''Portunus pelagicus'', also known as the flower crab, blue crab, blue swimmer crab, blue manna crab or sand crab, ''rajungan'' in Indonesian, and ''alimasag'' in Tagalog, Kapampangan, and Pangasinan, is a large crab found in the intertidal

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...

around most of Australia and east to New Caledonia.

Description

The males are bright blue in color with white spots and with characteristically long

cheliped A chela ()also called a claw, nipper, or pinceris a pincer-like organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through New Latin '. The plural form is chelae. Legs bearing a chela are called chelipeds. ...

s, while the females have a duller green/brown, with a more rounded carapace. The carapace can be up to wide.

Behaviour

They stay buried under sand or mud most of the time, particularly during the daytime and winter, which may explain their high tolerance to ammonium (NH₄⁺) and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

(NH₃). They come out to feed during high tide on various organisms 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 (''

Scylla serrata ''Scylla serrata'' (often called mud crab or mangrove crab, although both terms are highly ambiguous, and black crab) is an ecologically important species of crab found in the estuaries and mangroves of Africa, Australasia, and Asia. In their ...

''), they cannot survive for long periods out of the water.

Capture

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 ''

Callinectes sapidus ''Callinectes sapidus'' (from the Ancient Greek ,"beautiful" + , "swimmer", and Latin , "savory"), the blue crab, Atlantic blue crab, or regionally as the Chesapeake blue crab, is a species of crab native to the waters of the western Atlantic ...

''. This species is fished heavily and almost exclusively for meat consumption in the Persian Gulf with the females sold at higher prices than males. These characteristics, along with their fast growth, ease of larviculture, high fecundity and relatively high tolerance to both nitrate and

, (particularly ammoniacal nitrogen, NH₃–N, which is typically more toxic than ammonium, as it can more easily diffuse across the gill membranes), makes this species ideal for aquaculture. The species is commercially fished in Australia, and is also available to recreational fishers and regulated by various state governments. Relevant recreational fishing regulations for Australia (as of March 2016) are tabled below.

Ecology

''P. pelagicus'' commonly enters estuaries for food and shelter. Its

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...

is dependent on estuaries as the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e and early juveniles use these habitats for growth and development. Prior to hatching, the female moves into shallow marine habitats, releases her eggs and the newly hatched zoea I larvae move into estuaries. During this time they feed on microscopic plankton and progress from the zoea I stage to the zoea IV stage (approximately 8 days) and then to the final larval stage of

megalopa Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The ...

(duration of 4–6 days). This larval stage is characterised by having large chelipeds used to catch prey. Once the megalopa metamorphoses to the crab stage they continue to spend time in estuaries which provides a suitable habitat for shelter and food. However, evidence has shown that early juveniles cannot tolerate low salinities for extended periods, which is likely due to its weak hyper-osmoregulatory abilities. This may explain their mass emigration from estuaries to seawater during the rainy season. Male ''Portunus pelagicus'' are believed to become more territorial in colder water. This may explain why male crabs are rarely sighted within a close proximity to each other in more temperate waters; it also may explain why their female counterparts seem more prolific in these such areas.

References

External links

Fisheries Western Australia - Blue Swimmer Crab Fact Sheet
* * * {{Authority control Portunoidea Edible crustaceans Commercial crustaceans Crustaceans described in 1758 Taxa named by Carl Linnaeus