To paddle or not: context dependent courtship display by male blue crabs, Callinectes sapidus.

The nature of the courtship signalling used by a species is shaped by many factors, one of which is its habitat. Male blue crabs, Callinectes sapidus, have a courtship display in which they elevate their body by standing high on their legs, open their chelae and paddle their swimming legs. This courtship display is not reported in other swimming (portunid) crabs and is rarely expressed in laboratory experiments on male blue crabs. In this study, we characterised this display, which we call ;courtship stationary paddling', and distinguished it from other types of paddling. To explain the species specificity of courtship stationary paddling, we hypothesised that this behaviour is an adaptation to low visibility and abundant refuges in the habitat of blue crabs, and that this behaviour enhances chemical signalling when females are relatively inaccessible to males. We used particle imaging velocimetry to visualise water currents generated during courtship stationary paddling, showing that it created water currents directed away from the male and towards the female, thus enhancing chemical signalling. We also showed that males did not perform courtship stationary paddling when females were freely walking such that males could quickly contact and cradle carry them. Rather, males typically performed courtship stationary paddling only when females were inaccessible to them. These results indicate that courtship stationary paddling is a context-dependent behaviour, occurring only when females are not accessible to males, and suggesting that it evolved as an adaptation to life in habitats with many refuges and low visibility.