The Japanese spider crab ( Macrocheira kaempferi) is a remarkable marine creature, the largest arthropod in the world, dwelling in the waters surrounding Japan. Known for its immense size, with a leg span reaching up to 3.7 meters (12.1 feet) and a weight of up to 19 kg (42 lb), this crab is a subject of fascination and ecological importance. The Japanese name for this species is taka-ashi-gani, (Japanese: たかあしがに), literally translating to “tall legs crab”. While its fearsome appearance might suggest a voracious predator, the Japanese spider crab is, in fact, an omnivorous scavenger with a varied diet.
Taxonomy and Habitat
The genus Macrocheira contains multiple species. Two fossil species of this genus have been found, M. ginzanensis and M. yabei, both from the Miocene of Japan. The Japanese spider crab is primarily found off the southern coasts of the Japanese island of Honshū, ranging from Tokyo Bay to Kagoshima Prefecture. Outlying populations have been found in Iwate Prefecture and off Su-ao in Taiwan. These crabs inhabit the sandy, rocky bottom of the continental shelf and slope, typically at depths between 150 and 300 meters (492.13-984.25 feet), but they have been found at depths of 182.88 meters (600 feet). During mating season, they spend most of their time in shallower 50 meters (164.04 feet) water depths. Adults are found at depths between 50 and 600 m (160 and 1,970 ft). They like to inhabit vents and holes in the deeper parts of the ocean. The temperature preference of adults is unknown, but the species is regular at a depth of 300 metres (980 ft) in Suruga Bay, where the water generally is about 10 °C (50 °F).
An Omnivorous Diet
The Japanese spider crab is an omnivore, consuming both plant matter and animals. These slow-moving crabs don’t hunt, preferring to scavenge for dead animal or plant matter, though they may also eat live fish or invertebrates such as other crustaceans.
- Scavenging: As a scavenger, it plays a vital role in the marine ecosystem by consuming dead animals. They primarily feed on algae, mollusks, dead fish, and other decaying organic material found on the ocean floor. They have been known to also feed on small fish, shellfish, and even carrion.
- Plant Matter: The diet includes algae and other plant material found on the ocean floor.
- Opportunistic Feeding: While primarily scavengers, they may also consume live fish or invertebrates. They scurry along the floor and pick up mollusks like clams and mussels.
Feeding Behavior and Adaptations
The Japanese spider crab's feeding habits are closely linked to its environment and physical characteristics.
- Chelipeds: Upon picking up the object with the crab's slender chelipeds, the chelae are used to twist and tear off the organism, such as a worm tube or sponge, from the substrate on which it currently resides.
- Camouflage: This species is part of a group known as decorator crabs which adorn their shells with sponges or anemones for camouflage. Juvenile Japanese spider crabs will do this, but with few predators at the depths in which they live, adults of this species don’t have to dress to impress. The crab's bumpy carapace blends into the rocky ocean floor. To further the deception, a spider crab adorns its shell with sponges and other animals. Unlike other species of crab, such as the Chilean crab Acanthonyx petiveri, the Japanese spider crab does not specifically look for matching colors to blend into its environment; it simply camouflages in a way that disguises its entire structure. This is most likely because Japanese spider crabs are nocturnally active, so instead of trying to disguise themselves when catching prey, they are actually just trying to avoid predators at night. Once the material is picked up, it is brought to the crab's mouthparts to specifically orient and shape it before it is attached to the exoskeleton.
- Nocturnal Activity: Japanese spider crabs are nocturnally active.
Life Cycle and Diet
The life cycle of the Japanese spider crab significantly influences its dietary needs and feeding behavior.
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- Larval Stages: The Japanese spider crab undergoes two zoeal stages and a megalopa stage before it reaches adulthood. During the larval stages, M. kaempferi is found near the surface, as the planktonic forms drift with ocean currents. Each of these stages is influenced greatly by temperature, both in terms of survival and stage length.
- Adult Stage: Adults are found at greater depths and have a more varied diet, as described above.
Predators and Defense Mechanisms
Despite their size, Japanese spider crabs face threats from predators.
- Predators: While Japanese Spider Crabs do not have many natural predators due to their large size and tough exoskeleton, they are occasionally preyed upon by sharks, octopuses, and occasionally large fish like groupers.
- Defense Mechanisms: Japanese Spider Crabs have developed various defense mechanisms to protect themselves from predators. Their first line of defense is their large size and sharp spines on their legs, which serve as deterrents to potential threats. Additionally, they can camouflage themselves by covering their bodies with debris from the ocean floor, making them blend in with their surroundings. The Japanese Spider Crab also has a unique molting behavior that occurs for about 100 minutes, in which the crab loses its mobility and starts molting its carapace rear and ends with molting its walking legs. The Japanese Spider Crab has an armored exoskeleton that helps protect it from larger predators such as octopodes, but also uses camouflage.
Human Interaction and Conservation
Human activities have significantly impacted Japanese spider crab populations.
- Fisheries: They are sought by crab fisheries, and are considered a delicacy in Japan. The Japanese spider crab is "occasionally collected for food", and even considered a delicacy in many parts of Japan and other areas in the region. The fishery is centred on Suruga Bay. The crabs are typically caught using small trawling nets.
- Overfishing: The population has decreased in number due to overfishing, forcing fishermen into exploring deeper waters to catch them. The average size caught by fishermen is a legspan of 1.0-1.2 m (3 ft 3 in - 3 ft 11 in). Populations of this species of crab have diminished over recent years and many efforts are being made to protect them.
- Conservation Efforts: One of the primary methods of recovery of the species being used is restocking artificially cultured juvenile crabs in fisheries. Additionally, laws have been put into place in Japan that prohibit fishermen from harvesting spider crabs from January through April, during their typical mating season when they are in shallower waters and more vulnerable to being caught. Conservation efforts focus on preventing overfishing of these crabs.
General Characteristics
Apart from its outstanding size, the Japanese spider crab differs from other crabs in a number of ways. The first pleopods of males are unusually twisted, and the larvae appear primitive. The crab is orange with white spots along the legs. It is reported to have a gentle disposition despite its ferocious appearance.
- Size: Up to 12 feet (3.7 meters).
- Weight: Up to 42 pounds (19 kilograms).
- Speed: 0.12mph (0.19km/h).
- Lifespan: 12 years - 100 years.
- Habitat: Pacific waters.
- Geography: Japan.
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