Editor's Note:
This article was invited by the Yunhai Science Popularization Team to be original, first published on the public account "China Science Popularization Expo", thanks to the Chinese Academy of Sciences Science Popularization Communication Platform and Science Popularization China Sunac Program support.
The following article is from the China Science Expo, written by Yuan Ziming
Since the Middle Ages, knighthood has been a symbol of glory and loyalty:
For the treasured things to go to the soup,
Even the shattered bones are not resigned.
Noble chivalry has thus also fulfilled the romance in the hearts of countless people. In nature, this spirit is also constantly circulating, as can be seen in the trapezoidal crabs that are dedicated to protecting corals.
From left to right: Red-spotted trapezoidal crab Trapezia guttata; Red-spotted trapezoidal crab Trapezia rufopunctata; Trapezia garthi (Image from the author)
Part 1
Where the corals are, the trapezoidal crabs have a home
Trapezoidal crab Trapezia rufopunctata (Image: Ma Shaobo)
Corals are also never stingy when it comes to food.
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1
In polyps: Symbiotic zooxanthellae provide nutrients through photosynthesis
2
Nutrients: Stored in the form of lipids by polyps
3
Partial lipids: Excreted from the body along with mucus, it becomes the main food source for trapezoidal crabs
The symbiotic zooxanthellae in polyps provide nutrients through photosynthesis, which are stored by the polyps in the form of lipids, accounting for 30%-40% of the dry weight of their tissues.
Some of these lipids are excreted from the body along with mucus, which is the main food source for trapezoidal crabs.
Not only that, some cup-shaped corals even specially pack up the lipids to form a huge fat body in the tentacles, and when there is a trapezoidal crab symbiosis, these expanded fat bodies are constantly produced to provide a steady stream of catering services, and when the trapezoidal crab is removed, the fat body is no longer manufactured, which is simply a loving bento specially prepared by the kind coral princess.
Staghorn cup coral (Pocillopora damicornis), white clumps are fatty bodies (Stimson, 1984)
Part 2
You cook, I clean, and we have good years
Faced with the gift of coral, the trapezoidal crab could not be excused, and even brought its own tableware.
The stepping knuckles of the trapezoidal crab have a slime brush and a slime comb:
Slime Brush: Located at the end of the knuckles, it consists of several small spines that are used to agitate and stimulate the polyps to promote their mucus production, and there are also clusters of hairs behind the small spines for collecting mucus.
Slime comb: Consists of 3-6 rows of pinnate bristles used to concentrate and transfer mucus, eventually delivering the mucus to the mouth of the trapezoidal crab. Together, the two form a special tool for trapezoidal crabs to feast on mucus.
Trapezoidal crabs are also well versed in local conditions, and different symbiotic crabs tend to coexist with different corals, and their tableware is slightly different.
Trapezoidal crab
coral
Amount of coral mucus
"Cutlery" size
Trapezoididae
Cup coral family (Pocilloporidae)
less
Larger
Tetraliidae
Staghorn coral family (Acroporidae)
More
Smaller
Trapeziidae, which often symbiotic with less mucus, are symbiotic with smaller mucus, and their brushes and combs are larger in order to improve the efficiency of eating;
And its relative, tetraliidae, tends to coexist with the slimy-rich Staghorn coral family Acroporidae, and the tableware will be much smaller.
A-D: step foot knuckles of the trapezoidal crab family; H-K: step foot knuckles of the quasi-trapezoidal crab family; 1 slime brush, 2 mucus comb (Knudsen, 1967)
Of course, there is no reason to receive favors from people out of thin air, and human society pays attention to the reward of gratitude, and the same is true in nature.
In return, the trapezoidal crab is obligated to act as a flower protector of the coral. They clean up deposits, debris, and mucus secreted by some gastropods from the surface of corals.
The seemingly mundane cleanup is actually of great significance to the health of corals – the accumulation of dirt can cause a series of serious negative effects on the corals, and even large-scale deaths.
For those polyps that are deep in the branches, the removal of trapezoidal crabs has greatly improved their survival rates, and to some extent, it has also prevented the invasion of corals by epiphytes such as sponges, sea squirts and filamentous algae.
A mucus mesh made by the large snake snail Ceraesignum maximum. This slime is so deadly to corals that trapezoidal crabs can remove them (Credit: Hal Lescinsky)
Part 3
Even if the enemy is a hundred times stronger, it will also go to the soup for you
Taking care of the princess's household chores is important, but this is not the main job of a knight.
In the brutal deep sea, coral reefs face great threats at all times. Countless gastropods, echinoderms, and fish will feed on corals, like dragons, and look at corals. The weak Coral Princess could neither escape nor fight back, and could only be fed up with mermaid flesh.
At this point, the trapezoidal crab will transform into a brave knight to meet strong enemies, deter and attack those who try to prey on the polyps, forcing them to retreat.
Among the many predators, the most deadly to corals is the Acanthaster planci. These massive predators can grow to 25-35 cm, and in recent years they have repeatedly exploded, looting a large number of coral reefs like locusts, causing devastating blows.
In the presence of these predators, who are a hundred times larger than themselves, the indomitable golden spirit of the trapezoid crab is revealed, and when the long-spined starfish approaches the coral, they will not be afraid to charge, attacking the tube foot of the starfish with a sharp claw, or trying to break its thorn.
Although petite in size, through its dexterous hands and reckless attacks, trapezoidal crabs can often beat their fierce enemies to the ground and retreat. Studies of Coral Reefs on Guam and the Great Barrier Reef have shown that the protection of trapezoidal crabs can reduce coral damage by a factor of 2-3 by long-spined starfish.
"Your name is Longspinn Starfish?" The thorns are all broken for you" (gif, from: Smithsonian channel)
The bitter long-spined starfish will give priority to corals that do not have the protection of symbiotic crabs, and in corals with symbiotic crabs, they will adhere to the concept of "persimmons should pick soft pinches" and pick weaker ones.
For example, they would rather attack staghorn corals than touch cup corals, most likely because the quasi-trapezoidal crabs that symbiotic with staghorn corals are generally smaller than the trapezoidal crabs that symbiotic with the cup coral family.
Smaller trapezoidal crabs can only attack the tube feet of starfish, while larger trapezoidal crab species choose to directly break the spines of starfish.
For starfish, the long-lasting pain caused by the latter is obviously more difficult to digest. If symbiotic crabs are removed, starfish do not show a pronounced tendency to prey on corals.
From this point of view, the trapezoidal crab species are undoubtedly the knights of corals and one of the few symbionts that can effectively repel the long-spined starfish.
In Panama, cup-shaped corals protected by trapezoidal crabs survived the catastrophe of the eruption of long-spined starfish, and even gained a more dominant position in the decaying community, and the courage and strength of the knights were really effective and admirable.
Dark trapezoidal crab Trapezia septata (Image: Ma Shaobo)
Part 4
A knight on land who guards the princess
The knights who guard the princess not only exist in the deep sea, but also on land. The mutually beneficial symbiosis of corals and trapezoidal crabs is very similar to the relationship between Acacia drepanolobium and crematogaster mimosae in the African savanna.
The hollow spines of the acacia tree provide shelter for the ants and secrete sap for their consumption. Ants bite to protect acacia trees from herbivores.
Both are one as a princess and the other as a knight. One side provides sheltered harbors and hot meals, while the other side goes to the soup to protect the fire.
Whenever the dragon descended, these knights would block the princess, showing their fangs and offering the most violent fight for those who dared to invade.
bibliography
Shaobo Ma, Hazy Qu, Geng Qin, 2020, Coral Guard-Trapezoidal Crab. nature. 2020(04): 32-37.
Knudsen JW, 1967, Trapezia and Tetralia (Decapoda, Brachyura, Xanthidae) as obligate ectoparasites of pocilloporid and acroporid corals. Pacif. Sci. 21: 50-57.
Pratchett MS, 2001, Influence of coral symbionts on feeding preferences of crown-of-thorns starfish Acanthaster planci in the western Pacific. Mar Ecol Prog Ser 214:111−119.
Rinkevich B, Wolodarsky Z, Loya Y, 1991, Coral-crab association: a compact domain of a multilevel trophic system. Hydrobiologia 216: 279–284.
Stier AC, McKeon CS, Osenberg CW, Shima JS, 2010, Guard crabs alleviate deleterious effects of vermetid snails on a branching coral. Coral Reefs 29:1019−1022.
Stimson JS, 1984, Possible uses of lipid stored in coral tissues. Am. Zool. 24: 78A.
Weber JN, Woodhead PM, 1970, Ecological studies of the coral predator Acanthaster planci in the South Pacific. Mar. Biol. 1970, 6, 12–17.
Author: Yuan Ziming, Institute of Oceanology, Chinese Academy of Sciences
Editor-in-Charge: Liu Meijun, Beijing Normal University
Thank you to Master Ma Shaobo of the South China Sea Institute of Oceanology, Chinese Academy of Sciences for his help for this article!