Welcome back to 2023’s Month of Horror and we’re beginning this post with a bit of a story. As a young child my grandparents took me to the SeaLife Centre in Scarborough because of my love of animals. Young me and my younger brother saw a monster and became terrified so my grandparents had to rush us into the next section – that monster was actually the harmless Japanese spider crab. A creature from the deep they are remarkable and adaptable animals.
Taxonomy and Evolution
The taxonomic history for Japanese spider crabs has changed over its initial scientific description in the 1830s. Due to its similarity to the European spider crab it was initially placed in the same genus as it, Maja, and was moved to a different genus, Inchus, with the scorpion spider crabs. In the 1880s it was given its own genus which has stuck giving it the scientific name Macrocheira kaempferi. Crab evolution is interesting mostly because the crab body plan has independently evolved multiple times, and while the Japanese spider crab is a ‘true’ crab it has a different evolutionary history to the European spider crab. The Japanese spider crab is the only member still around of its genus with four extinct species being known – the oldest came from the United States around 30 million years ago, and the rest are found in Japan. A 2022 paper did find that Japanese spider crabs are indeed an independent genus from the rest of crabs.
Biology and Behaviour
Japanese spider crabs are massive and in regards to dimensions are the largest living arthropods. Most of this is in their legs so despite their size they are relatively light reaching between 16 to 20 kilograms (25-35 pounds), but that doesn’t diminish how big they are. Including their legs they can reach lengths of 3.7 metres (12.5 ft) so you can see why a 5-year old me was terrified seeing these giants. They do not start out life this way. Japanese spider crabs are microscopic when they hatch from their eggs and develop, moulting their shell to go larger. As these crabs get larger it takes longer to moult and can take several hours to moult for larger specimens. Also, as they get larger they move into deeper water and can potentially live for a century. Fully grown crabs will only go into shallow water when it is time to breed. In the winter the crabs will go into the shallows where the males will inject females with sperm from their claws, and the female will lay up to 1.5 million microscopic eggs!
The abdomen of Japanese spider crabs are pear-shaped, spiny, and orange colour on top. While the abdomen is stocky the legs are very fragile with very few joints which means they do not bend easily. It is very common to see spider crabs missing at least one leg, and they can survive with as little as three walking legs (they have eight walking legs). It is a long process but they can regrow missing legs during moults. The abdomen is also one of the two ways to determine the sex of the spider crab – females have wider abdomens whereas males have larger pincers.
Japanese spider crabs are also walking ecosystems. After they grow to a certain size they begin decorating themselves with anemones, sponges, algae, and seaweed. This is a mutually beneficial situation for all those involved. For the seaweed, anemones, and other organisms the crab will move them around giving them greater access to nutrients or food sources. At the same time the anemones and sponges help camouflage the crabs which helps them resemble rocks which will deter predators.
Distribution and Habitat
As expected from their name Japanese spider crabs are found in the waters off of Japan, specifically the southern islands of Japan. The furthest south they are found is the northern areas of Taiwan. Fully grown spider crabs are found in deep water, normally between 50 and 600 metres down where they only come near to the surface to mate and reproduce. It is understandable that living so deep down they prefer colder temperatures, from 6 to 16 degrees Celsius. However, they do not need to live in so deep waters so if they the right temperature and space they can easily be kept in shallower water – hence their popularity in aquariums. They are also nocturnal so being so deep down and covered in organisms helps them avoid predators.
Diet and Predators
Japanese spider crabs are gentle giants with their diet consisting of plants and the remains of dead animals. Moving slowly they are not really predatory, but are known to occasionaly go after bottom dwelling animals like sea cucumbers. The large size of the adults mixed with their camouflage and nocturnal habits means they have very few predators, but smaller ones can be eaten by large fish. Humans also have fished for spider crabs with their legs being a delicacy. The larvae are most at risk of predation with even filter-feeders being able to eat them – a tiny fraction of the million eggs laid will survive.
Conservation and Threats
Due to their deep habitat the IUCN has not been able to classify Japanese spider crabs. Their numbers are theorised to be dropping as catches each year have been shrinking, and it is rarer to find the super-giant crabs, like the one pictured in the AMNH. Warming oceans and unsustainable fishing practices will have severely restricted Japanese spider crab numbers. A fishing ban during the winter has been put in place to help preserve numbers during the breeding season, and the ease of breeding them in aquariums has made captive breeding for eventual release a possibility. However, as long as fishing keeps happening the Japanese spider crab will remain in peril.
Bibliography:
- ‘Japanese spider crab’, National Geographic, [Accessed 15/10/2023]
- Animalogic, ‘Japanese Spider Crab: The Original Kaiju’, YouTube.com, (09/04/2021), [Accessed 15/10/2023]
- William Riebel, ‘Japanese spider crab’, Animal Diversity Web, (2011), [Accessed 15/10/2023]
- Daniele Guinot, Peter Davie, Ling Ming Tsang, and Peter Ng, ‘Formal re-establishment of Macrocheiridae Dana, 1851 (Decapoda: Brachyura: Majoidea) for the giant spider crab Macrocheira kaempferi (Temminck, 1836) based on a reappraisal of morphological and genetic characters’, Journal of Crustacean Biology, 42:2, (2022)
- Peter Davie, Crabs: A Global Natural History, (Princeton, NJ: Princeton University Press, 2021)
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