Suspected mechano- and chemosensory structures of Temora longicornis (Copepoda: Calanoida)

The antennulary (A1) setae of Temora longicornis Müller are suspected of being mechanosensory. In the present study, the fine structure of the antennule, setae and pegs was examined by scanning and transmission electron microscopy. Two setae arise proximal to each antennule segment junction. The supraaxial (Type 1) seta of each pair is innervated by a pair of ciliary dendrites and has the morphological characteristics of a mechanoreceptor. The sub-axial (Type 2) seta may be chemosensory, and is innervated by a varying number of dendrites, but always more than two. The antennulary pegs do not appear to be innervated. T. longicornis is directionally sensitive to water disturbances, and this is consistent with the physical structure of the setae. Integumental structures on the body and other non-feeding appendages are described. These observations support previous behavioural experiments which suggested the antennule to be the major site for mechanoreception in copepods; however, mechanoreceptors must also occur on other parts of the body.