Mar. Freshwater Res., 2001, 52, 1357–65 © CSIRO 2001 10.1071/MF01103 1323-1650/01/081357 Structural plasticity in the olfactory system of adult spiny lobsters: postembryonic development permits life-long growth, turnover, and regeneration Paul J. H. Harrison, Holly S. Cate, Pascal Steullet, and Charles D. Derby Department of Biology, Georgia State University, PO Box 4010, Atlanta Georgia, 30302-4010, USA email: cderby@gsu.edu Abstract. Caribbean spiny lobsters (Panulirus argus) rely on their sense of olfaction for many behaviours. Growth of their olfactory systems, and maintenance of olfactory function, is ensured by structural change that occurs continuously throughout life. In this paper, we review recent studies on postembryonic development in the olfactory system of P. argus and several other decapod species. Major structural change occurs in both the peripheral and central olfactory systems; it includes addition and loss of olfactory receptor neurons (ORNs), aesthetasc and other sensilla, and interneurons associated with the olfactory lobes of the brain. From these studies it is clear that continuous growth and turnover of olfactory tissue is a normal process in decapod crustaceans. In addition, we describe for the first time mechanisms that enable the peripheral olfactory system of spiny lobsters to regenerate after injury. We monitored the regeneration of olfactory tissue using in vivo incorporation of the cell proliferation marker 5-bromo-2′-deoxyuridine (BrdU). Our results show that regeneration after partial antennular amputation, which reduces the length of the antennule and thereby the number of ORNs, occurs as a result of up- regulation of the normal mode of ORN addition and down-regulation of loss. In contrast, localized injury to aesthetasc sensilla, which causes the associated ORNs to degenerate but does not reduce antennular length, is followed by local regeneration of olfactory tissue. Extra keywords: olfaction, olfactory receptor neuron, neurogenesis, proliferation, invertebrate, crustacean, lobster, moult Introduction Lobsters rely on olfactory cues for many behaviours (Atema 1995, 1996; Ratchford and Eggleston 1998, 2000; Derby et al. this issue), so it is imperative that they retain olfactory function throughout life, particularly because many lobsters have indeterminate growth and an indeterminate life span (Hartnoll 1981). Recent studies have shown that persistent postembryonic development of the olfactory system occurs in lobsters and other decapod crustaceans (Harzsch and Dawirs 1996; Sandeman and Sandeman 1996, 2000; Schmidt 1997; Sandeman et al. 1998; Harrison et al. 1999, 2001; Harzsch et al . 1999; Schmidt and Harzsch 1999; Schmidt and Hansen 2000). These animals continually add olfactory tissue to accommodate their increasing body size (Laverack 1988), and additional mechanisms enable them to combat factors that can lead to a loss of olfactory function (Sandeman et al. 1998). These factors include, for example, the natural senescence of olfactory receptor neurons (ORNs), localized damage to peripheral receptors, and in extreme cases, complete loss of the peripheral olfactory organs. Lobsters have chemoreceptors over their entire bodies, but the main olfactory system consists of aesthetasc sensilla (located on the antennules) and associated ORNs, whose axons project to olfactory lobes in the brain (Fig. 1) (Ache and Derby 1985; Schmidt and Ache 1992, 1996). The antennules are bifurcated appendages, with lateral and medial flagella. Each flagellum is composed of segments, or annuli, and aesthetascs extend from the ventral surfaces of annuli in the distal part of the lateral flagellum (Fig. 1A–B). In the spiny lobster Panulirus argus, each aesthetasc is innervated by a group of ORNs that form a cluster (approximately 300 ORNs per cluster) at the aesthetasc’s base. The aesthetasc and its associated cluster of ORNs form a discrete unit, referred to here as the olfactory sensory unit (Fig. 1B). Each sensory unit is thought to function similarly, on the basis of evidence that a similar proportion of ORNs in each cluster is activated by any given odor (Steullet et al. 2000b; Derby et al . in press). A subadult spiny lobster