10 Facts about Nudibranchs, the Sea’s Poisonous Party Slugs

2. Some nudibranch also steal photosynthetic algae from jellyfish

Like its fellow mollusc the giant clam, nudibranchs eat photosynthetic algae. Nudibranchs store the algae in their bodies while the algae absorbs sunlight and turns it into energy. This energy feeds the nudibranch. These nudibranchs typically live in coral reefs near the ocean surface, where the algae can catch plenty of sunlight. In many cases, the zooxanthellae lives in a cnidarian, such as a jellyfish. The nudibranch obtains the zooxanthellae when it eats the cnidarian.

The lettuce nudibranch merges chlorophyll from algae into its own tissue

Tridachia crispata, called the lettuce nudibranch, eats photosynthetic algae for energy. Not only does it use the sunlight the algae converts into energy, it also puts the algae’s chlorophyll to good use. The lettuce nudibranch lets the chlorophyll into its own tissue. The chlorophyll gives this nudibranch its green color. It also photosynthesizes to turn water and sunlight into energy for the nudibranch.

3. Gymnodoris nudibranchs eat smaller nudibranchs

Some larger species of nudibranchs will prey on smaller species. For example, nudibranchs in the Gymnodoris genus hunt smaller nudibranchs by sniffing out the small prey’s slime trail. Certain species will swallow the smaller nudibranch whole. Others use radula, which is a sharp, tongue-like apparatus in the mouth, to rip their prey to shreds before swallowing.

4. The cerata also absorbs oxygen so the nudibranch can breathe

The nudibranch’s long, hair-like cerata aren’t just for attacking. They also help the creature breathe. Doric nudibranchs have gills that they can retract around their anus.  While Aeolids only have their cerata to help them breathe. The cerata absorbs oxygen from the surrounding water in a respiratory exchange, so that the nudibranch can breathe.

5. Nudibranchs are molluscs that shed their shells as larvae

Nudibranchs are molluscs, but lack one defining characteristic of the group: They don’t have a mantle. Mollusks usually have soft bodies, and their shells are an important defense. Nudibranchs have shells as larvae, but shed them as they develop into adults. Instead, their soft bodies are exposed. Without a shell, nudibranchs are more agile, and don’t have to waste energy rebuilding their defense.

6. Since Nudibranchs don’t have shells, their vibrant colors are a defense mechanism

Nudibranchs are adapted to deter predators without the protection of a hard shell. Their soft bodies come in hundreds, if not thousands, of different colors and patterns.  Some are just bright red, while others white with electric blue stripes. The cerata are not the same color as the body, so they always stick out. The bright coloring is a warning to predators that the nudibranch is toxic, and should not be eaten. Even some non-toxic nudibranchs are colorful, as it tricks predators into thinking they are poisonous.

Nudibranchs also get their colors from eating colorful corals and anemones

As a nudibranch eats a coral or anemone, it will absorb the prey’s colors. The nudibranch uses this coloring as camouflage. The nudibranch will be hidden from predators, and prey won’t notice it munching down.

The blue dragon nudibranch uses countershading to hide from prey

The blue dragon nudibranch has an interesting adaptation to hide from predators. Since this small, soft nudibranch lives in open waters, it could be an easy target. The blue dragon’s underside looks blue, and its upper side looks grey. But the blue dragon floats upside down. So from above, the blue looks like the water. And from below, the grey blends in with the sky. The blue underside also protects the nudibranch from the sun’s UV rays.

7. Nudibranch eyes are a quarter of a millimeter wide, so they rely on their horns to sniff out their environments

The average nudibranch’s eyes are only a quarter of a millimeter wide. The slugs are mostly blind.  They can only make out light and dark. Nudibranchs have two horns on their heads that many people mistake for eyes. These horns are called rhinophores, and they are more like noses. The rhinophores are chemical receptors that alert the nudibranch of nearby food and potential mates.

8. Nudibranchs have both male and female sex organs, and fertilize each others’ eggs

Nudibranchs are hermaphrodites. They each have both male and female sex organs. Nudibranchs use chemical cues to find mates. When two slugs pair up, they both extend their male organs to exchange sperm. Each nudibranch uses their mate’s sperm to fertilize their eggs. Many nudibranchs lay eggs in elaborate, ribbon-like spirals. After mating, some nudibranchs shed their male appendage, as retracting it takes more energy.

The mottled sea hare lays an estimated 25 million eggs at once

Some species of nudibranch only lay a few eggs. Others, however, lay much more. The nudibranch Aplysia fasciata, called the mottled sea hare, lays up to 25 million eggs at once! These eggs are not laid in vibrant red ribbons. Instead, the eggs are cream colored, and all together look like noodles!

9. The blue dragon nudibranch swallows an air bubble so it can float on the ocean surface

Unlike most nudibranchs, the blue dragon swims (well, floats) on the ocean surface. This nudibranch swallows an air bubble and then holds it in its stomach to stay afloat. The blue dragon’s cerata are along its wing-like appendages, instead of its back. But since the dragon is so small, it is at the mercy of the weather. After a storm, it's not uncommon for hundreds of blue dragons to be washed up on shore.

The blue dragon’s primary prey is the much larger Portuguese Man O’ War

The blue dragon is only 3 centimeters long.  But it preys on much larger creatures. One example is the Portuguese Man O’ War. The Man O’ War grows between 2-10 feet long. The tiny nudibranch doesn’t eat an entire Man O’ War, of course. Instead, the nudibranch grazes on the jellyfish’s tentacles. And in true nudibranch fashion, steals its nematocysts.

10.  Allegedly, two nudibranch species make noises that are audible to humans

In 1884, English naturalist Philip Henry Gosse claimed that a Professor Grant heard nudibranchs producing sound. Grant kept Eolis punctata and Tritonia arborescens in an aquarium. He claimed that when the nudibranchs were active, they made single strokes at one or two minute intervals. Grant described the noise as “like the clink of a steel wire on the side of the jar.” But scientists today have not inquired further into these observations.

Pokemon or party slugs? Either way don’t pick them up

Many people think blue dragon nudibranchs look like legendary pokemon, but that doesn’t mean you want to catch ‘em. Humans who have handled washed up blue dragons on the beach have been stung. And it hurts. Like, really, really hurts. Remember, the Portueguese Man O’ War has actually killed humans, and blue dragons have just enough of that deadly venom to make humans wish they were dead.

• Scientists aren’t certain how many nudibranch species are endangered/threatened.
• Some researchers have observed that water pollution and ocean acidification are making some species rarer to find.
• The destruction of coral reefs due to climate change threatens hundreds, and maybe thousands of nudibranch species. Nudibranchs rely on healthy coral ecosystems for habitats and food.