Sea slugs, also known as nudibranchs, are a group of soft-bodied marine gastropod mollusks that shed their shells after their larval stage. Many nudibranchs have striking colors and shapes and toxic or distasteful chemicals in their bodies that they use for defending themselves from predators. There are over 3,000 known species of nudibranchs worldwide, and they are found in oceans from the poles to the tropics. While most nudibranchs are harmless to humans, there are some species that can deliver a painful or even potentially dangerous sting if touched. In this article, we will go over some of the most toxic and dangerous sea slugs.
Blue dragon (Glaucus atlanticus)
The blue dragon, or Glaucus atlanticus, is a small species of sea slug that lives pelagically, floating upside down on the ocean’s surface. It can be found in temperate and tropical waters throughout the world. The blue dragon’s dorsal side is silver-grey in color, while its underside is a bright blue. It gets its name from its blue underside and the way it appears to float through the water like a dragon or mythical serpent.
The blue dragon is one of the more toxic nudibranch species. Its stinging cells come from ingesting the lethal Portuguese man o’war (Physalia physalis), a jellyfish-like siphonophore, and storing the stinging cells in the tips of its finger-like cerata on its back. The man o’war’s stinging cells contain potent venom that can cause excruciating pain, whip-like welts on the skin, muscle cramps, and other symptoms in humans. Contact with a blue dragon can result in an immediate and painful sting that could last anywhere from minutes to hours. Though not typically life-threatening, the sting is extremely uncomfortable and can ruin a beach outing.
Sea wasp (Chironex fleckeri)
The sea wasp, Chironex fleckeri, is widely considered the most venomous marine animal in the world. It is a species of box jellyfish found in coastal waters off Northern Australia and Southeast Asia. This dangerous sea slug has a cube-shaped bell that can grow up to 30cm wide, with 15 long tentacles hanging from each corner. The tentacles can reach up to 3 meters (10 feet) in length.
| Tentacle Length | Potential Sting Potency |
|---|---|
| Up to 30cm | Mild sting |
| 30-100cm | Extremely painful |
| 1-3 meters | Potentially fatal |
The sea wasp’s venom is considered to be among the most powerful and deadly in the world. It can cause excruciating pain, muscle cramps, and cardiac arrest within minutes of being stung. There is an antivenom available, but without rapid medical treatment, the fatality rate from sea wasp stings can be up to 90%. Only box jellyfish antivenom can treat a sea wasp sting.
Purple striped jelly (Pelagia noctiluca)
The purple-striped jelly, Pelagia noctiluca, is a luminescent jellyfish found widely distributed in the Mediterranean Sea, Aegean Sea, and Atlantic Ocean. This jellyfish has a bell that ranges from 3 to 12 cm wide, typically with a purple or mauve coloring and distinctive white stripes around the bell. The tentacles vary from 2 cm to 1 meter long and pack a powerful sting.
This jellyfish’s sting is extremely painful and can cause symptoms like severe burning, red welts, vomiting, muscle cramps, breathing difficulties, and heart problems in severe cases. Contact with the long tentacles of Pelagia noctiluca requires rapid first aid to limit injury, and can potentially be fatal, especially for young children.
While antivenom does not exist for this jellyfish, basic first aid like removing tentacles, rinsing the sting with saltwater (not freshwater), and hot water immersion can help reduce pain. Seeking emergency medical care is recommended.
Janolus fuscus
Janolus fuscus is a species of nudibranch found off the Pacific coast of North America, from Alaska to California. This toxic sea slug has a white or cream-colored translucent body with brownish spots and can grow up to 80mm long. When threatened, it can release a poisonous mucus from glands located all over its body.
The mucus contains a toxin called spiculessin that can induce headache, nausea, vomiting, muscle weakness and even neurological symptoms like seizures in humans if ingested or contacted. While it won’t cause death, the symptoms are highly uncomfortable, begin within 30 minutes of exposure and can last for hours. Rinsing thoroughly with water, supportive medical care, and pain management are the recommended treatments.
Sea hare (Aplysia parvula)
Sea hares are a family of ring-shaped nudibranchs that can release an inky purple secretion when threatened. Aplysia parvula is a Caribbean species that can grow up to 30cm long. Its ink contains concentrated toxic chemicals like aplysiatoxin that can irritate the skin, eyes, and mucus membranes on contact.
If the ink makes contact with human eyes, it can cause intense burning pain, blurred vision, and temporary blindness. On skin, it can induce rashes, blisters and swelling. Washing any skin exposed to the ink for 10-15 minutes while removing any particles can help minimize irritation. There are no known fatalities associated with sea hare ink.
Glaucus marginatus
Glaucus marginatus is a species of small blue sea slug similar to the blue dragon. It feeds on venomous siphonophores like the Portuguese man o’war, sequestering their stinging cells for its own defense. This 1-3cm nudibranch packs a surprisingly powerful sting from its finger-like cerata that can cause instantaneous pain, welts, nausea and muscle cramps in human victims.
While not fatal, the sensation of being stung by Glaucus marginatus has been compared to suddenly receiving a hot burn or electric shock. The severity depends on the amount of exposed skin, with only brief touches resulting in mild stinging that fades within an hour. More extended contact causes increased effects lasting up to a few days. Rinsing the area with saltwater and pain medication are recommended first aid steps.
Conclusion
While the vast majority of the thousands of nudibranch species are harmless, there are a few toxic species that can deliver painful and potentially dangerous stings if touched by humans. This includes the blue dragon, sea wasp, purple-striped jelly, Janolus fuscus, sea hare, and Glaucus marginatus. If you happen to get stung by any sea creature while near the ocean, immediately rinse the affected area and seek medical assistance when possible. Being aware of the most venomous species before diving in can help avoid any unfortunate encounters. Proper precautions allow us to safely enjoy the beauty of these unique sea slugs in their natural habitats.