|Male P. pelagicus|
Cancer pelagicus Linnaeus, 1758
Portunus pelagicus, also known as the flower crab, blue crab, blue swimmer crab, blue manna crab or sand crab, also known as alimasag in Tagalog, is a large crab found in the intertidal estuaries of the Indian and Pacific Oceans (Asian coasts) and the Middle-Eastern 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, East Asia, Australia, Persian Gulf and New Zealand.
The males are bright blue in colour with white spots and with characteristically long chelipeds, while the females have a duller green/brown, with a more rounded carapace. The carapace can be up to 20 centimetres (7.9 in) wide.
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 (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), 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 Callinectes sapidus.
These characteristics, along with their fast growth, ease of larviculture, high fecundity and relatively high tolerance to both nitrate and ammonia, (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.
P. pelagicus commonly enters estuaries for food and shelter. Its life cycle is dependent on estuaries as the larvae 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 (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.
- Nicholas Romano & Chaoshu Zeng (2007). "Ontogenetic changes in tolerance to acute ammonia exposure and associated histological alterations of the gill structure through the early juvenile development of the blue swimmer crab, Portunus pelagicus". Aquaculture 266: 246–254. doi:10.1016/j.aquaculture.2007.01.035.
- Nicholas Romano & Chaoshu Zeng (2007). "Acute toxicity of sodium nitrate, potassium nitrate and potassium chloride and their effects on the hemolymph composition and gill structure of early juvenile blue swimmer crabs (Portunus pelagicus Linnaeus, 1758) (Decapoda, Brachyura, Portunidae)". Environmental Toxicology and Chemistry 26 (2007): 1955–1962. doi:10.1897/07-144R.1.
- Nicholas Romano & Chaoshu Zeng (2007). "Effects of potassium on nitrate mediated changes to osmoregulation in marine crabs". Aquatic Toxicology 85 (3): 202–208. doi:10.1016/j.aquatox.2007.09.004. PMID 17942166.
- Nicholas Romano & Chaoshu Zeng (2006). "The effects of salinity on the survival, growth and haemolymph osmolality of early juvenile blue swimmer crab, Portunus pelagicus". Aquaculture 260: 151–162. doi:10.1016/j.aquaculture.2006.06.019.
|Wikimedia Commons has media related to Portunus pelagicus.|
- "Flower crabs, Portunus pelagicus". Online guide to Check Jawa. wildsingapore.com. 2003.
- Leo W. H. Tan & Peter K. L. Ng (1988). "Flower crabs, Portunus pelagicus". Seashore Life in Singapore.