If they want to "go shopping" during the day, they have to "explode"! The "survival wisdom" of slugs

Produced by: Science Popularization China

Author: Gu Ming Di Lian (popular science creator)

Producer: China Science Expo

The sun shines through the blue sea and sprinkles on the coral reefs on the shallow seabed. In the clear water, the colorful corals are like gems in a fairy tale world, they are bright and dazzling, and they sparkle with colorful lights. They sway gently with the water flow, as if dancing in the dancing sunlight, full of vitality and energy. Small fish and shrimps play in the corals, their smart bodies swaying, stirring up the water flow, pushing them forward to find food; occasionally, there are snails crawling on the reefs, occasionally poking their heads out of their hard shells, leisurely nibbling seaweed.

There are many different kinds of animals living in coral reefs.

(Image source: Wikipedia)

However, this beautiful scenery does not belong to the soft and slow-moving sea slugs without shells. In the bright daytime, they can only hide in dark corners, because once they are discovered by the enemy, they have only two choices, either to fight back or become a delicious meal for predators.

Even though most sea slugs have bright warning colors to alert predators that they contain unpalatable "food repellents", this can only make themselves unpalatable but cannot actually deter predators.

However, there is a class of beautiful creatures called "leaf sea slugs" (Phyllidiidae) that are not bound by such constraints. They start to stroll around the coral reefs in the daytime, without paying any attention to the fish and shrimp around them.

In the beautiful yet cruel seabed, how can these fragile creatures be so gorgeous? What unique way do they survive?

The leaf sea slug Phyllidia varicosa has a bright body color that forms a sharp contrast with its surroundings. They walk around in such a swagger that there must be a secret to protect themselves from being eaten.

(Image source: Wikipedia)

The meat is like rubber, tough and hard to eat

The body shape of leaf sea slugs is similar to that of other sea slugs. Their bodies are long and oval, with a bulging back and a flat belly. Without the protection of an outer shell, their flesh is directly exposed, but they are not as fragile as other sea slugs.

The meat of common mollusks is as soft as squid, scallops or conchs, but the meat of leaf sea slugs is completely different. They are full of tough fibers and connective tissues, and the mantle, feet and body wall covering the whole body are also embedded with a large number of calcium needles, forming a triple hardening system of spicules-fibers-muscles. It feels like a ball of tough rubber with a lot of tiny fish bones inside, which is hard, abrasive and elastic. Ordinary predators are completely unable to bite it and will retreat after a little attempt.

Phyllidia picta, its flesh can be seen to have a "frosted" texture, is opaque, and appears to be relatively hard.

(Image source: The Best Nudibranch Hunter)

In addition to its tough flesh, the back of the leaf sea slug is completely covered by a mantle, which is particularly tough and has many hard calcified nodules, just like a turtle's carapace, completely covering the feet and fragile gills underneath, forming a solid line of defense. No matter how cunning the predator is, it is difficult to penetrate this hard shell and taste the leaf sea slug's equally unpalatable flesh.

This is true 360-degree protection.

Coincidentally, not only is the flesh of the leaf sea slug like rubber, the distribution of calcium spicules in its flesh also blocks the light, making its flesh opaque. Coupled with the presence of hard nodules, it also looks like a rubber toy, so the leaf sea slug also has the common name of "rubber sea hare".

The ventral side of the leaf sea slug shows that the hard mantle completely protects the feet and surrounding feathery gills.

(Image source: the seaslug forum)

The mouth is like a medicine tube, strangely taking food and spitting out acid mist

Not only is it physically difficult to swallow, but in order to avoid being eaten by enemies, leaf sea slugs have also developed powerful chemical defenses.

The tubercles on their backs are brightly colored with yellow, white, pink, and green, which are quite conspicuous against the dark and cold background of the mantle. There is no doubt that this color and pattern are a warning to potential predators, indicating that they are poisonous. This poison is not the "anti-feeding poison" of other sea slugs that makes fish feel sick, but a "fish-killing poison" that will kill you if you eat it. Even if you don't eat it, but just inhale it in excess, you will be poisoned and die.

The toxicity of leaf sea slugs comes from the reprocessing of toxins from sponges. They feed exclusively on hard, toxic sponges that are widely distributed in the ocean (mainly the South China Sea sponge Axinyssa and the orange axis sponge, etc.). However, leaf sea slugs do not have the radula that other mollusks often have, which can chew food, and cannot bite off food.

The leaf slug Phyllidia varicosa feeds on an orange sponge. Its mouth is located on the underside of its body, making it difficult to see while feeding.

(Image source: The Garden Island)

Phyllidia ocellata feeding sponge

(Image source: Hakai Magazine)

Instead, they use a strategy similar to that of venomous spiders, using glands to produce acidic, enzyme-containing digestive juices, which they spray onto their food through specialized long tubular mouthparts, corroding and digesting it outside the body. Sponges that come into contact with the digestive juices are almost completely destroyed, leaving only a fibrous skeleton. The leaf sea slugs can then suck the partially decomposed juices for nutrition and extract the chemicals in them to protect themselves.

The toxins ingested from the sponge are further processed in organs such as the digestive glands and eventually refined into highly toxic diterpenoid isonitrile and formamide derivatives, which are stored in the nodules on the back in the form of milky white venom. With this highly toxic venom as a defense, the leaf sea slug is like carrying a "time bomb" with it, ready to send it to uninvited guests at any time.

Sex is like a volcano, it explodes and releases poison to kill fish

When it encounters an enemy or the environment changes suddenly or it is stimulated, the leaf sea slug will "explode" and quickly spray a large amount of venom from the nodules in all directions. In an instant, the sea water around it is filled with highly toxic, spicy and irritating substances, and everyone who sees it will retreat, fearing that they will be poisoned to death by the poisonous mist if they are a second late. The leaf sea slug itself will also consume a lot of physical strength, and it will take a long time to recover its ability to spray venom.

Due to the beautiful colors and strange shapes of leaf sea slugs, they are sometimes captured by some unscrupulous merchants and sold to customers who want to raise new and novel things.

However, in many cases, the ending of the story is completely tragic - after entering the fish tank, the frightened leaf sea slugs began to spit out clouds of mist, and the small fish tank did not give other creatures any space to hide, so they were quickly killed. Even these volatile poisons would be released from the water into the room, filling the whole house with a pungent smell, and even with ventilation, the smell of the poison would not dissipate for several days.

Phyllidiella pustulosa releasing milky venom

(Image source: the seaslug forum)

Phyllidia flava releasing venom. You can see that the venom is very sticky and difficult to get rid of once it gets on your body.

(Image source: the seaslug forum)

Although it's not feasible to keep leaf slugs in the home, they are a hot topic of research in the scientific community. The toxins they produce target marine life and are not too harmful to larger humans, but have potential value as antibiotics or anti-cancer agents.

In order to better use these powerful molecules to cure some serious human diseases, leaf sea slugs are kept in laboratories in major countries, where they are well fed and well-fed, and of course, the disadvantage is that they have to be "frightened" from time to time to induce them to release venom for research.

His reputation spread far and wide, and imitators abounded.

With its strong toxicity, leaf sea slugs are famous and are immune to all fish. Their unique and conspicuous appearance - many bright nodules and dark body color - has also become a symbol of underwater killers, and people will retreat when they see them. In this regard, some sea slugs and animals that are not sea slugs have also come up with strange ideas. They modify their bodies and imitate the body color of leaf sea slugs in order to protect themselves.

Other sea slugs can mimic leaf slugs relatively simply by changing their body color, and although their gills sometimes give them away, they are still a good thing.

William's mimic sea slug (Aldisa Williamsi), which imitates the leaf sea slug.

(Image source: The Sea Slug Forum)

Species from other invertebrate phyla have also jumped on the leaf slug bandwagon, including the sea squirt-eating mimic sea flatworm Pseudoceros imitatus, which bears a striking resemblance in color, pattern, and size to the common and highly venomous leaf slug Phyllidiella pustulosa.

The body shape of ordinary flatworms is similar to that of sea slugs, with both having oval, flat and soft bodies. The only difference is that flatworms are flatter and the shape of the sensory horns on their heads is also different from that of sea slugs. Flatworms have short, fleshy, unlobed antennae that fork forward, while sea slugs have long, erect antennae above their heads with feather-like lobes. Mimic sea flatworms not only mimic the color of sea slugs - black skin with dense white nodules, but also change their sensory antennae, rolling them into upright black cones to make them more like the olfactory horns of leaf sea slugs.

Although it is still different from a real sea slug, for a fish passing by in the water looking for food, they will at most take a brief glance at the flatworm. In this short time window, it is almost impossible for the fish to tell the difference, and the flatworm survives.

From a distance, the two animals (the mimicked leaf sea slug Phyllidiella pustulosa (right) and the flatworm that mimics it, Pseudoceros imitatus (left)) are more similar in appearance.

(Photo credit: DORIS - FFESSM)

One of the best mimics is a sea cucumber called Pearsonothuria graeffei, a scavenger that methodically travels across the ocean's sandy bottom every day, using its spoon-shaped oral tentacles to push any decaying particles it can find into its mouth and expelling waste and large amounts of seafloor sediment out its anus.

When they are young, the Gepi sea cucumber imitates leaf sea slugs. The young ones have bright colors, white or blue spots, dotted with black lines, and large and exaggerated orange or yellow nodules on their body surface. They are very similar to leaf sea slugs. They use mimicry to protect themselves from fish predators.

Larvae of the sea cucumber Gepi

(Image source: Wikipedia)

However, as they eat and grow, they will greatly exceed the maximum size of leaf sea slugs. The Guppy sea cucumber can grow to 30 cm, while the leaf sea slug Phyllidia varicosa they imitate is only about 10 cm. If the giant adult sea cucumber tries to disguise itself, it can only be said to be a cover-up. Therefore, the bright colors of the Guppy sea cucumber larvae will gradually fade as they age, turning into dark brown, unattractive adults.

Fortunately, while young sea cucumbers are helpless in the face of predators, as they age, they have enough material and energy to produce their own defenses: Cuvierian tubules with mild neurotoxins. When in danger, adult sea cucumbers aim their anus at the threat and shoot out these sticky white strands, entangling and anesthetizing the predator.

The unprepossessing adult Grips's sea cucumber (middle)

(Image source: Wikipedia)

Conclusion

In coral reefs, leaf sea slugs are the most common sea slugs that appear during the day. They are not only eye-catching with their unique appearance, but also amazing with their special defense mechanism. This "self-destruction" defense method not only guarantees their survival, allowing them to enjoy the sunshine during the day in the beautiful and cruel seabed, but also helps many weak creatures, allowing them to escape the natural law of the jungle by simply imitating. Although their survival strategy of spraying acid and poison is radical, it also shows the tenacious will of life.

Perhaps, every tiny creature in the world has interesting and amazing survival skills. Perhaps, every creature has its own unique story, waiting for us to discover and interpret.

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