Fighting Tooth and Claw

Velvet Swimming Crab, Necora puber

The Velvet Swimming Crab (Necora puber) is unique in a couple of senses. Firstly, it is the only member of the genus Necora, having been classified as such following reexamination of type material in 1987 (1). Secondly it, unusually for a swimming (or portunid) crab, occurs predominantly on hard substrates (2). It is also the largest swimming crab to be found in the waters surrounding Ireland and Britain, its final pair of pereiopods being flattened, fin-like to aid swimming. However its most striking features are its red eyes (lending it the alternative common name, “Devil Crab) and its velvety appearance. This is due to tiny hairs covering the carapace and appendages which allow a layer of algae to cover the crab. An important commercial crab (3), N. puber is usually found in shallow water to a depth of 20 m where it feeds on a variety of organisms including other crustaceans, molluscs and brown algae.

Velvet Swimming Crab, Necora puber, with swimming pereiopod on right

Interaction between individual crabs are more often than not antagonistic, with fights occurring between all sizes of crabs. They seem to prefer a solitary lifestyle and it is assumed that this is because large numbers of crabs increase the presence of predators and competition for food (4). In laboratory conditions (4), N. puber individuals were seen surprisingly not to engage in an escalation of violence, but rather in sporadic bursts. Fights between individuals were won in the majority of cases by the larger crab, yet smaller crabs never backed down from confrontation. This might seem a bit foolhardy on their parts, having little chance of winning, but under the laboratory conditions all crabs were in intermoult stage and had a hard outer shell. In vivo, fights may be one by smaller crabs against larger but softer ones.

Velvet Swimming Crab, Necora puber

References:

1. Holthuis, 1987. Zoologische Mededelingen 61 pp. 1-14
2. Norman and Jones, 1992. Estuarine, Coastal and Shelf Science 34 pp. 71-83
3. Robson et al., 2007. Food Microbiology 24 pp. 419-424
4. Thorpe et al., 1994. Behavioural Processes 32 pp. 235-246