The blue swimmer crab, also known as Portunus pelagicus, is a species of swimming crab that can be found in the Indo-Pacific region's coastal waters. With its delectable and succulent meat, it is a popular seafood item in Australia and is highly sought after. This species of crab falls under the classification of a decapod crustacean, which translates to having 10 legs and two claws. Its size can range from 12 cm across, and its weight can range from 120g to 500g when fully grown. Its large size and robust flavour make it a sought-after ingredient for various culinary preparations, including soups, stews, curries, and salads.

One of the most distinctive characteristics of the blue swimmer crab is its blue colouration and unique swimming abilities. The blue swimmer crab boasts a flat carapace that can measure up to 5 inches in length and has a pale blue hue with darker patches on its legs and claws, giving it an attractive appearance. The carapace also features eight long spines on each side, which serve as navigation aids while swimming and as a defence mechanism against predators.

This species of crab is also known for its exceptional swimming abilities, which are achieved by using its powerful pincers to push off the seafloor and flapping its back legs like paddles. The blue swimmer crab can reach speeds of up to 2 miles per hour and can travel up to 60 miles in a single day!

The blue swimmer crab primarily feeds on molluscs like clams or oysters but also feeds on small crustaceans and fish. It is an omnivore and feeds on both plants and animals, with its diet consisting mainly of small invertebrates like worms, clams, oysters, and other crustaceans. The blue swimmer crab is also known to feed on plant matter like algae and seaweed. This species is known for its aggressive behaviour when defending its territory or prey from other crabs or predators.

The blue swimmer crab can be found in the coastal waters of Australia, Southeast Asia, and the Indian Ocean, and its range extends along the coasts of Australia, New Guinea, Bali, Thailand, India, and the South China Sea region in China. The blue swimmer crab primarily inhabits estuaries and shallow bays with muddy or sandy bottoms, and can often be found hiding among shells on the seafloor or burrowing into mud flats during low tide.

The blue swimmer crab has become increasingly popular for commercial fishing due to its large size and sweet meat. It is not an aggressive hunter but feeds mostly on small organisms like shrimp, worms, molluscs, and other invertebrates. The blue swimmer crab is also known to scavenge for food by searching along the seafloor or scavenging dead animals in shallow water. Additionally, the blue swimmer crab has been known to dig up buried prey like clams or oysters with its claws.

During mating season, male blue swimmer crabs will hunt larger prey like fish or other crustaceans to demonstrate their strength to prospective mates. However, throughout the rest of the year, they are more likely to stick with smaller prey items that are easier to catch with their pincers.

The blue swimmer crab reproduces via external fertilization, where the male approaches a female and transfers sperm to her abdomen using his modified first pair of legs. The eggs are then released into the water by the female, where they develop into larvae over several weeks. After hatching, the larvae drift with ocean currents and settle to form juvenile crabs after around 10 days, depending on temperature and food availability.