Achaeus japonicus, sometimes known as the orang-utan crab, is a crab of the family Majidae (spider crabs or decorator crabs) has been observed in tropical waters of the Western Pacific Ocean from Indonesia to Fiji, as well as in coastal waters of Japan. With a carapace of only about 2 centimetres or 0.8 inches in diameter, it has relatively long arms, which are thickly covered with fine hairs, red or reddish brown in colour, and often laden with small bits of debris for further camouflage. It is frequently, but not always, found in association with the bubble coral Plerogyra sinuosa. 
Humann and DeLoach classify the orang-utan crab as "Oncinopus sp. 1" and assert it was "formerly classified" as Achaeus japonicus, though they describe their own genus identification as "tentative."
- Peter Davie (2010). "Achaeus japonicus". World Register of Marine Species. http://www.marinespecies.org/aphia.php?p=taxdetails&id=441846. Retrieved February 20, 2012.
- T. Gosliner, D. Behrens & G. Williams (1996). Coral Reef Animals of the Indo-Pacific. p. 231.
- "Orang-utang crab (Achaeus japonicus)". Crabs of Japan. Marine Species Identification Portal. http://species-identification.org/species.php?species_group=crabs_of_japan&menuentry=soorten&id=843&tab=beschrijving. Retrieved February 19, 2012.
- Matteo Guardini & Massimo Boyer. "Orang-utan crab". World Database of Marine Species. http://www.seadb.net/en_Orang-utan-crab-Achaeus-japonicus_54.htm. Retrieved February 19, 2012.
- Paul Humann & Ned DeLoach (2010). Reef Creature Indentification: Tropical Pacific. p. 223.