ARE PADDLE-LIKE PEREIOPODS A PHYLOGENETICALLY APPROPRIATE CHARACTER WITHIN THE BRACHYURA?

Possession of paddle-like articles of the fifth pereiopods has long been believed to be a defining character for the Portunidae, the swimming crabs, which include the blue crab, a common commercial species on the east coast of North America. The fifth pereiopods are the last pair of walking legs in all Decapoda, the shrimp, lobsters, and crabs. In the Portunidae, they are variously used for swimming, burrowing, and covering the body with a thin layer of sediment to hide from predators or to lie in wait for prey. However, not all Portunidae possess paddle-like or flattened articles of the fifth pereiopods. Furthermore, the nature and development of the paddle-like structures are quite variable within the subfamilies of the Portunidae as currently defined. For example, some members of the family possess a broadly ovate dactyl, which is the terminal article of the appendage. Others possess an obovate dactyl and a broadly ovate manus, the penultimate article of the appendage. In some taxa, the merus, which is the fourth from the last article of the appendage, is ovate, short, and flattened, whereas in other taxa, it is long, slender, and tubular. Further compounding the issue, in some taxa the fourth pereiopod possesses flattened and/or obovate dactyls and mani, and this condition can even extend to the second and third pereiopods. Other taxa within the Portunidae possess elongate, tubular articles of the second through fourth pereiopods with no evidence of flattened, ovate, or paddle-like articles. Clearly, there is wide variation in the development of the so called paddle-like fifth pereiopods within the Portunidae. To further complicate the matter, members of other families not closely related to the Portunidae also exhibit general flattening and paddle-like dactyls of the fifth pereiopods. These families include the Matutidae and the Raninidae in addition to the extinct Carcineretidae, which may be closely related to the Portunidae. Members of the former two families typically use the paddles for back-burrowing into the sediment. Thus, the question arises: is the possession of paddle-like appendages phylogenetically informative, or is the character homoplasic, arising as a result of the environment to which the organism is adapted?