"The family Portunidae are distinguished from the rest of the round-fronted (cyclometope) crabs by the adaptation of some of their legs for swimming, to which end these limbs are transformed into flattened paddles. The result is often to confer upon the crabs a power of darting at high speed through the water, which would hardly be credited by those who have not watched them. Corresponding to this mobility they have a thin flattened form of body, enabling them to pass sideways through the water, and a lightness gained at the expense of the protective cuticle. These peculiarities give the swimming crabs a strikingly different bodily form from the heavily-built, slow-moving Xanthids, which is moreover accompanied by an equally marked difference of habitat. The Xanthids are usually to be found on the reef or shore exposed to the full force of the breakers. In this position the lightly-built swimming crabs would be dashed to pieces against the rocks. Their proper haunt is a space of quiet waters, such as the lagoon of a coral atoll, and as these places are, in the tropics, generally bottomed with white or greyish coral sand, on which the crabs lie, and in which they often hide their bodies, they frequently mimic it by their pale greyish color, often in a manner as striking as that in which flat-fish resemble the shingly bottom they live on (Borradaile, 1902; pg. 199)."
"The family is highly variable and varietal and is probably undergoing rapid evolution in many directions (Borradaile, 1902; pg. 199)."
"At the same time the swimming crabs are by no means entirely confined to a bottom of coral sand even in the tropics. In deep water, where rocks are not associated with danger, they are found on every kind of bottom in about equal numbers, and here, if they hide, it must be under stones. They even occur, though not so very often, on the reef. But the individuals found in this position, may possibly have strayed from the lagoon with the outgoing tide. Probably, when more is known about the lives of the species, it will be found that certain of them maintain their existence on the reef by sheltering under stones or in blocks of coral, where if anywhere they are always found, and that others- certainly the bulk of individuals- prefer the lagoon. In their habits these crabs are active and intelligent, escaping capture with cleverness. The lagoon forms usually keep close to the sand and do not rise more than a few feet into the water, but others swim as boldly and strongly as fish (Borradaile, 1902; pg. 199)."
"The bodies of most Portunidae are adorned or protected with sharp thorns or teeth, and it is on such characters as the number and size of these and the shape of the lobed front that the species are generally distinguished, though in most cases enough is not known of their habits to make it possible to say whether, and if so how, these be of use to the animals (Borradaile, 1902; pg. 199)."
Portunid crabs are characterised by the flattening of the fifth pair of legs into broad paddles, which are used for swimming. This ability, together with their strong, sharp claws, allows many species to be fast and aggressive predators.
Its members include many well-known shoreline crabs, such as the European shore crab (Carcinus maenas), blue crab (Callinectes sapidus), and velvet crab (Necora puber). Two genera in the family are contrastingly named Scylla and Charybdis; the former contains the economically important species black crab (Scylla serrata) and Scylla paramamosain.
The circumscription of the family varies, with some authors treating "Carcinidae", "Catoptridae" and "Macropipidae" as separate families, and others considering them subfamilies of a wider Portunidae. Swimming crabs reach their greatest species diversity in the Pacific and Indian Oceans.