15 Sea Slugs You Won’t Believe Are Real

When we think of slugs, we often picture slimy brown garden creatures. Under the sea, however, you can find slugs so colorful and unique they seem to have been transplanted from another planet entirely. Some are masters of camouflage, while others display bright, bold colors to warn potential predators of their toxicity. As this Instagram video shows, sea slugs truly come in every color of the rainbow.

Sea Slugs and Nudibranchs

With at least 3,000 known species found worldwide, nudibranchs are one of the largest groups of sea slugs on Earth. Most nudibranchs are less than an inch long. They breathe using unique external structures on their backs — either feather-like gills or finger-like cerata. Atop their heads are two ear- or horn-like protrusions called rhinophores, which are sensory organs that help nudibranchs smell and taste to find food.

1. Goniobranchus daphne

This intriguing sea slug can grow up to an inch long. Its body is mostly white, appearing somewhat translucent or creamy, and is covered in tiny red spots. A distinct red border with a yellow stripe runs along the edge of its body, finished with a very thin, clear line right at the very rim. It has red-tipped rhinophores (sensory organs) and solid red gills arranged in a circle on its back end. You can find Goniobranchus daphne in both shallow coastal waters and slightly deeper areas where Chelonaplysilla violacea also lives. The slug takes in poisonous chemicals from these sponges and uses them as a defense against potential predators.

2. Rostanga arbutus

Also called the strawberry rostanga, this sea slug lives in New South Wales, Eastern Australia. It reaches about 0.4 inches in length and is often visible at low tide on rocks and algae. Its body is a striking orange-red and is covered in tiny, clear bumps. Beneath these bumps is a second layer with scattered dark brown patches and speckled markings. Some strawberry rostangas have a white patch between their dark-tipped rhinophores. Their gills are a watery orange color, lightly dusted with brown. These slugs also lay bright orange eggs.

3. Doriprismatica atromarginata

Sometimes referred to as the cheesecake nudibranch, Doriprismatica atromarginata has unique, ruffled edges like a fancy dessert. These sea slugs can grow up to 2.4 inches long, but most typically range from 1.2 to 2 inches. Their bodies vary in color from creamy-white to pale brown with a thin black line along the skirt-like edge. Adding to their elegant appearance are their black-tipped rhinophores and a chic black edging on their simple gills. Doriprismatica atromarginata lives throughout the tropical and subtropical regions of the Indian and Pacific Oceans.

4. Placida dendritica

Commonly known as the branched sapsucker, Placida dendritica stands out as a sacoglossan rather than a nudibranch. Like other sacoglossans, it feeds on coenocytic green algae and can temporarily store the algae’s chloroplasts within its own body. Although the slug is naturally white, its dark green digestive system branches throughout its body like a tree. This unique internal structure provides effective camouflage and gives the slug its distinctive overall coloring. Branched sapsuckers are tiny and measure only 0.2 to 0.3 inches long.

5. Glaucus atlanticus

Glaucus atlanticus goes by many names: the blue glaucus, blue angel, blue sea slug, blue sea dragon, blue ocean slug, dragon slug, and sea swallow, to name a few. It only grows up to 1.2 inches long, but don’t let its small size fool you. This tiny blue sea dragon preys on venomous animals like the infamous Portuguese Man-O’-War and keeps the stinging cells to make its own body toxic and dangerous. They can deliver a painful sting to humans and are sometimes found washed ashore. These vibrant blue sea slugs live in the Indian, Atlantic, and Pacific Oceans, where they often float upside down along the water’s surface.

6. Hypselodoris obscura

Found exclusively in Southeastern Australia, Hypselodoris obscura can grow up to 2.4 inches long. Its body color varies from gray to a deep, dark blue with striking yellow and navy blue spots. Its gills and rhinophores are dark red with a touch of blue at their base. There are two primary color variations for this species. The first type has a translucent gray background color, while the second form is distinctly bluer. These bluer slugs are primarily found in the northern parts of their geographical range.

7. Hypselodoris bennetti

Found in New South Wales and southeastern Australia, Hypselodoris bennetti is one of many local sea slugs with vibrant red and orange spots. These eye-catching creatures can grow up to nearly 2 inches long, though most are typically closer to 1.4 inches. Their bodies come in shades of purple, blue, pink, or white with a vivid yellow banding along the edge.

8. Hancockia uncinata

Hancockia uncinata is a rather rare sea slug species that grows only 0.5 inches long. Its body is usually pale pink or green with tiny white spots. Textured ridges run down the length of its rhinophores, which also have swollen tips. Another unusual feature is the slug’s oral veil (the flap around its mouth), which has two side extensions, each equipped with three or four finger-like projections. Being quite rare, Hancockia uncinata has been most frequently seen in the Bay of Biscay. However, there have been a few scattered sightings in other locations, including Naples (Italy), Spain, Mediterranean France, and Northwest Ireland.

9. Sagaminopteron ornatum

Sagaminopteron ornatum is often called the bat-wing sea slug because of its large, wing-like side flaps, known as parapodia. It was first discovered in Japan’s Sagami Bay, but its range extends from southern Japan across much of Australia. Unlike many other sea slugs, bat-wing sea slugs have a small, internal shell. Their bodies are a striking purple color with wide side lobes edged in orange. These slugs typically grow to about 1.2 inches long.

10. Phyllodesmium macphersonae

Phyllodesmium macphersonae is a tiny sea slug that lives in the tropical Indo-West Pacific and Australia. It has a translucent body, measuring between 0.6 to 0.8 inches long. Although its body has many scattered brown splotches, it is also covered with iridescent patches that create a beautiful, shimmering bluish-purple appearance. The upper parts of its cerata (finger-like projections on its back) and tentacles are white, often with a touch of yellow below, followed by a distinct blue band. Phyllodesmium macphersonae feeds on octocorals and can retain the symbiotic algae from its prey. This allows the slug to harness solar energy, enabling it to supplement its nutrition with energy directly from the sun.

11. Tayuva lilacina

Commonly known as the spotted foot nudibranch, Tayuva lilacina is a master of disguise, making it incredibly hard to spot. It blends in perfectly with its surroundings, hiding seamlessly under rocks and on coral rubble. Despite its elusive nature, this nudibranch is quite large, growing to 3 or 4 inches long. It has faint, dark oval shapes on its back that make it look just like sand and rubble on the seafloor, enhancing its camouflage. It also has distinctive horn-like rhinophores and frilly gills.

12. Dendrodoris nigra

Dendrodoris nigra, also known as the common black sea slug, lives in the tropical and warm temperate waters of the Indo-West Pacific. Adults are typically black, but young slugs can be a surprising orange or pink, eventually changing to black as they mature. Many adult slugs have clusters of white spots on their backs. Some of these are color markings, but others are small, white glands that produce a milky, acidic liquid. Common black sea slugs usually grow to be 2 to 3 inches long, and occasionally have a striking pink or red border along the skirt-like edge of their bodies.

13. Hypselodoris kanga

This vibrant sea slug makes its home across the western Indian Ocean all the way to Hong Kong. You’ll often find it in rocky areas and among coral rubble, especially where sponges are growing. Also known as the spotted blue sea slug, Hypselodoris kanga has a narrow, bluish body with short dark blue lines and bright orange or yellow spots. Although striking, it is quite small, usually only about an inch long, with a distinct tail and large, flower-like gills on its back.

14. Melibe australis

Melibe australis looks a bit like a tiny, wobbly mushroom, even though it truly is a sea slug. You can find these unusual creatures in shallow waters along Australia’s coast, from Cape Naturaliste in Western Australia all the way to northern New South Wales. They grow up to 1.2 inches long and like to hang out on brown algae and under rocks. Melibe australis has a slender, somewhat rounded body covered in small, branch-like bumps. It also has five pairs of cerata (finger-like projections on its back). These cerata can easily fall off if the slug feels threatened. Some slugs are transparent with just a few white and brown specks, while others can be orange, pink, and white. When handled, Melibe australis produces a strange odor that people have described as sweet, musky, and even acrid.

15. Janolus flavoanulatus

Found throughout the Western Pacific and Indian Oceans, Janolus flavoanulatus typically grows up to 2 inches long. Its body is a translucent white, but it’s easily identified by its strikingly colored cerata, which have vibrant rings of violet-purple, bright blue, brown, and yellow. These bright colors act as a warning to predators. But if that’s not enough, Janolus flavoanulatus can also roll into a tight little ball to protect itself, much like a hedgehog. While curled up, it can also secrete acids from its cerata, offering both offense and defense in one clever maneuver.

The post 15 Sea Slugs You Won’t Believe Are Real appeared first on A-Z Animals.