with aragonitic shells, confronts an escalating danger brought about by the dual forces of ocean acidification and climate change. Human activities have triggered ocean acidification, leading to a shift in the ocean’s chemical composition, resulting in a decline in pH levels and a reduced abundance of calcium carbonate, which is crucial for these creatures’ survival.

These sea slugs, which depend on aragonite, face significant vulnerability as aragonite exhibits high solubility in seawater. With the diminishing availability of calcium carbonate, their shells gradually thin and become delicate, rendering them susceptible to both predators and environmental stressors. As a consequence, the marine food chain is impacted.

The vast Southern Ocean, encircling the frigid Antarctic continent, finds itself in a precarious state with the looming threat of ocean acidification. The colder waters here possess a unique ability to absorb CO2 at an alarming rate, depleting aragonite levels and posing a grave challenge for sea butterflies and other marine inhabitants native to this region.

Furthermore, the altered climate patterns, causing a rise in ocean temperatures, intricately interfere with their ability to reproduce and disperse.



I'm so fascinated about animals and want to share with you these amazing, interesting, amusing and beautiful creatures. Hope you enjoy it!

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