Cycloglossum: The happiness of the “richest animal” is beyond your imagination!

Produced by: Science Popularization China

Author: Komeichiren

Producer: China Science Expo

Compared with the abundant food and clothing in human society, most animals in nature are very "poor". They can hardly guarantee even basic food and water sources, often starving, and have to spend most of their lives looking for food and avoiding natural enemies, with almost no quality of life to speak of.

In nature, there is a group of sea slugs called "Sacloglossa" that have unlimited food. No animals compete with them for food, and no animals even eat them. They don't have to worry about their livelihood at all. They can be said to be the "richest" among animals. How happy are their lives?

The richest man also has a history of entrepreneurship - he almost died by eating grass and mud

In human society, many successful entrepreneurs have had a start-up phase where they started from scratch, and the "richest man" in nature - the sac-glosseted sea slug is no exception.

In the Jurassic period, the cystoglossidae sea slugs were the absolute “poor”. They belonged to the class of molluscs called gastropods, and looked like snails, but were tiny in size, and had no thick shells, but only a large, thin, bubble-like cylindrical shell. This physical sign made it difficult for them to resist attacks, and the only way to survive was to burrow into the sand to escape predators.

Sand is not a rich place, and there are only two kinds of food that are easily available to sea slugs: the excrement of other animals and food that no animal is willing to eat. However, there are still many bottom-dwelling organisms, such as worms, echinoderms and other snails, competing with them for the nutrients in the waste.

Therefore, early cystoglossi were limited to eating food that most animals disliked: the underground parts of hard, filamentous algae (Udoteaceae).

Calcium fan algae belongs to the green calcium algae, which is a very special type of algae. It can grow to more than 10 cm in size, but it is a single-celled organism, that is, the entire algae is a single huge cell. This giant cell has a hard calcified cell wall and stores calcium carbonate in its internal tissues, making it as hard as coral and inedible. Therefore, few animals can feed on Calcium fan algae.

Calcarea

(Image source: iNaturalist)

Fortunately, although the cystoglossids are weak, they are good at gnawing. Although the ancestors of the cystoglossids have evolved triangular teeth that are harder and sharper than those of ordinary mollusks, the calcareous algae are too hard, and they can only slowly scrape off a section of the cell wall of the calcareous algae to create a gap and eat the cytoplasm inside.

Unfortunately, the algae will not "sit and wait for death". After being attacked, their chloroplasts will take on the task of thrombocytes: the chloroplasts of algae are thick, hard, elastic, and difficult to digest, and can quickly block the gap to form a wound plug. Subsequently, the algae will quickly secrete calcium to reconstruct the cell wall and prevent the outflow of cytoplasm.

Faced with the "defense response" of the calcium fan algae, the early sac tongues could only nibble on some chloroplasts, wound plugs and cytoplasm before the gap healed. These things were not easy to digest, but they had to accept them reluctantly in order to survive.

The radula of molluscs is covered with a large number of small teeth, which can be used to chew like a file.

(Image source: Wikiwand)

However, even so, there are still other hungry animals competing with the cystoglossa, and some other gastropods, echinoderms and fish can also break through the physical defenses of the calcareous algae with their huge bodies and sharp teeth.

Under the combined effects of natural disasters and biological predation, the filamentous calcareous algae gradually disappeared, and the early cysts that fed on them also lost an important food source and subsequently lived a life of hunger.

Today, only the Cylindrobullidae family survives from the early cystoglossids, and they still hunt for rare calcareous algae on the seafloor. Due to the scarcity of food, the species and number of Cylindrobullidae are very limited.

Cylindrobullidae is the most primitive and ancestral family of sacral glossids, and it allows us to imagine what the early sacral glossids looked like. Image credit: Marshall Islands

However, in the face of disaster and starvation, some sac tongues did not sit still and wait for death. They tried to drill out of the sand and look for new food sources. Their efforts paid off, and the sac tongues finally found fern algae, and have since forged an indissoluble bond with it.

Who can compete with the “chemical master” fern?

Caulerpa is similar in structure to Chlorella, and is also a large single-celled organism with a cell wall enclosing the cytoplasm. This similar structure helped the cystoglossus adapt to the new food.

Unlike green calcium algae, which have calcified cell walls and rely on physical means to resist predators, fern algae are soft and easy to be eaten.

However, because ferns innovatively use xylan and mannan as cell walls to provide them with structural support and physical protection, but there are no symbiotic microorganisms in all animals that can decompose xylan or mannan. The cell walls composed of these two types of polysaccharides are like a layer of plastic film and are almost indigestible. After eating, they will disappear the next day just like enoki mushrooms.

Even with sharp teeth, animals' digestive systems find it difficult to get nutrients past the outer cell wall, which means any animal that relies on gnawing but cannot separate the cell wall from the cytoplasm cannot use it as its main food source.

Sea grapes of the genus Caulerpa (Caulerpa lentillifera)

(Image sources: Free Vectors, Stock Photos & PSD)

At the same time, fern algae accumulate and secrete fern toxins similar to ciguatera toxins, which accumulate around the algae and reach concentrations so high that even seawater cannot dilute them. In this way, fern algae can drive away and poison the animals and plants that approach them, and their toxicity and smell make herbivores "change color when they see them."

Unless there is really no other food to eat, no animal would want to eat the fern algae that secretes a large amount of toxins, and other algae cannot compete with the poisonous fern algae, which makes the number of fern algae very large. However, to put it another way, once an animal overcomes these two difficulties, it is equivalent to obtaining an unlimited source of food.

Under the macro lens, you can see the internal cytoplasm and chloroplasts of fern algae wrapped by the cell wall.

(Photo credit: Vaucheria litorea – AquaPortail)

"The Chosen Plant" - Moving Forward Hand in Hand

The cystoglossidae are a natural advantage. For the cystoglossidae, their original feeding method is to scrape off the cell wall and eat the chloroplasts and cytoplasm inside. Therefore, the cell wall components of the fern algae have almost no effect on them.

However, the cytoplasm of fern algae is highly viscous and difficult to flow. It is difficult to get enough cytoplasm out by slowly grinding out a notch. For this reason, the sac tongue evolved a special large, single-pointed, dagger-shaped bayonet-shaped radula and a barrel-shaped, muscular mouthpart suitable for sucking. They can directly cut a large hole in the cell wall with the blade-like teeth, and then stick the mouthparts to the perforation to suck the internal cytoplasm.

This way of eating is similar to people sucking jelly - using a blade to open the plastic film on top, and then using a straw to suck the jelly liquid inside. In human thinking, this seems simple, but for sacral tongues, this skill is enough for them to stand out among a group of animals that cannot "open the lid" and enjoy the delicacy alone.

Most living sacloglossa have large, dagger-like teeth that can pierce cell walls and suck out the fluids inside.

(Image source: Reference [6])

The cysts have also found an effective way to overcome the deadly toxins secreted by ferns and algae.

Because they have to take on the task of forming wound plugs, the chloroplasts of fern algae have thick walls and a strong, highly elastic outer membrane, making them difficult to digest. The weak digestive system of the cystoglossidae cannot digest them quickly, so the chloroplasts can only remain in the digestive tract for a long time.

However, the indigestible chloroplasts provide a service for the cystoliths to remove toxins. In response to their own toxins, the chloroplasts are given "antidotes" by the fern algae - enzymes that deal with the fern algae toxins, which prevent the toxins from harming the chloroplasts themselves.

Thus, a strange coincidence was formed between the two. After the sac tongue eats the chloroplasts, due to indigestion, the chloroplasts survive in the body for a longer period of time, and while the chloroplasts are still active, they pass the "antidote" to the sac tongue, and continue to use the enzymes in the body to help the sac tongue resist the toxin.

A long-legged snail feeding on sea grapes

(Photo source: SEASLUG.WORLD)

With the help of chloroplasts, the sac tongues are no longer afraid of the toxins of the fern algae. They can even isolate, concentrate and modify them in their bodies, making their own bodies toxic, so that predators will retreat. The toxins used by the fern algae to drive away the enemy have become the "magic weapon" of the sac tongues. No animal competes with them for food, and no animal can eat them. From then on, the sac tongues enjoyed a safe, wealthy and happy time.

Never forget the old favors, and move forward hand in hand

Although they are already "rich" enough, the sac-glossum seems to understand the principle of "one should never forget the well digger" and tries hard to "repay" the chloroplasts for helping them resist toxins.

Some cystoglossids abandoned the protection of their shells and built a "new home" for chloroplasts in their bodies. They shed their shells, exposing the skin on their backs, and their digestive glands extended some tiny branching tubules, which continued to branch and extend to the mantle on their backs, forming a branching structure like that of human lungs, specifically used to store chloroplasts.

With this structure, the chloroplasts can continue to remain active inside the tube, still carry out photosynthesis as usual, provide the sea slug with sugars as nutrition, and help resist toxins longer until the end of their natural lifespan after they leave the cell nucleus.

Through the translucent gills, you can clearly see the granular chloroplast storage area inside.

(Image source: Nudibranch Domain)

The two life forms, from plants and animals, seemed to be a match made in heaven. Chloroplasts provide energy for the cystoglossids, allowing them to leave the fern and algae where they live, and take their immobile chloroplasts on a "travel on a whim".

After acquiring chloroplasts that could survive for a long time, the shell-less sactail-tongues tried a large number of new foods. In this process, the sactail-tongues also underwent a variety of adaptive evolutions and reached a new technological peak in retaining chloroplasts.

The foldable leaf sea slug (Elysia chlorotica) of the order Sacralose has genes for chloroplast-related proteins horizontally transferred from algae, so it can sustainably maintain chloroplasts, giving them a nearly unlimited lifespan. Once it has eaten algae and ingested the necessary chloroplasts, the leaf sea slug can continue to produce chlorophyll, maintain photosynthesis, and meet all nutritional needs.

Elysia chlorotica

Solar-Powered Sea Slugs Becoming Too Rare to Study

The powerful photosynthetic ability also gives the Sea Slugs the same willfulness as plants. The Sea Slug (Elysia marginata) of the Sea Slugs family has developed a unique skill of rebirth after being decapitated by relying on photosynthesis. They can disconnect their heads from a small groove connected to the body by themselves, and the head retains part of the ventral foot, which can crawl. In about 20 days, the head can grow a new body without parasites. Before the digestive system grows, the sea slug will rely on photosynthesis to obtain nutrients and complete a perfect rebirth.

The split-back sea beetle grows a whole new body

(Image source: Daily briefing: Sea slugs cut off own heads to grow a fresh body)

Conclusion

When you want to eat, you can just open your mouth and food will come to you. When you want to stay at home, no one will disturb you. You can live without eating and you don’t have to rush for a living. When you want to travel, you can just go. You can “go on the road” with unlimited energy anytime and anywhere. If you feel sick and weak, you can throw it away and replace it with a new one... Are you envious of the happiness of these little sea slugs?

Editor: Ying Yike

References:

[1]Jensen K R. Behavioral adaptations and diet specificity of sacoglossan opisthobranchs[J]. Ethology ecology & evolution, 1994, 6(1): 87-101.

[2]Marín A, Ros J. Chemical defenses in Sacoglossan Opisthobranchs: Taxonomic trends and evolutionary implications[J]. Scientia Marina, 2004, vol. 67, num. Supl. 1, p. 227-241, 2004.

[3] Händeler K, Grzymbowski YP, Krug PJ, et al. Functional chloroplasts in metazoan cells-a unique evolutionary strategy in animal life[J]. Frontiers in Zoology, 2009, 6(1): 1-18.

[4] Hirokane Y, Miyamoto A, Kitaura J, et al. Phylogeny and evolution of functional chloroplast retention in sacoglossan sea slugs (Gastropoda: Heterobranchia)[J]. Organisms Diversity & Evolution, 2022, 22(2): 419-429.

[5]Jensen K R. Evolution of the Sacoglossa (Mollusca, Opisthobranchia) and the ecological associations with their food plants[J]. Evolutionary Ecology, 1997, 11(3): 301-335.

[6]Takano T, Hirano YM, Trowbridge CD, et al. Taxonomic clarification in the genus Elysia (Gastropoda: Sacoglossa): E. atroviridis and E. setoensis[J]. American Malacological Bulletin, 2013, 31(1): 25-37.