Bida Personal Blog: Portunus pelagicus

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 estuaries of the Indian and Pacific OceansMiddle-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, Japan, Australia and New Zealand. (Asian coasts) and the

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 cm 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 NH₄⁺ and NH₃ ^[1]. 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 (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 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 ^[2] ^[3] and ammonia ^[1], (particularly NH₃-N, which is typically more toxic than NH₄⁺, as it can more easily diffuse across the gill membranes), makes this species ideal for aquaculture.

P. pelagicus is not strictly marine as it commonly enters estuaries for food and shelter. Furthermore, 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 ^[4]. This may explain their mass emigration from estuaries to seawater during the rainy season.