The coelotrich: form and function of an unusual sensillum in Lowrya (Ostracoda : Myodocopina : Cypridinidae)

This study reports a previously undescribed and unique sensillum. Some spec ies of Lowrya Parker 1998 (Ostracoda: Cypridinidae) possess sensillae. of a n unusual type arranged in a single row parallel to, and near, the anterior , ventral and posterior margins of the external surface of each carapace va lve. These sensilla are here termed coelotrichs, and are each centred withi n an almost spherical cavity in the carapace. The cavity forms at the base of a depression, and the depression in turn lies within a sunken channel on the exterior carapace surface. The opening of the coelotrich cavity bears a 'plug', through which only the widened, divided base of a seta passes. Th e seta is very fine and stiff throughout most of its length, and exhibits n o pores. For most of its length the seta is orientated parallel to the cara pace surface, shielded within the sunken channel. The coelotrich is probabl y a velocity detector, acted upon by the viscous drag of the surrounding fl uid. Its function is probably to detect vibrations in the substrate, such a s acoustic motion in the surrounding liquid, or steady fluid drainage motio n.