Burrow ventilation in the tube-dwelling shrimp Callianassa subterranea (Decapoda: thalassinidea). I. Morphology and motion of the pleopods, uropods and telson.

The morphology of the pleopods, uropods and telson of the tube-dwelling shrimp Callianassa subterranea have been studied using dissection microscopy and scanning electron microscopy. The kinematics of these appendages were examined by motion analysis of macro-video recordings of ventilating shrimps in transparent artificial burrows. The pleopods show the usual crustacean biramous anatomy, but all segments are rostro-caudally flattened. The protopodite bears a triangular medially oriented endopodite and a scoop-shaped exopodite. The contralateral endopodites are linked by the appendix interna, ensuring a perfect phase relationship between contralateral pleopods. The outer rims of the exopodites are fringed with long fern-leaf-like plumose setae bearing flattened setules. These setae have very mobile joints and can be turned caudally. The slits between contralateral endopodites have rims of leaf-like setae as well. Setae of the same leaf-like type fringe the uropods, but these are non-motile. The telson has a narrow fringe of leaf-like setae, locally interrupted by long mechanoreceptory setae. A shrimp, wandering through the burrow or resting, holds its pleopods against the abdomen with the exopodites and their setal fringes retracted medially. The uropods are folded medially as well, probably to reduce the shrimp's drag. During ventilation, the uropods are extended against the tube wall, leaving only a small opening for flow ventral from the telson, and the pleopods beat at a frequency of approximately 1 Hz (0.9+/-0.2 Hz). Fourier analysis of pleopod kinematics showed that the motion pattern of the first flow-generating pleopod pair (PP1) consisted mainly of the first harmonic (75 %) and to a lesser extent of the third harmonic (20 %), resulting in almost perfect sinusoidal motion. The motion patterns of PP2 and PP3 could be modelled by assigning pure sinusoids with a 120 degrees phase shift and a rostro-caudal ranking to the three pairs of pleopods.