Masters of Disguise: Sea Slugs That Look Like Anything But
- Waverly S.
- Sep 18
- 10 min read
Olin, North Carolina
Water covers more than seventy percent of Earth’s surface, so there are bound to be many interesting marine animals. However, one might not expect so many underwater animals to resemble other entities. From land animals, to plants, to mythical creatures, sea slugs, underwater masters of disguise, are known to have a variety of appearances that might deceive the untrained eye. Read more to learn more about some of the most interesting sea slugs that are oddly reminiscent of anything but sea slugs.
What Is a Sea Slug?
Before you begin reading this article, it might be helpful to understand what a sea slug is. There is much debate as to what counts as a sea slug, as “sea slug” is a common term, not a scientific one. Some individuals only consider nudibranchs, soft-bodied molluscs with external gills, to be sea slugs, while others more broadly consider any marine animal that superficially resembles a land slug to be a sea slug, and there are many opinions that fall somewhere in between these two extremes. While not all of the animals included in this list are universally considered to be sea slugs, they have all been deemed sea slugs by a reliable source.
Eastern emerald elysia (Elysia chlorotica)
To start off this article, let’s learn about one of the most convincing lookalikes: Elysia chlorotica. Elysia chlorotica not only looks like a leaf, but it can photosynthesize like one, too! Elysia chlorotica is a member of the clade Sacoglossa, which are often known as “sap-sucking sea slugs” due to the very behavior that gives this sea slug the ability to photosynthesize: kleptoplasty, the “theft” of plastids. When E. chlorotica feeds upon Vaucheria litoria, a type of algae, it makes a hole in the algae’s cell wall and sucks out all of the contents of the cell, including its plastids, which are organelles that can carry out photosynthesis. After ingesting the algae’s plastids, E. chlorotica stores some of them within its
diverticula, which are pouches within its digestive system. However, plastids are not the only thing that these sea slugs keep from V. litoria! When ingesting the contents of algal cells, E. chlorotica also can keep V. litoria’s genes for managing plastids, taken from the algae’s cell nucleus! Remarkably, these genes can also be passed down to the offspring of E. chlorotica, although the chloroplasts are not.
Scientists have two main theories as to why E. chlorotica resembles a leaf: to camouflage and to assist with photosynthesis. Despite the theory that E. chlorotica’s unusual appearance might keep it camouflaged from dangerous animals, this species of sea slug is not known to have any predators. Instead, these sea slugs die shortly after laying eggs. It might surprise you to learn that Elysia chlorotica sea slugs in the juvenile stage of life do not look much like leaves at all! Until E. chlorotica young consume V. litoria, they are brown with red spots. The sea slugs do not enter the adult stage until
consuming algae, after which they will usually lose their red spots and turn green. E. chlorotica sea slugs are only green due to the presence of plastids within their bodies, and they will turn grey if there is a lack of functioning plastids within them. As far as photosynthesis goes, E. chlorotica’s adaptations make them so effective at the process that they can go without eating for months at a time, simply allowing their plastids to generate all of the energy that their bodies need. E. chlorotica’s leaf-like structure gives it an optimal surface area for photosynthesis, allowing it to absorb solar energy through much of its thin, flat body, similar to a leaf itself. In addition to this, much of E. chlorotica’s life is spent floating in the water to receive the maximum amount of solar energy possible.
Blue dragon (Glaucus atlanticus)
As the name suggests, Glaucus atlanticus sea slugs are often compared to dragons, as well as a variety of fictional characters based on the mythical creatures. Interestingly, G. atlanticus not only looks like a dragon, it behaves like one, too. While dragons use their wings to fly above other creatures, G. atlanticus uses buoyancy. Blue dragon sea slugs are known to swallow air bubbles, which they keep in a sac in their stomachs, allowing them to float at the water’s surface. When floating at the surface of the ocean, they float upside down, facing the blue side of their body towards the sky to blend in with the ocean from the perspective of predators from above, and facing their grey side towards predators below to blend in with what the bright ocean’s surface looks like from underwater in a form of coloration called countershading. Despite the fact that only the bottom of a blue dragon is blue, the species is well known for its blue color. The genus Glaucus is named after a blue-skinned sea god of ancient Greek mythology, although it should be noted that G. atlanticus is not the only species to fall under this genus.
Also like a dragon, G. atlanticus has a dangerous attack. Blue dragon sea slugs feed on a variety of venomous creatures whose venom does not affect them, but they most notably prey upon the Portuguese man o’ war, whose nematocysts, or stinging cells, they store within their cerata, the finger or feather-like projections that are often highlighted when comparing them to other creatures. Blue
dragon sea slugs have a more potent concentration of nematocysts than their prey do, allowing them to paralyze animals up to three hundred times their size. They sting for both hunting and self-defense, and unlike a Portuguese man o’ war, which stings when its nematocysts are triggered, blue dragons make the conscious decision to sting, and they can release all of their nematocysts at once if they feel threatened. Blue dragons have sharp, serrated teeth that they use to tear chunks out of their prey, and they have a thick layer of mucus that protects them from being stung, along with hard discs in their stomachs to further protect them. Due to the nature of their floating behaviors, blue dragon sea slugs are moved around by wind and currents, and they are often washed up onto shores, where they are sometimes difficult to recognize due to being dried out by the Sun.
Sea hare (Aplysiidae)
The term “sea hare” refers to any organism that is a part of the family Aplysiidae. Sea hares are known for being larger than most other sea slugs, with an average adult length of seven inches. They facilitate this unusual growth by eating up to one third of their body weight, which can take several hours of feeding every day. The black sea hare (Aplysia vaccaria) is widely agreed upon to be the largest discovered species of sea slug, although estimates of the size of the largest documented specimen vary greatly, from around eight to thirty-nine inches. The diets of sea hares are thought to be very important to their lives. Many scientists believe that the color of the algae that a sea hare eats influences the color of its body, and amongst species that produce ink, the color of its ink. Some sea hares can release ink when they feel threatened, which has the interesting property of containing chemicals that can mimic a food source, causing a predator to focus on catching a food source that is not there, and giving the sea hare a chance to escape. Sea hare ink contains both ink and opaline from two different glands within the
body of a sea hare, giving the ink another unique quality. Opaline, which is sticky, can get stuck on the chemosensory organs of predators, inhibiting their ability to detect their prey, and allowing the sea hare to slip away while the predator struggles to remove the substance from its body. Finally, sea hares can also accumulate toxins from the algae they eat, which they secrete through their skin, making them toxic to many animals. The combination of all of these defense mechanisms makes it so that sea hares are rarely preyed upon.
The primary reason that these sea slugs are compared to hares is the ear-like appearance of their rhinophores. These organs can sense chemicals in the water around them in a process that is similar to the sense of smell that many other animals have. This ability allows sea hares to detect potential food, mates, and predators. Sea hares are also known for their distinctive parapodia and eggs. Some species
of sea hares both crawl and swim to move through their environment, and when they swim, they use their flowy parapodia that resemble wings. Finally, sea hares are known for their long, thin eggs, masses of which are often said to look like spaghetti. Sea hares can lay up to eighty million of these eggs, which have been found in yellow, pink, and green, which is, once again, influenced by the color of algae that the sea hare eats, and if a sea slug eats a different type of algae, the color of its eggs could change within forty-eight hours.
Mystery mollusc (Bathydevius caudactylus)
Bathydevius caudactylus was only discovered by the Monterey Bay Aquarium Research Institute (MBARI) in February of the year 2000, and only deemed a new species in 2024, making it the most recently discovered species of sea slug. MBARI scientists spent over twenty years researching the species after first discovering it, and they found many unique features that set it apart from other sea slugs. B. caudactylus is the only known sea slug to live in the deep water column, although it anchors to the seafloor when it releases eggs, which, as a hermaphroditic species, all B. caudactylus sea slugs are able to do. The researchers also found several distinct physical features of the species. B. caudactylus sea slugs have a hood, which they can use to move and to trap crustaceans to eat. Sea slugs of this species can propel themselves backwards to get out of danger by rapidly closing their hood in a process that is similar to the way a jellyfish pulses its
bell. B. caudactylus sea slugs will often flex their bodies up and down to swim when needing to move, or, otherwise, simply drift along with the currents.
Uniquely, the mystery mollusc also has finger-like projections on its tail. In fact, the species name “caudactylus” comes from the Latin word meaning tail and the Greek word meaning finger. While researching the species, scientists discovered that when threatened, these sea slugs can detach these projections from their body in a process called autotomy, likely to distract the predator from hunting it and giving it a chance to escape. This effect is intensified by another noteworthy characteristic of B. caudactylus: its bioluminescence. The bodies of these sea slugs contain luminous
granules that allow them to light up in patterns that are compared to starry skies, increasing the effectiveness of using autotomy to create a diversion for predators. There are only three known species of sea slugs that exhibit bioluminescence, contributing to the confusion of scientists. According to MBARI, the “devius” section of the name “Bathydevius” was a result of the fact that the many mysterious qualities of the species initially confused researchers, making the sea slugs “devious” creatures.
Which sea slug was your favorite? Are there any sea creatures that you would like to read about in the future? If so, be sure to leave a comment down below.
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