Portunus pelagicus, also known as the ''flower crab'', ''blue crab'', ''blue swimmer crab'', ''blue manna crab'' or ''sand crab'', is a large crab found in the intertidal of the and Pacific Oceans and the coast of the Mediterranean Sea. The name "flower crab" is used in east Asian countries while the latter names are used in Australia. The crabs are widely distributed in eastern Africa, Southeast Asia, Japan, Australia and New Zealand.
The males are bright blue in colour with white spots and with characteristically long , while the females have a duller green/brown, with a more rounded carapace. The carapace can be up to 20 in width.
They stay buried under sand or mud most of the time, particularly during the daytime and winter, which may explain their high tolerance to and . They come out to feed during high tide on various organsims such as bivalves, fish and, to a lesser extent, macroalgae. They are excellent swimmers, largely due to a pair of flattened legs that resemble paddles. However, in contrast to another portunid crab , they cannot survive for long periods out of the water.
The species is commercially important throughout the Indo-Pacific where they may be sold as traditional hard shells, or as "soft shelled" crabs, which are considered a delicacy throughout Asia. The species is highly prized as the meat is almost as sweet as the blue crab, although ''P. pelagicus'' is physically much larger.
These characteristics, along with their fast growth, ease of larviculture, high fecundity and relatively high tolerance to both nitrate and ammonia . This may explain their mass emigration from estuaries to seawater during the rainy season.