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Nudibranchs: The Fascinating World of Sea Slugs and Their Venomous Secrets


Stinging cells, produced by cnidarians such as jellyfish and anemones, are one of the most effective defenses of ocean creatures. This protection method can keep almost any predator at bay, not the kaleidoscopically colored sea slug. Discover the incredible world of sea slugs who commit serial crimes with stinging cells.

What is a nudibranch?

Nudibranchs, commonly known as sea slugs, are a group of shellless marine mollusks. Their name means ‘naked gills’ and refers to the breathing apparatus outside their soft bodies. These snails can be brightly colored, with hues ranging from bright purple and vivid blue to fluorescent orange, earning them common names including “dragon” and “clown.”


Theft of stingers

Nematocysts are poisonous stinging cell organelles produced by cnidarians such as jellyfish. They are a highly effective defense mechanism that can keep several threats at bay. But the sea slugs of the Aeolid group are virtually impervious to these barbed spears despite their soft bodies. Aeolid animals use a coating of chitin, a glucose derivative, at the front of their digestive system that can ward off cnidarian attacks. Any damage caused is quickly repaired.

Reflecting blows and showing no weakness, the nudibranch will continue to hunt cnidarians, most commonly sea anemones, hoping for a well-deserved meal.

Instead of digesting the nematocysts, the slug stole them for itself. The obtained cell organelles are then recycled into the flamboyant tentacle-like cerata on their backs. Now, if ever in danger, the organelle-enhanced stinging nudibranch re-fires the hijacked nematocysts as if they were always their own.

Sea bunny

Sea hares, the species Jorunna parva, may look like rabbits but are a type of sea slug. These one-centimeter creatures belong to a group known as dorid nudibranchs.

Sea bunny

Blue dragon sea slug

Glaucus atlanticus – the sea swallow, blue angel, or blue dragon – looks unusual. The cerata of this species are not on the back but are positioned like wings, extending to the sides, with ‘feathers’ of different lengths. This otherworldly-looking species, first described in 1777, floats on the surface of the water on its back and is carried across the ocean by currents and wind. Like many other nudibranchs, the tern uses the poison of its prey. But this little slug also takes on colonies of floating cnidarians. These include the Portuguese man-o-war, a colonial open-sea animal closely related to jellyfish whose sting can sometimes be fatal to humans. But the tern is picky about keeping its sting cells. Instead of taking all it can get, the slug only keeps the larger of the two main types of nematocysts produced by their hovering prey. This ensures that the slug has maximum power when stinging its predators. 

Blue dragon sea slug

Life in fragile technicolor

Life in fragile technicolor Sea swallows look delicate, let alone in glass. In the late nineteenth century, Leopold Blaschka and his son Rudolf created over 10,000 detailed glass models of marine invertebrates, including a replica of G. atlanticus. Museum conservator Chelsea McKibbin worked on the model sea swallow before it was displayed at the museum. Chelsea says, “They were very famous scientific, anatomical glassblowers. No one since has been able to replicate what they created. They are awesome.”At 5.5 cm long and 3.75 cm wide at its widest point, the model is close to the size of a live sea swallow. “When you take a live animal out of the water, it withers and collapses. They are then stored in alcohol, which can cause further shrinkage, so they end up quite small.” Museum conservator Chelsea McKibben worked to fix the finger structures of the fine glass model of Blaschka. The model was made from several separate and exceptionally fine pieces of glass and needed some conservation before it could be displayed. The “glass” method for attaching the pendants is animal glue – so you can see the brown glue. It’s applied to glass, which is a smooth material, so over time, some of the pendants fell off,” says Chelsea.

“I reattached them with preservative glue. One of the back appendages was also missing, so I had to replicate it.’ Using monofilament, similar to the fishing line, Chelsea produced prototypes and, finally, a final reconstruction of the missing “wing,” which she finished by painting the same shade as the original model. It sounds easy, but it took me about a month to reach this point. Although the replica is displayed next to the original model, it will not be permanently attached. Chelsea explains, “I want to ensure we’re portraying the specimen accurately, but I don’t want to mount it directly because it’s not an original piece. “I’ll stick the last replica on a small pin that can be attached to the mount so it looks like it’s attached.

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