Ultrastructure of the antennal sensilla of Acetes (Crustacea, Decapoda, Natantia, Sergestidae)

Abstract

Many mesopelagic shrimps, including all Sergestidae and some Penaeidae, have long second antennae with similar patterns of setation. The morphology of the antenna and the ultrastructure of five types of antennal setae of the sergestid Acetes sibogae australis are described. The basal portion of the antenna of Acetes is normally held at about right angles to the long axis of the body; the antenna has a right-angle bend about a third of the way along its length and its distal two-thirds trails parallel to the body as the animal swims. Distal to the flexure each antennal segment bears a pair of type 1 setae, which bear lateral setules forming an almost closed tube on the medial side of the antenna. Although they have elaborate tracts leading from them, we believe that the type 1 setae are uninnervated. Two kinds of type 2 setae project into the lumen of the tube formed by the type 1 setae; thick type 2B setae have abundant setules nearly occluding the lumen of the tube and occur only just beyond the flexure; thin type 2A setae have fewer setules and occur at widening intervals along the full length of the tube. Both kinds are innervated by four neurones, three of which bear a ciliary dendrite, one a paraciliary dendrite. These four dendrites lose their microtubular structure proximal to the setal base but the tubule containing their amorphous extensions is fastened to the base of the seta by an amorphous electron-dense material. Both kinds of type 2 setae have a well-developed scolopale and are almost certainly mechanoreceptors. The proximal portion of the antenna bears four types of setae. Type 3 setae are paired, unadorned cylinders with a pore at the tip; although externally similar the type 3 A setae are innervated by three dendrites and the type 3 B by eight to ten. The dendrites of both 3 A and 3B setae have a short ciliary segment and poorly developed scolopales. Type 3 setae are probably chemoreceptors. A single setule-bearing type 4 seta occurs at the distal end of each segment of the basal third of the antenna closely associated with a pair of type 3 setae. Type 4 setae are innervated by three neurones each bearing a ciliary dendrite. These dendrites fuse distally and are attached to the wall of the seta at its base by dense amorphous material. Type 4’s have a well-developed scolopale and are probably mechanoreceptors. Type 5 setae are setule bearing, located on the posterior side of the antenna, and larger than the type 4’s, but their internal structure is identical to that of the type 4’s and they are also presumed mechanoreceptors. There are four to seven pairs of sparsely setuled type 6 setae found only within the antennal flexure and paired, setule-bearing type 7 setae are located at the tip of the antenna. Both types are presumed mechanoreceptors but their ultrastructure was not investigated. Literature on the ultrastructure of crustacean sensilla is summarized and compared with the results of the present study. Setal function and arrangement are then discussed in terms of the known behaviour and ecology of sergestids.