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There has been much conflict between larval- and adult- based classification. Some support the use of a separate family for this species, but much further study is needed.
Originally M. kaempferi was incorrectly placed in the genus Maja before later being placed in Macrocheira. Today, this species is the only known surviving member of Macrocheira, and is regarded as one of the earliest-branching members of Majidae. For this reason, it is often referred to as a living fossil.
Not much is known about communication in Macrocheira kaempferi. They often scavenge for food alone, and there is little communication between members of the species, even when isolated with other spider crabs in aquaria. The antennae are greatly reduced. The eyestalks are also short and stubby. Because these crabs are not active hunters and do not have many predators, their sensory systems are not as acute as those of many other decapods in the same area.
Communication Channels: visual
Perception Channels: visual ; tactile
There is insufficient data concerning the conservation status for the Japanese spider crab. The catch of this species has declined considerably in the last 40 years. Some researchers have put forth a method for recovery which involves restocking with juvenile crabs artificially cultured in fisheries. In Japan, laws prohibit fishermen from catching M. kaempferi during mating season in the early spring, from January until April, in order to keep natural populations up and to give the species a chance to spawn.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
This species goes through two zoeal stages and one megalopa stage. The zoeal stages generally last between 12-37 days, a shorter duration than other crabs in the same region. The megalopa stage typically lasts an average of 30 days. During the first molt (the prezoeal stage) the hatchlings writhe about, eventually slowly drifting to the sea bed. Here, each hatchling thrashes about until it flicks up the spines on its carapace. This dislodges the cuticle, and allows it to wriggle out by twisting and pulling until it frees itself.
The optimal rearing temperature for all larval stages is between 15-18 degrees Celsius, while the survival temperature is 11-20 degrees Celsius. Larval stages can most likely be found at shallower depths, then later move to deeper waters. In Suruga Bay, the temperature at 300 meters is around 10 degrees, and only adults may be found at these depths. These survival temperatures are much higher than those of other decapod species in the region. In the lab, at optimum growth conditions, only around 75% survive the first zoeal stage. This number drops to around 33% for the second zoeal and megalopa stages.
Development - Life Cycle: metamorphosis
There are no known adverse effects of Macrocheira kaempferi on humans. They rarely come into contact with humans, and their weak claws are fairly harmless.
Macrocheira kaempferi is quite useful and important to the Japanese culture. The crabs are often served as a delicacy during the appropriate crab-fishing seasons and are eaten both raw and cooked. Because the walking legs are so long, researchers often use tendons from the legs or chelipeds. In some parts of Japan, it is popular to take and decorate the carapace. Macrocheira kaempferi is also common in aquaria because of its gentle disposition.
Positive Impacts: food ; body parts are source of valuable material; research and education
Macrocheira kaempferi is not an active predator, as it mainly scavenges the seafloor for dead and decaying matter.
While nearly all spider crabs tend to decorate their carapaces with sponges and other items, M. kaempferi does it less than others because it has so few predators and therefore no need for camouflage. Sponges provide camouflage and protection for the crab; the spider crab carries the sponge to new areas and possibly provides it with drifting food.
Mutualist Species:
Macrocheira kaempferi is an omnivorous scavenger. These large crustaceans generally do not hunt, but instead crawl along and pick at dead and decaying matter along the sea bed. This includes both animal and plant matter. They occasionally eat living kelp and algae. Although they move slowly, giant crabs occasionally hunt for small marine invertebrates that they can catch easily. Mariners used to tell tales of M. kaempferi dragging sailors underwater and feasting on their flesh. This is generally regarded as untrue, although it is certainly plausible that one of these crabs would feast upon the dead body of a sailor who had previously drowned.
Animal Foods: fish; carrion ; aquatic crustaceans; other marine invertebrates
Plant Foods: algae; macroalgae
Other Foods: detritus
Primary Diet: omnivore ; detritivore
The Japanese spider crab Macrocheira kaempferi is mostly limited to the Pacific side of the Japanese islands, Konshu and Kyushu, usually at a latitude between 30 and 40 degrees North. They are found most often in the Sagami, Suruga, and Tosa bays, as well as off the coast of the Kii peninsula. However, the crab has been found as far south as Su-ao, in Eastern Taiwan. This is most likely a one time event; it is possible a fishing trawler or extreme weather may have carried this individual much further south than its home range.
Biogeographic Regions: palearctic (Native )
Japanese spider crabs most often inhabit the sandy and rocky bottom of the continental shelf and slope at an average depth of 150-300 meters. They have, however been found at depths of 600 feet. During spawning season the crabs spend most of their time in shallower waters around 50 meters. In Suruga Bay, at depths of 300 meters, the temperature is around 10 degrees Celsius. Younger crabs tend to live in shallower areas with warmer temperatures.
Range depth: 50 to 600 m.
Average depth: 200 m.
Habitat Regions: temperate ; saltwater or marine
Aquatic Biomes: benthic ; coastal
Relatively little is known information regarding the longevity of this species. It is often reported that one of these crabs may live to be 100 years old in its natural habitat, but this may be conjecture. Other reports indicate that M. kaempferi generally live for over half of a century.
Typical lifespan
Status: wild: 100 (high) years.
Although not the heaviest, the Japanese giant spider crab is the largest known living arthropod. The well-calcified carapace is only around 37 centimeters long, but adult specimens can be nearly 4 meters long from one tip of one cheliped (a claw-bearing leg) to the other when stretched apart. The carapace of Macrocheira kaempferi is sub-circular and pear-shaped (pyriform), narrower towards the head. Females tend to have wider, although slightly smaller, abdomens than males. Spiny and stubby tubercles (growths) cover the carapace, which ranges from dark orange to light tan in color. It possesses no cryptic coloration and is unable to change color. The rostrum (an extension of the carapace above the head) is shaped into two slender spines that jut out from between the eyes. The base of the well-developed antennae is fused with the epistome (the area above the mouth).
The carapace tends to stay the same size throughout adulthood, but the walking legs and chelipeds lengthen considerably as the crab ages. Spider crabs are known for having long, spindly legs. Like the carapace, the legs are also orange, but may be blotchy and mottled with both orange and white. The walking legs of Macrocheira kaempferi end simply in inwardly-curving dactyls (the movable part at the tip of a walking leg). These assist the creature in climbing and hooking onto rock, but prevent it from picking up or grasping objects. In adult males the chelipeds are far longer than any of the walking legs, with the right and left chelipeds being of equal size. Females, on the other hand, tend to possess chelipeds that are shorter than the other walking legs. The merus (upper portion of the leg) is slightly longer than the palm (portion of the leg containing the unmoving part of the claw), but comparable in shape. The weak movable finger is small, taking up less than a quarter of the palm. Although long, the legs are often weak. One study reported that nearly three quarters of these crabs are missing at least one limb, most often one of the first walking legs. This is because the limbs are long and poorly-jointed to the body of the organism, and tend to come off due to predators and nets. Spider crabs can usually survive with up to 3 walking legs missing. The walking legs often grow back during the successive molts.
Range mass: 16 to 20 kg.
Range length: 3.7 (high) m.
Average length: 3 m.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes shaped differently
Many juveniles decorate their shells with sponges, kelp, or other objects to disguise themselves. However, most adults do not because their large size deters most predators. Although slow-moving, they use their claws against smaller predators.
Anti-predator Adaptations: cryptic
These spider crabs mate seasonally during early spring, from January through March. Mating behavior is rarely observed. Male crabs hold sperm in spermatophores, which are inserted into the female's abdomen using the first two chelipeds.
Even though juvenile stages are well-documented in laboratories, reproduction information concerning M. kaempferi in its natural habitat is sparse. Fertilization is internal. A female often lays up to 1.5 million eggs per season, but only a few survive. Eggs are around 0.63-0.85 mm in diameter. The hatching duration is around 10 days. The breeding duration is around one year, although exact times are not available.
Breeding interval: Giant Japanese spider crabs mate once a year, seasonally between January and April
Breeding season: Early spring
Range number of offspring: 1,500,000 eggs (high) .
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous
Females carry eggs on their backs and lower bodies during incubation until they hatch. In this way, the mother can stir the water with her back legs to oxygenate the eggs. After the eggs hatch, there is no parental investment and the larvae are left to fend for themselves.
Parental Investment: female parental care ; pre-hatching/birth (Protecting: Female)
