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)
The Japanese spider crab (Macrocheira kaempferi) is a species of marine crab that lives in the waters around Japan. It has the largest known leg-span of any arthropod. It goes through three main larval stages along with a prezoeal stage to grow to its great size.[2]
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.[3][4] Its diverse taxonomic history is an important part of what these creatures are and how they evolved to be what they are today. They are sought by crab fisheries, and are considered a delicacy in Japan. Conservation efforts aim to protect these creatures and their population from overfishing.[5]
The Japanese spider crab is similar in appearance to the much smaller European spider crab (Maja squinado), though the latter, while within the same superfamily, belongs to a different family, the Majidae.
The Japanese spider crab has the greatest leg span of any known arthropod, reaching up to 3.7 m (12.1 ft) from claw to claw.[6] The body may grow to 40 cm (16 in) in carapace width and the whole crab can weigh up to 19 kg (42 lb)[7]—second in mass only to the American lobster among all living arthropod species. The males have the longer chelipeds;[4] females have much shorter chelipeds, which are shorter than the following pair of legs.[3] 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.[1] The crab is orange with white spots along the legs.[8] It is reported to have a gentle disposition despite its ferocious appearance.[8] The Japanese name for this species is taka-ashi-gani, (Japanese: たかあしがに), literally translating to “tall legs crab”. It 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.[9] Its armored exoskeleton helps protect it from larger predators such as octopuses, but also uses camouflage. 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.[10] The way in which a spider crab is able to pick up and cover itself with such organisms is by following a specific routine behavior. 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.[11] 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.[11] 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.[11] 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. Then, through mechanical adhesion and secretions, the materials attach to the crab, and are able to regenerate, and colonize on the crab.[11]
Japanese spider crabs are mostly found off the southern coasts of the Japanese island of Honshū, from Tokyo Bay to Kagoshima Prefecture. Outlying populations have been found in Iwate Prefecture and off Su-ao in Taiwan.[3] Adults are found at depths between 50 and 600 m (160 and 1,970 ft).[3] They like to inhabit vents and holes in the deeper parts of the ocean.[12] 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).[13] Based on results from public aquaria, Japanese spider crabs tolerate temperatures between 6 and 16 °C (43 and 61 °F), but are typically maintained at 10–13 °C (50–55 °F).[13]
Female crabs carry the fertilized eggs attached to their abdominal appendages until they hatch into tiny planktonic larvae.[12] They can lay up to 1 egg per season, and this egg hatches in 10 days on average.[5]
Once hatched, this larva undergo four stages of development before they mature into adulthood.[2] The first, or prezoeal, stage lasts only a matter of minutes, with most molting within 15 minutes to enter the first zoeal stage.[14] it looks very different from its parents at this stage, with a small, transparent body. M. kaempferi undergoes two zoeal stages and a megalopa stage before it reaches adulthood.[2] Each of these stages is influenced greatly by temperature, both in terms of survival and stage length. The optimum rearing temperature for all larval stages is thought to be between 15 and 18 °C, with survival temperatures ranging from 11 to 20 °C.[15] At these temperatures, the zoeal stages can last 7 to 18 days, with the megalopa stage lasting 25 to 45 days.[15] Colder water is associated with longer durations in each stage. During the larval stages, M. kaempferi is found near the surface, as the planktonic forms drift with ocean currents.[15] This surface water ranges between 12 and 15 °C during the hatching season (January to March).[16] This is much warmer than the waters at depths below 200 m, where adults are found, with waters steadily around 10 °C. Optimal temperatures have a 70% survival through the first zoeal stage, which is greatly reduced to a 30% survival in the second zoeal and megalopa stages.[15]
The Japanese spider crab is an omnivore, consuming both plant-matter and animals. It also sometimes acts as a scavenger, consuming dead animals. Some have been known to scrape the ocean floor for plants and algae, while others pry open the shells of mollusks.[8][12]
The Japanese spider crab was originally described by Western science in 1836 by Coenraad Jacob Temminck under the name Maja kaempferi, based on material from Philipp Franz von Siebold collected near the artificial island Dejima.[17] The specific epithet commemorates Engelbert Kaempfer, a German naturalist who lived in Japan from 1690 to 1692 and wrote about the country's natural history.[18] It was moved to the genus Inachus by Wilhem de Haan in 1839, but placed in a new subgenus, Macrocheira. That subgenus was raised to the rank of genus in 1886 by Edward J. Miers.[1] Placed in the family Inachidae, M. kaempferi does not fit cleanly into that group, and it may be necessary to erect a new family just for the genus Macrocheira.[1] Four species of the genus Macrocheira are known from fossils:[19][20]
However, some evidence indicates that the genus Macrocheira does come from this family in some way due to its anatomical arrangements. This genus is similar in anatomical arrangement to the genus Oncinopus, seeming to preserve the earliest stage of anatomical evolution in the family Inachidae. The genus Onicinopus has a semihardened body, which allows the basal segment of the antennae, which articulates with the head capsule, to move.[21] The antennulae which are segmented appendages between and below the eye stalks are connected to each other.[21] Like Oncinopus, the genus Macrocheira also has a seven-segmented abdomen and a basal segment of antennae that is mobile. Macrocheira also has orbital parts, the eye socket and features around it, that are similar to differentiated genera. Another differentiating feature is the supraorbital eave.[21] It is part of the orbital region above the eyestalks. It projects laterally and becomes part of the spine. From the anatomical observations of this genus and others in the family Inachidae, Macrochiera was placed in the family Inachidae, descending from the genus Oncinpus and from it descending the genera Oreconia, Parapleisticantha, and Pleistincantha.[21]
M. kaempferi is a giant crab with a pear-shaped carapace that is 350 mm (13.7 in) when measured on the median line.[21] Its surface is covered in small spike-like projections or tubercles. The spine of an adult giant crab is short and curves outward at the tip. The spines in young giant crabs, though, are long compared to their carapaces, along with an uncurved spine.[21] This proportionality explains, as in other decapod crustaceans, that spine size decreases as specimens grow older.[21] As mentioned in the taxonomic section, this genus contains the family's primitive feature of a movable antenna at the basal segment, but "the development of a spine at the posterior angle of the supraocular eave, and the presence of intercalated spine and antennulary septum seem to attribute a rather high position to this genus." Lastly, differences are seen between the sexes. Adult males have very long front legs where the claws are located, but they are still shorter than the ambulatory legs of females, located in the back of the carapace.[21]
Temminck, in his original description, noted that the crab was known to the Japanese for the serious injuries it can cause with its strong claws.[4] The Japanese spider crab is "occasionally collected for food",[22] and even considered a delicacy in many parts of Japan and other areas in the region.[12][23] In total, 24.7 tonnes (54,000 lb) were collected in 1976, but fell to only 3.2 tonnes (7,100 lb) in 1985.[15] The fishery is centred on Suruga Bay. The crabs are typically caught using small trawling nets.[23] 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).[17] Populations of this species of crab have diminished over recent years and many efforts are being made to protect them.[23] One of the primary methods of recovery of the species being used is restocking artificially cultured juvenile crabs in fisheries.[15] 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.[23] This protection method seeks to keep natural populations growing, and enables time for juvenile spider crabs to go through the early stages of their lifecycle.[23]
The Japanese spider crab (Macrocheira kaempferi) is a species of marine crab that lives in the waters around Japan. It has the largest known leg-span of any arthropod. It goes through three main larval stages along with a prezoeal stage to grow to its great size.
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. Its diverse taxonomic history is an important part of what these creatures are and how they evolved to be what they are today. They are sought by crab fisheries, and are considered a delicacy in Japan. Conservation efforts aim to protect these creatures and their population from overfishing.
The Japanese spider crab is similar in appearance to the much smaller European spider crab (Maja squinado), though the latter, while within the same superfamily, belongs to a different family, the Majidae.
