Blue Swimmer Crabs (Portunus armatus) are amazing beasts when you see them underwater. As you get close to them, they spread their formidable claws challenging you to take them on. Here's a short video of one I recently saw at Seacliff, not far from where I live in South Australia. There are three interesting things to note here.
In the first sequence, you can see the crab open its mouth and run its palps over its antennae, as though it's tasting the water, perhaps testing what the large object (me!) nearby might be.
In the second sequence, the crab rapidly walks sideways ("crabwalks"!!) across the seagrass and sand to find some cover. The third sequence shows part of this slowed down to make it easier to see what's happening. There is a complex sequence of movements of the first three pairs of legs, as they flex and extend laterally. The last pair of legs are modified to create the "swimmer", a paddle like structure that you can see generating a rowing-like motion from time to time. The shape and angle of the paddle generates lift and thrust like an underwater wing.
Finally, as per its name, the crab is blue. Blue pigments are rare in animals. Most blue structures are so-called structural colours due to light interference effects. But in crustaceans, the blue is a pigment, crustacyanin. Crustacyanin is a multimacromolecular complex that contains the carotenoid, astaxanthin, bound to a protein. Carotenoids are plant pigments, such as those that colour carrots and many flowers, and are usually yellow, orange or red. The crustaceans get carotenoids from seaweeds in their diet. When bound to the protein, the colour of the carotenoid-protein complex is blue. Complex semi-crystalline structures in the cuticle help make the blue colour more intense. When crustaceans are cooked, the heat breaks down the crustacyanin complex and only the heat-resistant carotenoids remain, producing the characteristic orange-red colour of cooked crabs and crayfish.
