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Facts About Sea Slugs and Photos of 12 Beautiful Species

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about living things.

Nembrotha cristata is a sea slug that lives in Indonesia.The feathery structures on the top of the body are the gills.

Nembrotha cristata is a sea slug that lives in Indonesia.The feathery structures on the top of the body are the gills.

Beautiful and Interesting Animals

Sea slugs are a beautiful and intriguing group of invertebrates. They bear some resemblance to land slugs and belong to the same class as these animals. Unlike their terrestrial relatives, many sea slugs are colorful animals. They are classified in the phylum Mollusca and the class Gastropoda. They move like a terrestrial slug (with some exceptions) and have either no shell or a small internal one.

Sea slugs have a wide variety of features and have been placed in several categories within the class Gastropoda. I include some interesting and surprising facts about the animals in this article. I also show and describe species in the order Nudibranchia, the clade Sacoglossa, and the order Cephalaspidea. Each of these groups contains some lovely representatives.

Felimida purpurea is found in Portugal. Its body color ranges from light purple to reddish purple. Its upper surface is outlined in yellow..

Felimida purpurea is found in Portugal. Its body color ranges from light purple to reddish purple. Its upper surface is outlined in yellow..

Facts About Sea Slugs

External Features

Some noticeable features that are present in various species of sea slugs are listed below. A single animal contains some of the features, but not all of them.

  • Tentacles: long extensions at the front of the head that are sensitive to touch and perhaps smell
  • Rhinophores: short, horn-like structures at the front of the head and above the tentacles (if these are present) that are sensitive to smell
  • Cerata: multiple projections on the top of the body that act as respiratory structures and may be defensive structures as well, as described below
  • Gills: a feathery patch of branched structures on the top of the body that function in respiration and can be retracted

Sea slugs have eyes, but they can't form an image. They can distinguish light from dark, however. The eyes are small and are located on the top of the head near the base of the tentacles or rhinophores or at the front of the body in some species. They look like dark spots and are often hard to see. In land slugs, the eyes are located at the tip of tentacles instead of near their base.

Locomotion

As in terrestrial slugs, the bottom of a sea slug's body (known as the foot) secretes sticky mucus. The animal moves over the mucus via muscle contractions or by ciliary gliding. Cilia are hair-like structures.

Feeding

As the animal eats, the feeding apparatus emerges from the mouth to engulf food. Some species have a radula, which is tongue-like structure covered with tiny teeth. The teeth can scrape and cut food.

1. Flabellinopsis iodinea (Spanish Shawl)

The Spanish shawl is vibrantly colored. Its rhinophores are red, its cerata orange, and its body and tentacles purple. It's thought that there are two reasons why sea slugs have vivid colors and patterns. In some species, as in the Spanish shawl, the colors may warn predators that a species is venomous or toxic. In others, they may camouflage an animal against its background and protect it from an attack.

Flabellinopsis iodinea is found on the Pacific Coast of North America in both Canada and the United States. The animal belongs to a group known as aeolid nudibranchs. The sea slugs in this group have cerata and feed on animals in the phylum Cnidaria (or Coelenterata). The phylum contains animals such as jellyfish, sea anemones, and coral polyps. These animals contain cells called cnidocytes in their tentacles. A cnidocyte contains a stinging, harpoon-like structure called a nematocyst. The nematocyst injects a venom into the cnidarian's prey and kills it.

Aeolid nudibranchs have the impressive ability to safely obtain nematocysts from their prey and then store them inside sacs in their cerata. The storage sacs are called cnidosacs. The nematocysts pass from the sea slug's digestive tract into the digestive glands located in the cerata as they travel to the cnidosacs. If the nudibranchs are threatened by a would-be predator, they send the stolen nematocysts into their attacker.

A blue dragon that was stranded on a beach

A blue dragon that was stranded on a beach

2. Glaucus atlanticus (Blue Dragon)

Glaucus atlanticus belongs to the order Nudibranchia and is known as the blue dragon, the blue sea slug, the blue angel, or the sea swallow. It has some interesting features and behavior, even compared to other sea slugs. It's a small animal that may reach a little over an inch long. Despite its size, it has some impressive abilities.

The animal is pelagic, which means that it floats on the water's surface instead of crawling along the ocean floor. It swallows air to help it stay at the surface and is upside down as it floats. Its darker and bluer lower surface faces the sky and its silver or grey upper surface faces the ocean floor. This helps to protect it from predators. Viewed from above, the animal tends to blend in with the color of the water, Viewed from below, it blends in with the bright light of the sky.

In addition to the blue color, one of the first things that a viewer would notice would probably be the extensions from the animal's body. Each appendage on the body branches into cerata. The cerata are located on the side of the animal instead of on its back. They contain venom and can sting.

The animal doesn't produce its venom. It captures nematocysts from jellyfish and uses them itself. As the floating sea slug is transported by wind and water currents, it feeds on creatures that it encounters and stores their nematocysts. A common food is the Portuguese man o' war or the blue bottle (Physalia physalis). This animal is technically not a jellyfish or even a single organism, but it contains nematocysts.

Fjordia lineata is an aeolid nudibranch.

Fjordia lineata is an aeolid nudibranch.

3. Fjordia (or Flabellina) lineata

Systematics is the study of the way in which organisms are related to each other and of how they should be classified. There isn't universal agreement about relationships and the best classification system for sea slugs at the moment. The scientific name of a particular species is sometimes changed as more research is done and as more facts are discovered. A change in classification often requires DNA analysis to be performed.

One example of the confusion in classification is that Fjordia lineata is also known as Flabellina lineata and as Corphella lineata. The species can be found in the UK and in part of continental Europe. It has a translucent body decorated with white lines that extend along the body. The cerata are orange and have a white tip where the nematocysts are stored. As in the Spanish shawl and the blue dragon, the cerata contain nematocysts obtained from prey. Members of the phylum Cnidaria form at least part of the nudibranch's diet.

4. Chromodoris annae

Chromodoris is a genus that belongs to a group known as dorid nudibranchs. The members of this group have gills for respiration instead of cerata. As in cerata, oxygen enters the gills and carbon dioxide leaves them. The animals in the first two photographs in this article are also dorid nudibranchs.

Chromodoris annae lives in the Indo-Pacific area. The colorful animal is not as flat as it appears in the photo above. A side view would show more of the body. The species eats sponges and perhaps other stationary prey. Like some other species in this article, the animal is referred to by its scientific name instead of a common one.

The surface of the animal's body has a light blue center that is speckled with black. The blue area is almost completely surrounded by a black line, which is in turn surrounded by a white, yellow, or orange band. The surface of the animal is sometimes so vibrant that it looks as though it has been painted. The rhinophores and gills of the animal are orange.

Goniobranchus geminus (formerly known as Chromodoris geminus)

Goniobranchus geminus (formerly known as Chromodoris geminus)

5. Goniobranchus geminus (Gem Sea Slug)

The gem sea slug was classified in the genus Chromodoris until its DNA was studied. Scientists discovered that the species has significant genetic differences from the members of its former genus. It's now known as Goniobranchus geminus. The animal has been found in the Indian Ocean and in the Red Sea.

The mantle is the upper wall of a sea slug's body. It covers the internal organs. The edges of the mantle may form a "mantle skirt" that hangs over the sides of the body. The gem sea slug is a good example of an animal with a very noticeable mantle skirt. The edges of the skirt are bordered by colored lines. The top of the mantle is decorated with multiple spots. The dark spots are outlined with a white circle, which creates a dramatic appearance.

Glossodoris atromarginata in the Philippines

Glossodoris atromarginata in the Philippines

6. Doriprismatica atromarginata

Doriprismatica atromarginata lives in the Indo-Pacific area and eats sponges. It was formerly known as Glossodoris atromarginata. The edge of its mantle is highly folded and is usually outlined in dark brown or black. The gills and part of the rhinophores are also dark. When set against the white or yellow mantle, this creates a dramatic appearance.

The intensity of the brown stripe down the middle of the body varies. In some individuals, it's very faint. In others, it's dark and wide. The amount of dark pigment in the gills also varies. The gills often wave rhythmically as the animal moves, which probably improves gas exchange.

7. Nembrotha kubaryana

Nembrotha kubaryana is sometimes confused with N. cristata (which is shown in the first photo in this article). The latter species lacks the red border around the foot. N. kubaryana is known as the variable neon slug. In real life, the green and red areas on the animal sometimes have a neon appearance. Photographers say that this effect doesn't show up well in photos.

The animal has a black body decorated by green stripes. The rhinophores are red and black. The gills are green, red, or a mixture of these colors. There are red patches around the mouth and the front of the animal.

Ascidians (sea squirts) are part of the animal's diet. Some of these creatures are poisonous. Some people have claimed that N. kubaryana retains sea squirt toxins in its tissues, making it poisonous. I haven't seen reliable sources stating that this is the case for the species, except for a very old reference. It's probably a good idea to avoid touching the animal, however. Other species of sea slugs are believed to absorb toxins from their prey as well.

A lettuce sea slug that has a few green patches

A lettuce sea slug that has a few green patches

Large elysiid, up to 50 mm long and 30 mm wide with prominent parapodia which are very folded at the edge, with both primary and secondary semipermanent folds.

— Fact sheet from the Sea Slug Forum (in reference to the lettuce sea slug)

8. Elysia crispata (Lettuce Sea Slug)

The lettuce sea slug (Elysia crispata) is a sacoglossan with an additional feature of interest besides its ability to store chloroplasts. Parapodia are projections extending from the side of some sea slugs. In some animals, they resemble wings. In the case of Elysia crispata, the parapodia are folded over the upper surface of the animal. There are additional folds at the edge of the parapodia that look like ruffles. They create the impression that the animal is covered by curly lettuce leaves.

The lettuce sea slug is variable in color but is sometimes green, which increases the resemblance to a curly-leafed variety of lettuce. Some animals, like the one above, are multicolored. The species is found in the Caribbean Sea.

Sacoglossans are also called sap-sucking sea slugs. They feed by sucking out the liquid (or sap) of algae. The sap contains the chloroplasts from the cells of the algae. The chloroplasts are not digested and are stored in the sea slug's body, where they continue to function. The process in which an animal obtains chloroplasts from a photosynthetic organism and keeps them alive and functional is called kleptoplasty.

Elysia chlorotica is another sap-sucking sea slug that performs kleptoplasty. It also steals algal genes that control chloroplasts and incorporates them into its own DNA. I haven't seen any reports showing that Elysia crispata does the same thing.

9. Costasiella kuroshima (Leaf Sheep)

The leaf sheep or leaf slug (Costasiella kuroshimae) is another sacoglossan. Its interesting name is derived from the green leaf-like structures on its body, a face that is often white and resembles that of a cartoon sheep, and its overall shape. Like the lettuce sea slug, the leaf sheep steals chloroplasts from the algae that it eats and makes use of them in photosynthesis.

The animal is tiny and may be no larger than 5 mm in length. It has small, beady eyes and long and mobile rhinophores. The "leaves" are actually cerata and sometimes have reddish tips. The species is said to be rare. Not much is known about it, but it has become newsworthy due to its "cute" appearance. It's found in Japan and nearby areas.

10. Chelidonura varians

Chelidonura varians belongs to the order Cephalaspidea. The animals in this order are sometimes known as headshield slugs or as bubble snails. The latter name refers to the bubble shape of the internal shell. Chelidonura varians has a variety of common names, including the blue velvet sea slug and the blue velvet headshield slug.

The animal is mostly black in color but has blue lines on its body. It's around 2.8 inches in length. The front of its head is wide and hammer-shaped. There are two "tails" of unequal length at the rear end.

The blue velvet slug lives in the Indo-Pacific region and is a predator. Flatworms form part of its diet. Interestingly, the shape of its head is reminiscent of that of the hammerhead flatworm, which lives on land. The slug has sensory hairs at the front of its head, which help it to detect its prey. It's kept in some home aquariums.

A Fascinating Group of Animals

Many beautiful sea slugs exist besides the ones mentioned in this article. It was hard to choose just ten to describe. There are many unanswered questions about the animals. The group is fascinating to explore and is still providing surprises for researchers. Even animals that are much simpler than us (such as sea slugs) sometimes have important things to teach us. The underwater world is a valuable habitat for its inhabitants and for us.

References

  • Nudibranch facts from the Natural History Museum in the UK
  • Gills and defense in nudibranchs from the Living Oceans Foundation
  • How sea slugs steal the defenses of their prey from the Smithsonian Museum of Natural History
  • Spanish shawl entry from iNaturalist
  • Fjordia lineata entry from the Marine Life Information Network
  • Information about Glaucus atlanticus from the Smithsonian Magazine
  • Chromodoris annae entry at the World Register of Marine Species (This site also contains information about some of the other animals mentioned in this article.)
  • Nembrotha kubaryana entry at the Sea Slug Forum (The fact sheets on this site were created by the Australian Museum. Like the website mentioned above, this one contains information about some of the other animals referred to in this article.)
  • Elysia crispata information from Animal Diversity Web, University of Michigan Museum of Zoology
  • Facts about the leaf sheep sea slug from the University Museum of Bergen
  • Bubble shell entry from the Encyclopedia Britannica

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

Comments

Linda Crampton (author) from British Columbia, Canada on September 07, 2020:

They are impressive animals. Thank you for the visit and comment, Dora.

Linda Crampton (author) from British Columbia, Canada on September 07, 2020:

A "crayola box" is a good description of the animals! Thank you for the comment, Flourish.

Dora Weithers from The Caribbean on September 07, 2020:

These creatures are so beautifully designed; they're admirable! It's also interesting how well-equipped they are for their survival. Thanks for the lesson!

FlourishAnyway from USA on September 07, 2020:

These are both fascinating and beautiful. It would probably be people's first instinct to try to touch these odd creatures in the wild which, of course, would be a mistake. Thanks for presenting such offbeat and factual information to us. The world's creatures are like a Crayola box here.

Linda Crampton (author) from British Columbia, Canada on September 05, 2020:

Hi, Adrienne. They are lovely creatures. Thank you for the comment.

Adrienne Farricelli on September 05, 2020:

I would have never imagined sea slugs could boast such pretty colors! Thanks for bringing such creatures to life by writing this article on such fascinating marine gastropods.

Linda Crampton (author) from British Columbia, Canada on September 04, 2020:

Thank you for the visit and the comment, Mark.

Mark Tulin from Santa Barbara, California on September 04, 2020:

Nice job, Linda. Nice to learn about the variety of sea slugs.

Linda Crampton (author) from British Columbia, Canada on September 04, 2020:

Thank you very much for the comment, Fran. The animals can be a lovely sight.

fran rooks from Toledo, Ohio on September 04, 2020:

Oh, Linda, this is such an interesting article. Who would have thought sea slugs could be so beautiful. They are truly unique. Thanks for the info and loved the pics!

Linda Crampton (author) from British Columbia, Canada on September 04, 2020:

Thank you, Chitrangada. There are some lovely creatures in the ocean. It's interesting to explore them.

Linda Crampton (author) from British Columbia, Canada on September 04, 2020:

Thank you for the visit, Moondot. I appreciate your comment. There is a mysterious aspect to sea slugs. I think they're very interesting animals.

Chitrangada Sharan from New Delhi, India on September 04, 2020:

Beautiful and bright pictures of Sea Slugs, and wonderful informative article about them. Thanks for the education about these gorgeous aquatic creatures.

Thanks for sharing.

EK Jadoon from Abbottabad Pakistan on September 04, 2020:

Alicia, wonderful work. A very informative article; after watching these beatiful creatures of God, we can't deny that this world is full of mysterious creatures. All the sea slugs have very exquisite and bright colours but my favourite is Goniobranchus geminus. Keep up the good work. Thanks for sharing.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Thank you very much for such a kind comment, Penny.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Hi, Heidi. I like the reference to sea slugs as fantasy creatures! I appreciate your visit.

Penny Leigh Sebring from Fort Collins on September 03, 2020:

I have always loved images of sea slugs! And what a wonderfully written article to go along with them. Made my day!

Heidi Thorne from Chicago Area on September 03, 2020:

These all look like fantasy creatures! Or aliens. :) Wow, who knew these existed? Well, now we do because of this. Thanks for sharing another rarely seen corner of the animal world!

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Hi, Denise. Thanks for commenting. Sea slugs are found in a wide variety of locations, depending on the species. Some have been found in deep water and some in shallow water.

Blessings to you.

Denise McGill from Fresno CA on September 03, 2020:

This was so educational. Are they found in the deep sea or nearer to the shore? I've never seen photos like these before. They are so colorful.

Blessings,

Denise

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Hi, Rachel. Thank you for the visit and the comment.

Blessings to you, too.

Rachel L Alba from Every Day Cooking and Baking on September 03, 2020:

Hi Linda, How beautiful these animals are. I never even knew they existed. I really enjoyed the videos too. Thank you for sharing these beautiful animals under the sea and the information.

Blessings to you.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

H, Devika. Sea slugs can be surprising animals! Thank you for commenting.

Devika Primić from Dubrovnik, Croatia on September 03, 2020:

I am surprised by sea slugs and interesting and unique hub about the beautiful creatures. They have pretty colors and I had no idea of the facts you mentioned.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

I appreciate your comment, Peggy. Sea slugs offer many surprises. I suspect that there are more interesting and beautiful ones to be discovered.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Hi, Liza. I think that many sea slugs are attractive. It's enjoyable to learn about new ones. Thanks for the visit.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Hi, Pamela. It's interesting to think about what is hiding below the surface of the ocean. There's a whole new world there!

Peggy Woods from Houston, Texas on September 03, 2020:

The 10 sea slugs you featured here are stunningly beautiful. People who scuba dive undoubtedly get to see things like this and other wonders in our oceans, depending upon where they are diving. Your articles are always so informative and address subjects that add to our knowledge. Thank you!

Liza from USA on September 03, 2020:

Oh my gosh, I didn't know that sea slugs can be so pretty like the ones you've shared on the article, Linda. They are so fascinating and beautiful. I think my favorite one is number seven. It almost looks like flower petals on its body, except it can be poisonous. Thank you for sharing the intriguing article, Linda. It makes me appreciate what's inside the oceans.

Pamela Oglesby from Sunny Florida on September 03, 2020:

These slugs are interesting creatures and so colorful. You would never imagine these colorful creatures living in the ocean The video was interesting as well as watching the slug moving was also very interesting. Great article, Linda.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Thank you, Bill. I think the slugs are spectacular, too. Their colors are beautiful.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Thank you, Eric. I appreciate your visit and comment.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

I appreciate your visit, Bill. The ocean is an amazing place.

Linda Crampton (author) from British Columbia, Canada on September 03, 2020:

Thanks, John. They are beautiful creatures. The ocean contains some fascinating animals.

Bill Holland from Olympia, WA on September 03, 2020:

I always learn cool stuff from your articles. Makes me wish you posted more often. Great information, Linda, and the slugs are spectacular in their vibrant colors.

Eric Dierker from Spring Valley, CA. U.S.A. on September 03, 2020:

Excellent. This is definitely one for my son and I to go over together.

Bill De Giulio from Massachusetts on September 03, 2020:

Very interesting creatures. It amazing what lives below the surface of the ocean. They are so colorful. I was not familiar with any of these sea slugs, thank you for the education.

John Hansen from Queensland Australia on September 03, 2020:

A very interesting and educational article Linda. These creatures are stunningly beautiful and the photos and videos wonderful.

Linda Crampton (author) from British Columbia, Canada on September 02, 2020:

Thank you very much, Ankita.

Ankita B on September 02, 2020:

Wonderful article and lovely photos. I enjoyed reading about the sea slugs.

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