Sea Anemone: Damn, some "dirty stuff" has sneaked in...

Produced by: Science Popularization China

Author: Komeichiren

Producer: China Science Expo

In the two pictures below, there are other animals besides sea anemones. Can you find them?

(Image source: The Sea Slug Forum)

The answer is revealed:

The red circle shows the Jakobson gray-winged sea slug Phyllodesmium jakobsenae

The red circle shows the gray-winged sea slug Phyllodesmium rudmani

(Image source: The Sea Slug Forum)

Yes, the animals in the red circle are not sea anemones, but two sea slugs. Their body surfaces are covered with long protrusions and brown stripes, which are very similar to those of sea anemones in shape and color, and even the tentacle structure is imitated vividly, which makes them look like sea anemones and very difficult to be found.

Not every creature has the skill of camouflage. These two sea slugs belong to the same group - the genus Phyllodesmium under the suborder Phyllodes of the class Gastropoda, also known as gray-winged sea slugs for short.

When we think of sea slugs, we probably think of those brightly colored little critters. These sea slugs are poisonous, just like the brightly colored caterpillars, but the symbiotic graywing sea slugs hide themselves in colonies of sea anemones.

Today, let us learn about these "ghosts" hidden in the sea anemones.

Because it tastes bad, it is safe

Why does the gray-winged sea slug want to disguise itself as an anemone? This has to start with the anemone itself.

It is important to note that anemones do not arouse the appetite of animals. They belong to the phylum Cnidaria, which is a close relative of jellyfish. 97% of their bodies are water and have little nutrition. But as for food, nutrition is one aspect and taste is another. Compared with jellyfish, which have a crisp taste and can be made into a popular cold stingray, anemones have a very bad taste.

The body of anemone is different from that of jellyfish. They do not have a cap like that of jellyfish. Instead, they are composed of a slender base at the bottom and a larger, flat oral disc at the top. There is a circle of tentacles around the oral disc, like a mushroom with a bunch of small tentacles on top. When this "mushroom" encounters a threat, it will secrete a large amount of smelly mucus to wrap itself, making its taste like a slippery ball of snot, which is disgusting to animals.

In addition, the anemone's body is covered with stinging cells, which are distributed on their tentacles. Their structure is similar to a small harpoon, including a hair-like trigger and a slender needle-like stinger containing venom. When the anemone is stimulated by external factors, these stinging cells will quickly eject the stinger like a bee stinging a person.

This process occurs in just 700 nanoseconds, and the stinger will attack with an acceleration of up to 5,410,000g (g is the acceleration of gravity), penetrating the skin of the prey like a bullet, allowing the venom to quickly penetrate the prey or natural enemy and paralyze it. After releasing the stinger, the stinging cells will die, and the new immature stinging cells will quickly mature to replenish, so that the anemone has a means of self-defense at any time.

Therefore, the advantage of this camouflage adopted by the gray-winged sea slug is obvious: by imitating anemones that are not eaten by general animals, they can avoid being discovered and becoming prey to predators. But disguised as anemones also means that they must always live in anemone colonies, because once they leave the colony of anemones, the gray-winged sea slugs can be easily discovered. In the long-term symbiotic relationship with anemones, they have gradually evolved special skills and achieved "perfect utilization" of anemones.

The 100-armed gray-winged sea slug Phyllodesmium briareum (marked by the black arrow in the picture) feeds on the turf coral Briareum. Its gills are gray-brown, finger-like, and can be bent at will to mimic the turf coral of the same shape.

(Image source: Wikipedia)

Not only camouflage, but also eat

Gray-winged sea slugs have no shell and look fragile, but they are not as fragile as they appear.

Instead of a shell to protect them, a special epithelial cell can secrete a thick layer of mucus, which is like glue, transparent but extremely sticky, protecting them from predators and prey. Gray-winged sea slugs live in sea anemones, and they are the most convenient food for them because they are close to the water. However, before eating sea anemones, they must overcome the counterattack of their stinging cells.

Before feeding, the gray-winged sea slug releases a large amount of mucus around its head, forming a mucus barrier between itself and the anemone. Don't underestimate the blocking effect of mucus: the resistance of water is 800 times that of air, making the killing distance of a bullet that can fly more than a thousand meters in the air less than 1 meter in the water, not to mention the glue-like mucus.

The mucus also contains special chemicals that can neutralize the toxins from the stinging cells of the sea anemone. The shots from the stinging cells cannot penetrate and can only be consumed in the mucus layer, leaving behind lightning-like white trails and bubbles like bullets traveling through water.

When the gray-winged sea slug directly touches the anemone's mouth, there is a thick chitin protective layer to protect the mouth from injury. With such a set of combined punches, the anemone's defense is like nothing in front of the gray-winged sea slug, and they can feast on it.

Coleman's Gray-winged Sea Slug Phyllodesmium colemani feeding on a Tubipora musica coral. Only its mouth is visible touching the anemone bush on the right.

(Image source: The Sea Slug Forum)

The mature stinging cells of the sea anemone are very "brave". They react very quickly and release poisonous stings as long as they are physically touched, and then all die heroically. But the immature stinging cells have no resistance and can only be manipulated. For these "young prisoners of war", the gray-winged sea slug did not digest them, but took them in and raised them for its own use. Sea anemone: Eat my food, use my food, this is a bit of bullying.

Immature stinging cells will enter the digestive tract of the gray-winged sea slug as it eats, and then enter the digestive gland connected to it. The digestive gland branches on the dorsal side and extends all the way to the "gills" on the dorsal side, which are slender and tubular. In the picture, the "small anemones" on the back of the gray-winged sea slug are the gills. At the top of the gills, there is a small bag called a "cnidosac", which is connected to the thin tube of the digestive gland. Inside, there are special nutrient cells as attendants, waiting for the arrival of these immature stinging cells.

After settling in the nematocysts, they develop here and gradually mature under the care of the gray-winged sea slug. When the gray-winged sea slug is attacked, the stinging cells will also fight for them, shooting out poisonous stings to resist the enemy in return for not killing them. Of course, when the stinging cells die, the gray-winged sea slug will digest and absorb them again, recycling the nutrients it has paid. After all, there is no need to keep the dead stinging cells.

The "psychedelic gray-winged sea slug" Phyllodesmium acanthorhinum may feed on the gorgonian coral Acabaria. It has a transparent body color, and the orange-red digestive glands in its exposed gills are clear and bright. With the metallic blue background and bright white dots, it looks very sci-fi. Among them, the nematocysts that store its stinging cells are the yellow-white area on the top of each exposed gill.

(Image source: Sea Slug Phyllodesmium acanthorhinum in top 10)

Gray-winged sea slug: "Life as a sea slug" is like a play, all depends on acting skills

The gills with stinging cells are the most obvious and amazing part of the gray-winged sea slug. It can hide the gray-winged sea slug in the sea anemones and can also store stinging cells, all thanks to its special structure.

Each gill is composed of a large number of slender tubes arranged side by side, which are filled with blood and can exchange matter and energy; the base of the gill is slender cylindrical, like the stem of a sea anemone, and becomes wider and flatter towards the tip, consisting of a narrow light-colored area and a wide brown flat area around it, which is very similar to the color pattern of the tentacles of a sea anemone, alternating between light and dark colors under light, creating the illusion of tentacle-like shapes.

But in fact, the gills of the gray-winged sea slug do not have small tentacle structures like those of the anemone. They just create the "naked eye 3D" effect through color changes, but this is enough to be called lifelike.

Most sea slugs of the genus Phyllodesmium feed on the Xenia anemone, such as the Jakobsen's gray-winged sea slug Phyllodesmium jakobsenae, whose tentacles have a white-brown color pattern that closely mimics the tentacle morphology of the Xenia anemone.

(Image source: The Sea Slug Forum)

Richard's gray-winged sea slug Phyllodesmium lizardensis hiding among the glittering thousand-handed sea slugs.

(Image source: The Sea Slug Forum)

But to simulate the shape and spirit, it is not only necessary to have the same shape, but also the same color. Sea anemones have rich colors, red, white, brown, green and other colors complement each other. In addition to the pigments of the sea anemone's own tissues, these colors also come from the brown zooxanthellae that coexist in the sea anemone.

Zooxanthellae live in the anemone, and the anemone provides protection and nutrition for the zooxanthellae, allowing them to perform high levels of photosynthesis and produce organic substances such as sugars. Their production can meet 90% of the anemone's energy needs. The more zooxanthellae there are, the darker the anemone is; the less zooxanthellae there are, the lighter the anemone is. Even within the same species of anemone, different individuals can have different colors, so in order to hide seamlessly in an anemone, you must also learn the ability to change color.

But the gray-winged sea slugs, which are good at "recruiting", can always come up with their own countermeasures. In the process of eating anemones, they will obtain the pigments of the anemones, place them on their white bodies, and dye themselves the same color as the anemones.

After a period of time, the pigment will gradually decompose, the dye will fade, and their bodies will turn white again. At this time, if the gray-winged sea slug wants to "move" and change the anemone to eat, the white body will be dyed the color of the new anemone like a piece of white paper, making it perfectly integrated into the new environment.

Phyllodesmium rudmani feeding on a green sea slug

(Image source: The Sea Slug Forum)

Phyllodesmium rudmani feeding on a yellow slug

(Image source: The Sea Slug Forum)

Phyllodesmium rudmani appears white as it feeds on white anemonefish. The white color is the natural color of the sea slug, and the rest of the color comes from the anemones it feeds on.

(Image source: The Sea Slug Forum)

In addition to "co-opting" the anemone's pigments, the gray-winged sea slug also hijacks the zooxanthellae in the anemone's body and cultivates them on its body. Like the stinging cells, the zooxanthellae are also stored in its gills, but the two are kept in different locations. Remember the alternating white-brown color pattern on the top of the gray-winged sea slug's gills? In fact, the brown areas on the top of the gills are where the zooxanthellae are cultivated, and it is because of the presence of zooxanthellae that these areas appear brown.

The area where the gray-winged sea slug cultivates zooxanthellae has a special structure inside, consisting of a large number of tubes. The tubes branch out in large numbers at the top of the exposed gills and eventually form bubble-like sacs at the end, just like the trachea-alveolar structure of human lungs, which greatly increases the surface area of ​​the tubes. The zooxanthellae live in the small sacs at the top, forming rows of brown spots.

Simply put, the gills of the gray-winged sea slug are a garden for zooxanthellae. The pipeline system is equivalent to water pipes, allowing the gray-winged sea slug to provide nutrients such as carbon dioxide, nitrogen and phosphorus to the zooxanthellae. The sacs are equivalent to flower pots, where the zooxanthellae continue to carry out high levels of photosynthesis. They can survive in the body of the gray-winged sea slug for 11 days, producing sugars for it, and in some gray-winged sea slugs they can even provide about a quarter of the nutrition.

But zooxanthellae are also a sweet burden for them - they need sunlight to work. Therefore, we can sometimes see gray-winged sea slugs crawling out of their hiding places, coming to the outside of the anemone bush, stretching their gills like sunflowers, swaying with the water flow to face the sun, so as to provide more sunlight for the zooxanthellae to photosynthesize.

At this time, they have reached an agreement with the anemone not only in shape and color, but also in movement - the latter will also wave its tentacles towards the sun and sway gently with the water flow. The predators and prey, who would even fight each other when they meet normally, now bask in the sun together harmoniously for the sake of the same delicate and useful zooxanthellae "princess", and may even communicate with each other and complain about the difficulty of "raising algae".

Detail of the gills of a gray-winged sea slug, showing the brown areas composed of numerous brown globules and tubes that contain zooxanthellae.

(Image source: The Sea Slug Forum)

Phyllodesmium longicirrum, a long-bearded gray-winged sea slug, feeds on the leather coral Sarcophyton trocheliophorium. It is one of the most spectacular solar sea slugs, with large and flat gills compared to its body, like solar panels on a satellite in orbit around the Earth. The brown area contains zooxanthellae, and the white branches are ducts from the digestive gland, which transport the zooxanthellae from the stomach to the brown area.

(Image source: The Sea Slug Forum)

Of course, most of the time, the gray-winged sea slug is still an outright enemy of the anemone. After eating and drinking in the anemone group and "growing up", the gray-winged sea slug will begin to reproduce. They lay large eggs, about 0.1~0.2mm in diameter, which develop into free-swimming, non-feeding planktonic larvae. These planktonic larvae will become crawling larvae within a few minutes and begin to feed on the anemone.

However, it is difficult for the larvae to float too far in these few minutes. As a result, most of them land on the anemone group where their parents are and continue to feed on the same group of anemones.

I guess these anemones are devastated, and they don’t want to be fleeced even if they can…

Conclusion

Gray-winged sea slugs are highly specialized marine organisms that not only have the amazing ability to mimic sea anemones, but can also incorporate the immature stinging cells of sea anemones to protect themselves, and take in the zooxanthellae of sea anemones for photosynthesis, making perfect use of sea anemones. These strategies allow them to hide themselves, protect themselves, and obtain energy in the marine environment, making them a very "successful" group.

Their unique way of survival shows us the magic of nature: a creature can imitate other creatures vividly without any intelligence. This is the "miracle" of natural evolution.

Editor: Sun Chenyu

References:

[1]Burghardt I, Schrödl M, Wägele H. Three new solar-powered species of the genus Phyllodesmium Ehrenberg, 1831 (Mollusca: Nudibranchia: Aeolidioidea) from the tropical Indo-Pacific, with analysis of their photosynthetic activity and notes on biology[J]. Journal of Molluscan Studies, 2008, 74(3): 277-292.

[2]FitzPatrick SK, Liberatore KL, Garcia JR, et al. Symbiodinium diversity in the soft coral Heteroxenia sp. and its nudibranch predator Phyllodesmium lizardensis[J]. Coral Reefs, 2012, 31: 895-905.

[3]Moore E, Gosliner T. Additions to the genus Phyllodesmium, with a phylogenetic analysis and its implications to the evolution of symbiosis[J]. The Veliger, 2014, 51: 237-251.