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Sea Hares, Angels, and Butterflies: Fascinating Gastropods

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

Clione limacina is a sea angel.

Clione limacina is a sea angel.

Interesting Gastropods in the Ocean

Some fascinating animals live in the ocean. These include marine gastropods, which are relatives of terrestrial slugs and snails. Three types of gastropods in the ocean are sea hares, sea angels, and sea butterflies. The animals have some interesting features, including wing-like structures called parapodia that enable them to swim.

Sea hares are relatively large and bulky compared to the other animals. They have an internal shell. They sometimes release a liquid known as ink when they're disturbed. Sea angels are small animals with no shell and have a delicate and gelatinous body. Sea butterflies are usually tiny animals that have an external shell. In some species, the shell resembles that of a snail.

A California sea hare with the parapodia wrapped around its body (The tentacles on the right are at the front of the animal.)

A California sea hare with the parapodia wrapped around its body (The tentacles on the right are at the front of the animal.)

Sea Hare Facts

All of the marine gastropods described in this article belong to the phylum Mollusca and the class Gastropoda, like their land relatives. Sea hares belong to the clade Anaspidea within the class Gastropoda.

Sea hares are herbivorous animals that are usually found in shallow water. They have a tongue-like structure called a radula in their mouth. It's covered with tiny teeth that provide a rasping or cutting action when the animals are feeding.

On a solid surface, the animals exhibit a crawling motion. There's a flap-like extension on each side of their body that is called a parapodium. The parapodia enable an animal to swim. The flaps are wrapped around the body when they aren't in use. Sea hares can sometimes look like a big blob, especially when they're removed from the water, but they become beautiful creatures when they swim.

The video below shows an Atlantic black sea hare "flying" through the water. The scientific name of the animal is Alypsia morio. The California sea hare belongs to the same genus but a different species.

Sense Organs of the Animals

Two pairs of tentacle-like structures are attached to a sea hare's head. The upper pair are called rhinophores. They are thought to be responsible for the group's name because they reminded early observers of a hare's ears. They contain receptors that are very sensitive to scents. The oral tentacles around the mouth appear to detect a variety of stimuli.

A sea hare has small eyes that can't form an image but can differentiate between light and dark. They are located near the base of the rhinophores. The surface of the body is sensitive to touch and perhaps other stimuli.

Facts About Sea Hare Ink

There has been some debate about the function of a sea hare's ink. It's released when the animal is under stress, so it's seems to be part of a defense strategy. It's not the animal's only defense mechanism. The animal's body is covered with mucus containing chemicals that irritate some of its predators. The ink seems to be released as a last resort.

A team of researchers at Georgia State University is studying the ink and its effects. They say that the ink varies in chemical composition and color and in the way that it affects potential predators. The liquid is often unpleasant for predators and is frequently sticky. In one species of spiny lobster, it sticks to the animal's antennae and apparently blocks its sense of smell. In the lab, the lobsters exposed to the ink stopped their attack on a sea hare and focused on cleaning their antennae. In the wild, this could give the animal time to escape.

It doesn't seem to be easy to stimulate a sea hare to release its ink. The video above was the least objectionable one that I could find with respect to treatment of the animal in an attempt to make it release ink. It doesn't show all of the details of the animal's stimulation, though.

A front view of a California sea hare

A front view of a California sea hare

The California Sea Hare

The California sea hare (Aplysia californica) is variable in color. It's sometimes red or a mixture of red, pink, and other colors, but it may also be brown. It's also known as the California brown sea hare. I think the animal in the photo above has a lovely mixture of colors.

The species eats red algae, sea lettuce (a type of green alga), and eelgrass. The adult lives in shallow water in California and Mexico. It can reportedly reach a length of seventeen inches, but most individuals are about half this length.

Researchers say that the pigment in the sea hare's ink comes from molecules in the algae in its diet. The color's dependence on diet could explain why some people say that the animal's ink is red while others say that it's purple. The pigments in the diet are also said to be responsible for the fact that the color of the animal's surface varies.

Eggs of a sea hare in Scotland

Eggs of a sea hare in Scotland

Reproduction in Aplysia californica

Sea hares are hermaphrodites, which means they have both male and female reproductive organs. The animals need a mate in order to obtain sperm and reproduce, however. Self-fertilization doesn't occur. The fertilized eggs are laid in gelatinous strings that are sometimes said to resemble spaghetti. Larvae hatch from the eggs and later become adults.

The California sea hare displays some interesting behavior during mating. When it's time to reproduce, the animals often gather in lines or circles. A mating chain is formed. It's sometimes known as a "daisy chain." Sperm travels along the chain.

The behavior of each animal depends on its position in the chain. The front animal acts as a female. The others alternate between action as a male and a female, passing sperm to the animal in front of them like a male and receiving sperm from the animal behind them like a female.

A Sea Angel Named Clione limacina

Sea angels belong to the clade Gymnosomata. They have no shells. Clione is not the only genus of sea angel, but it seems to have been the best studied so far. Clione limacina lives in the Arctic. The adults are no longer than 4 cm. The animal is transparent except for an orange region in the front part of the body and at the tip of the tail.

Like sea hares, C. limacina has a radula. The feeding apparatus also contains hooks and tentacles. The apparatus is normally hidden but emerges as the animal feeds. An important component in the animal's diet is Limacina helicina, which is a sea butterfly and is described below.

The animal in the photo above has a dark visceral mass (the last colored section in the front half of the body). The specimen in the first photo in this article has a light visceral mass. Researchers say that the dark color indicates that an animal has eaten recently. The visceral mass contains the digestive system.

They're born swimming in the ocean, they grow up swimming in the ocean, and they spend all of their lives in the middle of the water never touching the bottom.

— Rob Jennings, University of Massachusetts (with reference to sea angels)

More Facts About Clione limacina


C. limacina spends its entire life swimming in the ocean. Though the species can't retreat into a shell, it has another form of protection from predators. It contains chemicals that taste bad. It seems that predators quickly discover the bad taste and leave their potential prey alone.


One animal that shares the sea angel's habitat has learned to take advantage of the bad-tasting chemicals. A small animal called a hyperiid amphipod picks up a C. limacina and carries it around on its back. This makes the amphipod taste unpleasant and helps to protect it from predators.


Sea angels are hermaphrodites but must mate with another member of their species to exchange sperm. Once this has happened, the eggs are fertilized and then laid in gelatinous strips. As in sea hares, the eggs hatch into larvae, which eventually become adults.

Limacina helicina: A Sea Butterfly

Sea butterflies are tiny animals in the clade Thecosomata. They usually have an external shell, which is either long, straight, and narrow or coiled like the shell of a snail. Both types are shown in the last video in this article. Corolla spectabilis is one sea butterfly that lacks a shell. It has an internal pseudoconch instead, which is gelatinous.

Limacina helicina is a common sea butterfly in the Arctic. The adult is up to 8 mm long. It feeds in a different way from members of the clade Gymnosomata. The wings are covered by a mucus net, which traps plankton as the animal swims. The animal periodically eats the net in order to feed on the plankton. It has a radula, like the animals described above.

Effects of Ocean Acidification on Shells

Carbon dioxide from the atmosphere is absorbed by the ocean. The chemical reacts with water to produce acid, which lowers the pH. The rising level of atmospheric carbon dioxide is of great concern for multiple reasons, including the fact that it's causing ocean acidification.

Researchers have found sea butterflies with shells that have been eroded and damaged, such as the one in the photo below. The shell of Limacina helicina is made of a substance called aragonite, which is a form of calcium carbonate. This form is easily damaged by an acidic solution.

Some interesting research suggests that animals in the Greenland Sea have at least some ability to repair the damage cause by ocean acidification. The significance of this discovery with respect to the overall problem is unknown.

Researchers found that some L. helicina specimens in the Greenland Sea had patches in their shells where they had lost all of the original material. The animals had protected themselves by thickening the inner shell wall in these patches. The scientists also found that one animal whose shell was repeatedly damaged produced the equivalent of four times the normal amount of shell as a replacement. It would be interesting to known how common or widespread the ability to repair shell damage is.

Ocean acidification diminishes the availability of carbonate ions, the building blocks of calcium carbonate, making calcification of shells and skeletons in marine organisms more energetically demanding and unprotected shells susceptible to dissolution.

— Victoria L. Peck et al, Nature Communications

Acid damage (white areas and missing ridges) on a sea butterfly shell

Acid damage (white areas and missing ridges) on a sea butterfly shell

Potentially Painful Creatures

Since sea butterflies are so tiny, it might be thought that they would have no direct effect on our lives. This isn't always the case. In September 2020, residents of North Carolina reported a painful experience. When they walked on the beach in shallow water, they experienced a stinging sensation. Despite the pain, they weren't being stung. Instead, they were being jabbed by the long, needle-like shells of a sea butterfly. An Ocean Rescue Coordinator quoted in the last reference below said that "the lack of currents and wind" enabled the animal to congregate in the area. The species of sea butterfly that caused the problem wasn't reported.

The Fascinating World of Undersea Life

The world below the ocean surface is fascinating. Sea hares, sea angels, and sea butterflies are interesting members of the ocean community. Many questions about their lives still need to be answered. The animals are intriguing from a biological point of view. Since the ocean is very important for life on Earth, some of the effects of the sea slug group might be significant with respect to our lives. Exploring this possibility could be important.


  • Information about the California sea hare from the Aquarium of the Pacific
  • Discoveries related to the ink from io9 at Gizmodo (including quotes from a scientist)
  • An interview with Rob Jennings, a researcher who studies sea angels, from The Naked Scientists
  • Information about Clione limacina from Arctic Ocean Diversity
  • Facts about Limacina helicina from Arctic Ocean Diversity
  • Ocean acidification and the shell of L. helicina from Nature Communications
  • Corolla spectabilis facts from the Sea Slug Forum (Fact sheet written by a malacologist at the Australian Museum)
  • A painful problem caused by sea butterflies in North Carolina from CBS

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on September 23, 2020:

Thank you for the comment, Mary. I like the names that have been given to some marine animals, too. They are quite imaginative.

Mary Norton from Ontario, Canada on September 23, 2020:

Sea angels. What an exciting name. The underwater world is full of many interesting creatures, most of them unfamiliar to us. I am glad you posted this.

Linda Crampton (author) from British Columbia, Canada on September 15, 2020:

Thank you very much for the kind comment, Liza. I appreciate it a great deal!

Liza from USA on September 15, 2020:

I find myself in a different setting when I'm reading your article, mostly because it takes me to the atmosphere. It was amazing! These sea animals are astonishing and unusual. Thank you for sharing another excellent article, Linda!

Linda Crampton (author) from British Columbia, Canada on September 15, 2020:

Thank you very much for the interesting comment, Dora.

Dora Weithers from The Caribbean on September 15, 2020:

Linda, I am always fascinated by the mysterious habits of these creatures: for example, the ink from the sea hares as well as their strange reproductive system; the sensations in humans caused by the tiny sea butterfly. These thoughts exercise my brain. Thanks!

Linda Crampton (author) from British Columbia, Canada on September 14, 2020:

I've never heard of using squid ink in a sauce before. Thanks for the comment and for sharing the interesting information, Adrienne.

Adrienne Farricelli on September 14, 2020:

I always learn something new when reading your interesting articles. I knew about squid releasing ink when disturbed, indeed, in Italy we used to make a yummy spaghetti sauce made of squid ink, but never heard of sea hares releasing ink too.

Linda Crampton (author) from British Columbia, Canada on September 13, 2020:

Thanks, Liz. I think organisms in the ocean have interesting features.

Liz Westwood from UK on September 13, 2020:

This is a very interesting and well-presented article. I have learnt a lot about these sea creatures.

Linda Crampton (author) from British Columbia, Canada on September 13, 2020:

Thank you for such a kind comment, Devika. I appreciate your visit very much.

Devika Primić from Dubrovnik, Croatia on September 13, 2020:

Linda This is amazing! Your thorough research is informative and well-written.

You share unique hubs and always to the point and more fascinating each time.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 12, 2020:

I wholly agreed.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Hi, Denise. No, they're not new creatures. I think they are being publicized more than they once were. In addition, I suspect that because some of them are small, they weren't discovered or appreciated until technology reached the required level. I'm glad we know about them now.

Blessings to you.

Denise McGill from Fresno CA on September 12, 2020:

It seems every time I open one of your Hubs I learn something new. Why is it I have never seen some of these fascinating sea creatures ever before? Are they relatively new or was there just no interest before? Makes me wonder if I've had my head in the sand.



Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

I appreciate your comment, and I agree with your statement, Peggy. Everything is interrelated. I wish this idea was more widely considered and discussed.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 12, 2020:

Linda, you're welcome.

Peggy Woods from Houston, Texas on September 12, 2020:

Once again, you have written a fascinating article about creatures in the sea that I never knew existed. Obviously, they have a function, and I hope that further studies will enable us to learn more about them and the health of our oceans and the planet in general. Everything is interrelated in one manner or another. Thanks for your research and for locating those videos for us to watch.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Hi, Maren. There are many fascinating animals and plants on Earth. I hope they survive for a long time to come, but their treatment is often worrying. Thanks for the visit.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Hi, Bill. Yes, I think more effort needs to be made to investigate and protect marine life. As always, I appreciate your visit and comment.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Thank you very much, Ankita

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

I appreciate your comment, Miebakagh. I think a lot of the animals in the ocean are curious and worth investigating.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Thank you for the visit and the comment, Bill. There is a lot to learn about undersea life. I think that investigating the area is important.

Linda Crampton (author) from British Columbia, Canada on September 12, 2020:

Hi, Pamela. There are a lot of unusual creatures being discovered in the ocean. It's an interesting place. Thanks for the comment.

Maren Elizabeth Morgan from Pennsylvania on September 12, 2020:

Wow, I love learning that these delicate creatures exist. Thank you!

Bill Holland from Olympia, WA on September 12, 2020:

There is such a fascinating world out there for us to witness and learn by. Instead, our government spends time worrying about fracking and off-shore drilling. When will we ever learn? Probably not in my lifetime, I'm afraid. Great information, Linda! Thank you!

Ankita B on September 12, 2020:

Excellent article. I enjoyed reading about the three marine creatures. It was a great explanation with lovely photos.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on September 12, 2020:

Hi Alicia, the ocean is an interesting world. And I had not heard of these diverse animals. The information your article release is interesting and wonderful. I found the sea angels beautiful and curious. Thans.

Bill De Giulio from Massachusetts on September 12, 2020:

It’s amazing what goes on below the surface of the oceans. I was not at all familiar with Sea Hares, Angels or Butterflies so thank you for the education once again. We have a lot to learn about the oceans and it’s impact and importance to our lives. Great job, Linda. Very interesting article.

Pamela Oglesby from Sunny Florida on September 12, 2020:

This is yet another sea creature that was unfamiliar to me. I liked watching them swim and the ink was actually kind of pretty. I found your article to be very interesting. I like that you write about unusual creatures and I learn from you.

Linda Crampton (author) from British Columbia, Canada on September 11, 2020:

Hi, Ann. I think marine animals are fascinating to explore. As you say, there are many little-known species in the ocean. There's an intriguing world below the water's surface. I appreciate your visit and your comment.

Ann Carr from SW England on September 11, 2020:

What amazing creatures! The oceans have such a variety of little-known species and, as with all your articles, you educate us with your detailed research and knowledge. These all look beautiful but I like the sea-hare the best - maybe because I love hares!


Linda Crampton (author) from British Columbia, Canada on September 11, 2020:

Thank you very much, Umesh. Blessings to you.

Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on September 11, 2020:

Splendid and exhaustive. Well researched article. Stay blessed.

Linda Crampton (author) from British Columbia, Canada on September 11, 2020:

Thank you, Flourish. I appreciate your comment very much.

FlourishAnyway from USA on September 11, 2020:

Sounds like those sea butterflies like NC and don’t want to go home. This was an excellent and well-written article.