Antennular sensory organs in cyprids of balanomorphan cirripedes: standardizing terminology using Megabalanus rosa

Cirripedes are one of the major groups of fouling organism in the marine environment. The cyprid can, before a permanent attachment, actively explore and walk on the substratum using its antennules in a bipedal fashion without leaving the surface. Studying the structure of the cyprid antennule is therefore important for understanding the events that culminate in biofouling by barnacles. There are at present no complete, standardised accounts of the structure of the cyprid antennules in thoracican barnacles, and moreover, the existing accounts vary in their use of terminology. This article describes the cyprid antennule of the barnacle Megabalanus rosa. This barnacle species is common in E Asia, and the cyprids have previously been used in several biofouling studies. All externally visible setae on the antennules have been mapped; these comprise both chemosensors with a terminal pore, a putative aesthetasc-like seta and mechano-sensory setae. More setae were found on the attachment disc than in previous scanning electron microscope-based studies, but not all structures that can be seen with transmission electron microscopy were visible. The disc itself seems to have a variable surface area, which could assist in exploring rough surfaces. The various lengths of the antennular setae, coupled with the disposition of the segments, enable the cyprid to cover a wide swath of substratum during exploratory walking. A new terminology is proposed for cyprid antennular setae, which will form a basis for future comparative and functional studies of cirripede settlement.