Spider crab

The giant spider crab is the largest known species and can live up to 100 years. The Japanese name for the species is taka-ashi-gani, which literally translates as “crab with high legs.” Its bumpy shell blends into the rocky ocean floor. To enhance the illusion, the spider crab decorates its shell with sponges and other animals. Although these creatures frighten many with their arachnid appearance, they are still an amazing and exciting marvel hidden in the deep ocean.

Origin of the species and description

Photo: Spider Crab

Photo: Spider Crab

The Japanese spider crab (タカアシガニ or “legged crab”), or Macrocheira kaempferi, is a species of sea crab that lives in the waters around Japan. It has the longest legs of any arthropod. It is a fishery item and is considered a delicacy. Found two fossil species belonging to the same genus, ginzanensis and yabei, both from the Miocene period in Japan.

Video: Spider crab

There were many controversies during the classification of the species based on larvae and adults. Some scientists support the theory of a separate family for this species and believe that further research is needed. Today this species is the only known surviving member of the Macrocheira, and is considered one of the earliest branching members of the Majidae. For this reason, it is often referred to as a living fossil.

In addition to one extant species, a number of fossils are known that once belonged to the genus Macrocheira:

  • Macrocheira sp. — Pliocene Takanabe Formation, Japan;
  • M. ginzanensis — Miocene form Ginzan, Japan;
  • M. Yabei — Miocene Yonekawa Formation, Japan;
  • M. teglandi — Oligocene, east of Twin River, Washington, USA.

The spider crab was first described in 1836 by Koenraad Jakob Temminck under the name Maja kaempferi, based on Philipp von Siebold's materials collected near the artificial island of Dejima. The specific epithet was given in memory of Engelbert Kaempfer, a German naturalist who lived in Japan from 1690 to 1692. In 1839, the species was placed in a new subgenus, Macrocheira.

This subgenus was elevated to genus status in 1886 by Edward J. Myers. The spider crab (M. kaempferi) has fallen into the family Inachidae but does not quite fit into this group, and it may be necessary to create a new family exclusively for the genus Macrocheira.

Appearance and Features

Photo: Animal Spider Crab

Photo: Animal spider crab

The Japanese giant spider crab, although not the heaviest in the underwater world, is the largest known arthropod. A well-calcified carapace is only about 40 cm, but the total length of adults can be almost 5 meters from one tip of the cheliped (clawed claws) to the other when stretched. The shell has a rounded shape, and closer to the head it is pear-shaped. The entire crab weighs up to 19 kg – second only to the American lobster among all living arthropods.

Females have a wider but smaller belly than males. Spiny and short tubercles (growths) cover the carapace, which varies from dark orange to light brown. It does not have a mysterious coloration and cannot change color. The continuation of the shell on the head has two thin spikes protruding between the eyes.

The carapace tends to remain the same size throughout adulthood, but the claws elongate significantly as the crab's age increases. Spider crabs are known for having long, thin limbs. Like the carapace, they are also orange, but can be spotted, with patches of both orange and white. The walking claws end in inwardly curved movable parts at the tip of the walking limb. They help the creature climb and cling to rocks, but do not allow them to lift or grab objects.

In adult males, the chelipeds are much longer than either of the walking legs, with the right and left claw-bearing chelipeds being the same size. On the other hand, females have chelipeds shorter than other walking limbs. The merus (upper part of the leg) is slightly longer than the palm (the part of the leg that contains the fixed part of the claw) but is comparable in shape.

Although the legs are long, they are often weak. One study reported that nearly three-quarters of these crabs were missing at least one limb, most commonly one of the first walking legs. This is because the limbs are long and poorly connected to the body and tend to come off due to predators and nets. Spider crabs can survive if they lack up to 3 walking legs. Walking legs can grow back during the next molts.

Where does the spider crab live?

Photo: Japanese Spider Crab

Photo: Japanese Spider Crab

The habitat of the Japanese arthropod giant is limited to the Pacific side of the Japanese islands of Honshu from Tokyo Bay to Kagoshima Prefecture, usually at a latitude of 30 to 40 degrees north latitude. They are most often found in the bays of Sagami, Suruga and Tosa, as well as off the coast of the Kii Peninsula.

The crab has been found as far south as Su-ao, in eastern Taiwan. This is most likely a random event. It is possible that a fishing trawler or extreme weather helped move these individuals much farther south than their home range.

Japanese spider crabs most often live on the sandy and rocky bottom of the continental shelf at a depth of up to 300 meters. They like to hide in vents and holes in the deep parts of the ocean. Temperature preferences are unknown, but spider crabs are regularly found at depths of 300m in Suruga Bay, where the water temperature is around 10°C.

It is almost impossible to meet with a spider crab, because it roams in the depths of the ocean. Based on research in public aquaria, spider crabs can tolerate temperatures of at least 6-16°C, but are comfortable at 10-13°C. Juveniles tend to live in shallower areas with warmer temperatures.

What does a spider crab eat?

Photo: Large spider crab

Photo: Large spider crab

Macrocheira kaempferi is an omnivorous scavenger, consuming both plant matter and animal parts. It is not an active predator. Basically, these large crustaceans tend not to hunt, but crawl and collect dead and decaying matter along the seabed. By their nature, they are detritivores.

The diet of the spider crab includes:

  • small fish;
  • carrion;
  • aquatic crustaceans;
  • marine invertebrates;
  • algae;
  • macroalgae;
  • detritus.

Algae and live shellfish are sometimes eaten. Although giant spider crabs move slowly, they are able to prey on small marine invertebrates that they can easily catch. Some individuals are ocean floor cleaners from decaying plants and algae, and some open shells of mollusks.

In the old days, sailors told frightening stories about how a terrible spider crab dragged a sailor underwater and feasted in the depths of the ocean on his flesh. This is not believed to be true, although it is likely that one of these crabs will be able to feast on the dead body of a sailor who drowned earlier. The crustacean has a gentle disposition despite its ferocious appearance.

The crab has been known to the Japanese for a long time because of the damage it can do with its strong claws. Often caught for food, it is considered a delicacy in many regions of Japan and other parts of the world.

Personalities and lifestyles

Photo: Sea Spider Crab

Photo: Sea Spider Crab

Spider crabs are very calm creatures, spending most of their days looking for food. They roam the seabed, moving at ease over rocks and bumps. But this marine animal does not know how to swim at all. Spider crabs use their claws to tear things apart and attach them to their shells. The older they get, the larger their size. These spider crabs shed their shells, and new ones grow even larger with age.

One of the largest spider crabs ever caught was only forty years old, so it is not known how big it could be them when they reach 100 years old!

Little is known about the communication of spider crabs among themselves. They often forage alone and there is little contact between members of this species, even when they are isolated and kept in aquariums. Since these crabs are not active hunters and do not have many predators, their sensory systems are not as sharp as those of many other decapods in the same region. In Suruga Bay, at a depth of 300 meters, where the temperature is about 10°C, only adults can be found.

The Japanese variety of crabs belongs to the group of so-called “decorator crabs”. These crabs are so named because they collect various objects in their environment and cover their shells with them as a camouflage or protection.

Social Structure and Reproduction

Photo: Red Spider Crab

Photo: Red spider crab

At 10 years old, the spider crab becomes sexually mature. Japanese law prohibits fishermen from catching M. kaempferi during the early spring mating season, from January to April, to conserve natural populations and allow the species to spawn. Giant spider crabs mate once a year, seasonally. During spawning, crabs spend most of their time in shallow water about 50 meters deep. The female lays 1.5 million eggs.

During incubation, females carry the eggs on their backs and lower body until they hatch. The mother stirs the water with her hind legs to oxygenate the eggs. After hatching, parental instincts are gone and the larvae are left to fend for themselves.

Female crabs carry fertilized eggs attached to their abdominal appendages until tiny planktonic larvae hatch. The development of planktonic larvae is temperature dependent and takes 54 to 72 days at 12–15 °C. During the larval stage, young crabs do not look like their parents. They are small and transparent, with a rounded legless body, drifting in the form of plankton on the surface of the ocean.

This species goes through several stages of development. During the first molt, the larvae slowly drift towards the seabed. There, the cubs rush about in different directions until they click on the spikes on their shell. This allows the cuticles to move until they are released.

The optimal rearing temperature for all larval stages is 15-18°C, and the survival temperature is 11-20°C. The first stages of larvae can be traced to shallower depths, and then the growing individuals move to deeper waters. The survival temperature of this species is much higher than that of other decapod species in the region.

In the laboratory, under optimal growth conditions, only about 75% survive the first stage. At all subsequent stages of development, the number of surviving young decreases to about 33%.

Natural enemies of the spider crab

Photo: Giant Japanese Spider Crab

Photo: Giant Japanese Spider Crab

The adult spider crab is large enough that it has few predators. He lives deep, which also affects security. Juveniles try to decorate their shells with sponges, algae, or other items suitable for camouflage. However, adults rarely resort to this method because their large size deters most predators from attacking.

Although spider crabs are slow moving, they use their claws against small predators. The armored exoskeleton helps the animal protect itself from larger predators. But even though these spider crabs are massive, they still have to beware of the occasional predator like the octopus. Therefore, they really need to camouflage their huge bodies well. They do this with sponges, kelp and other substances. Their mottled and uneven carapace is very similar to a stone or part of the ocean floor.

Japanese fishermen continue to fish for spider crabs even though their numbers are declining. Scientists fear that its population may have declined significantly over the past 40 years. Often in animals, the larger it is, the longer it lives. Just look at the elephant, which can live for over 70 years, and the mouse, which lives up to an average of 2 years. And since the spider crab reaches sexual maturity late, there is a chance that it will be caught before it reaches it.

Population and species status

Photo: Spider crab and man

Photo: Spider Crab and Man

Macrocheira kaempferi is a fairly useful and important crustacean in Japanese culture. These crabs are often served as a treat during their respective fishing seasons and eaten either raw or cooked. Since the legs of the spider crab are very long, researchers often use the tendons from the legs as a subject for study. In some parts of Japan, it is customary to take and decorate the animal's shell.

Due to the mild nature of the crabs, spider crabs are often found in aquariums. They rarely come into contact with humans, and their weak claws are fairly harmless. There is insufficient data on the status and population of the Japanese spider crab. The catch of this species has declined significantly over the past 40 years. Some researchers have proposed a recovery method involving restocking with juvenile fish farmed crabs.

A total of 24.7 tons were harvested in 1976, but only 3.2 tons in 1985. The fishery is concentrated to Suruga. Crabs are caught using small trawl nets. The population has declined due to overfishing, forcing fishermen to move their fisheries to deeper waters to find and catch the expensive delicacy. Collecting crabs is prohibited in the spring, when they begin to breed in shallow water. Numerous efforts are now being made to protect this species. The average size of individuals caught by fishermen is currently 1–1.2 m.

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