Japanese Spider Crabs: All About The Giant and Scary Crabs

It is time to learn about the Japanese Spider Crabs or Macrocheira kaempferi in scientific terms, which is the largest crustacean in the world

The body of this crustacean is quite similar than that of a spider, but much bigger. They grow up to 37 cm in length and they have an amazing leg span of up to 3.8m, which is their most outstanding characteristic. The weight of this species is roughly 20 kgms

Japanese Spider Crabs Main Characteristics

This crab is a decapod crustacean, which belongs to the group of spider crabs, so it is characterized for having a triangular shell and long legs.

The Japanese spider crabs lack of sense of vision, but they compensate this lack, with their quite developed sense of hearing.

The legs of the Japanese spider crab can reach more than two meters in length, the frontal being the longest ones. So, the legs in conjunction with their body can reach a total span (from cheliped to cheliped) of  3.8 meters in diameter, and about twenty kilograms in weight.

There have been not confirmed reports about certain specimens that reach almost seven meters in total span when they are totally  expanded. They can survive losing even three legs, which many times grow again when they molt. They lose them due to fights against their  predators , or when they get trapped within nets left by human .japanese spider crabs: anatomy

These weird but inoffensive crabs, live mainly in the  waters of the Pacific Ocean near Japan, onthe sandy bottom of the continental shelf, at an average depth of 150 to 300 meters. This location makes very difficult to see them or to capture them. However, since this weird crab is one of the most exclusive products for the Asian gastronomy; several efforts are carried out by the fishermen to fulfill their tasks.

According to the scientists the pincers of these crabs are even more powerful than the jaws of the crocodiles, which in conjunction with the anticoagulant that they employ against their predators, make of them a sort of lethal weapon.

In terms of color the Japanese spider crab is orange, while its legs often have white spots on them..

The Japanese spider crabs usually carry out a sort of camouflage practice known as cripsis. This is a defense mechanism that allows certain organisms to pass unnoticed to the senses of other animals, which may be both their prey, and their predators.

In the case of the Japanese spider crabs they adhere to their bodies, different kinds of remains that they find in the sea such as sponges, anemones, and other animals that they cement to the tops of their carapaces.

This sort of camouflage remains attached to their bodies for long time, but when they move to a different location they get rid of their old decoration, to collect new remains to adhere to their bodies. That is why in some parts of the world they are also called masked crabs, or decorative crabs.

This weird and even scary creature is also considered as a living fossil, since it is an ancient species of crab; in fact it is the only living species of the genus Machocheira

Another distinctive characteristic of this species lies in its eyes, which are strategically located in the front portion of their bodies. Moreover, they also have two stick-shaped horns, which are usually much longer when they are young, but then they atrophy as the crabs grow. .

The juvenile Japanese spider crabs usually have hair and thorns on their shells, which they lose as they develop. Although these crabs inhabit mostly in the deep waters of the Pacific and the coasts of Japan, they move to shallower waters of no more than 50 meters of depth to perform the spawning.

Another distinctive feature of this species, is given by its head and thorax which are fused, forming this way the cephalothorax and the abdomen. They also possess articulated oral appendices comprised by maxillipeds. The Japanese spider crab has special appendages called pleopods, which they employ to swim, and they breathe through their gills.

Although this animal can swim very well, it moves with the help of its legs. Like all crabs, it also molts its exoskeleton, making several changes as it grows. During the molting process they are quite susceptible to the attacks of their predators, since its exoskeleton is very soft at this moment.

(See our article Crabs for a more detailed information about how crabs molt their exoskeleton )

And now let´s learn more about them

 Japanese Spider Crabs Common Habitats

They like the vents and holes of the deeper areas of the oceans, which makes of them a difficult prey  for the fishermen So it’s not a surprise that this crab is a delicacy in Japan.
Although this animal has been the protagonist of several unbelievable histories around the terrestrial globe, most of them belong to the nourished imaginative mind of those who created them. This species only lives at the  Japanese archipelago´s pacific area, at depths which range between one hundred fifty (150) and three hundred (300) meters deep.

For this reason, the stories that sailors and navigators usually tell about the Japanese spider crabs, lack of support and are quite unlikely.

They can only be fished by employing trawls, and most of the time it happens in the bays of Sagami, Suruga, and Tosa; and on a few occasions, around the Izu Islands. This crab is a culinary delicacy, so it is persistently searched; but in recent years its catches has decreased considerably, so several initiatives have been created to protect it.

One of the protective measures that have been developed, forbid hunting these crabs during spring, since that’s their spawning season. However, in the locality of Wakaya its capture is allowed, because it is carried out for repopulating purposes.

Due to the kind behavior of this animal it is usually captured for ornamental purposes, and to be raised in aquariums. That is why they are exported to various parts of the world for later exhibition.

In this sense, we can mention the case of two creatures,  that  have just arrived  at the Sea Life Centre in Manchester from north Weymouth. They have become in the biggest attraction of this marine center, being described   as «absolutely ginormous», «really cool» and «scary» by young visitors.

Japaneese Spider Crabs Alimentation Habits

Although many people fear this species, not only because of its appearance, but also because they have called it a man-eater; this affable creature feeds mainly on marine creatures..

It has an omnivorous diet, which allows it to consume both plant and animal matter; and sometimes even acts as an eliminator of dead animals in the aquariums where they live. Just as they tear the depths of the ocean with their huge legs, searching for plants and seaweeds, they can also open the shells of different kind of mollusks that they find in their way..

Under no circumstances they can be considered as man-eater, since the Japanese crabs, contrary to what some people may believe, have an affable and docile character.

It is the speculations of some people, what have increased the fear that some other people feel for this animal, because of its weird appearance, and above all because of the intimidating size of its legs..

But those who have had a body to body contact with this crab, attest that it is a rather a shy animal, that even allows itself to be caressed without any kind of aggressive behavior against people.

Japanese Spider Crabs Reproductive Methods

As in the case of most crabs, the Japanese spider crabs are oviparous. During mating the male carries the female , during five or six hours, even for up to three days in some cases.

The females carry the eggs adhered to the pleopods until they hatch. The males do not have a copulatory organ as such, but appendage or chelipeds which perform the same function. The male transfers spermatophores to the female through such appendage, so that  the new offspring can be procreated. The development of the new offspring consists of several larval stages, which begin with the earliest one called Zoea, where the eyes, abdomen, pleopods, the telson and the pereiopods are developed. This larval stage lasts between 12 and 37 days.

After several molts the new offspring go to the next larval stage called Megalopa, where its general structure changes. The rostral and the posterior spine disappear, while the carapace becomes rounded and the cephalic appendages, the periopodos and pleopods begin to resemble those of the juvenile crabs. During the Megalopa, the japanese spider crabs are only swimmers, but they have three pairs of maxillipeds designed exclusively to capture the food. After several molts and changes they finally become in adults.

As other species such as the yeti crab, the male Japanese spider crab attracts his partner by employing pheromones, and the mating takes place just after the female has molted her shell, since it is softer in this stage… Then the females store the sperm for a long time before fertilizing them. Once fertilized, the female carries the eggs attached to her abdominal appendages, and once the development has been completed, the female releases the freshly hatched larvae one hundred and fifty meters deep.

These spider crabs mate seasonally during  spring, from January until March. We do not know for sure why this happens at this time of the year, which coincides often with the tides.

The larvae must find their food, however, they are provided with enough yolk that allows them to continue living for a while. During this stage they are quite vulnerable, and for this reason several actions have been taken, to prohibit its fishing during this season.

When the Japanese spider crabs become finally in juvenile specimens, they must continue with their continuous molts, until getting into the adult stage.

The cycles of molt are coordinated by a hormonal process. This fact allows their bodies to be prepared, by softening the old shell which becomes also partially eroded, once the signs of a new shell appear. This process requires a big ingest of water, so that the crab can expand to the maximum and cracks its carapace.

The whole process requires an arduous work, and in case the crab gets stuck in its old shell, it will die. Once the molt has finished, the new shell is soft and weak, for this reason, the Japanese spider crab often hides, to be safe from its predators.

Let´s keep learning about this weird and lovely creature

Weird Stories about the Japanese Spider Crabs

Due to the enormous size of this animal, and its appearance quite similar to that of a horrendous spider, hundreds of supposed anecdotes of fishermen and navigators have been told. According to such tales, they had the misfortune to run into one of these animals, but for their good luck, they could get out alive of the terrifying encounter.

The first encounter occurred in the Bering Sea in 2006, when a group of crabs´ fishermen lead by Phil McGraw; had a close encounter with a Japanese spider crab.

It was a cold morning, like those they were already used to have in the sea, but with the difference that it was the last day of collecting fishing cages to return home.

According to the speech of Phil McGraw; a weird creature came from the cage, when they were preparing to withdraw the last basket. At first, they thought that it was a good fishing, because of the resistance offered by the rope. However, when they finally saw what it was, all of them were out of breath.

According to Phil McGraw, there was a huge Japanese spider crab caught in the basket, which at first, they confused with the other type of crab they were looking for. But they realized immediately, that it was a different creature; which in turn was devouring the other crabs.

They just did not know how to react against such animal, which exceeded them in two bodies of height. It also had a creepy pincer that could split in two whoever it wanted. Following, they tried to chase away the crab by intimidating it with an oar of the fishing boat, but the immense creature broke it as if it were a toothpick.

Everyone was terrified of such a demonstration of power, because now they knew that this sort of monster could kill them immediately.

Then, they  ​​threw the cage with the crabs back into the sea, in order to make the creature disappear, and they were quite right, since after seeing how its food returned to the sea, the giant crab immediately jumped also after it, leaving the fishermen with a frightening experience that would accompany them for the rest of their lives.

Phil Mcgraw was the only one of them who kept working in the sea, but he never saw that creepy animal again. He stated once, that if it hadn’t been a group experience he would have thought that it was just a terrible nightmare.

We Must Be Responsible of What We Say….

This is a fantasy story without any sustenance, but sometimes people just don´t know the damage they can cause to a species, by spreading this type of misinformation.

Nowadays, the social networks, the press and television lend themselves to creating false positives, which just alarm the most gullible people and get them to hate certain species.

Maybe someone could say that it is only a joke, but this is the way how rumors arise becoming later in true, especially for those who don´t  know about these topics. These actions often result in a very bad publicity for the marine species, which are the most disadvantaged in this situation.

In many occasions these turn out to be a ruse by those interested in fishing and commercializing these animals, in order to justify their actions.

Since the Japanese spider crab is a species that lives in the depths, and there is not much knowledge about it, some people have obtained a special permit, to capture them, in order to study their behavior.

However, the poaching of this species has not been totally controlled, and the Japanese spider crab and its eggs; are still captured in this way. Moreover, this practice is very dangerous for this species, since it becomes almost impossible for them to grow and develop.

Since the Japanese spider crab is an exquisite meat for the Asian culinary art, it turns out to be even more attractive its capture, given the benefits that the commercialization of this animal entails.

What will they do when the Japanese spider crab no longer exists? Perhaps they realize that there were other species that also serve as food. And why are they unable to notice it at this moment?, Very simple, because the other foods are not as expensive as the delicious meat of the Japanese spider crab.

It is time to become aware of the moderate consumption of this species, to be able to enjoy of them as long as they continue to exist.

You should not get carried away by all what the social media say about the supposed attacks and damages for the human being, because they are just sales and advertising strategies.

In some other occasions, these tales are just funny stories of people who just want to become famous or who even just tell them  without a bad purpose.

Thus we must always keep in mind, that in the same way that we have only one body…. we also have only one earth, and even the most weird creatures or species are necessary  to maintain the proper echobalance. We can enjoy of all the wonders of nature, but we also must respect them

«When the last tree has been cut, the last fish has been fished and the last river has been poisoned, maybe we will understand that we just can´t eat money»… Indigenous Proverb

This is what we can enjoy from our world, if we follow the proper measures


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