Evolutionary Changes in the Olfactory Projection Neuron Pathways of Eumalacostracan Crustaceans JEREMY M. SULLIVAN * AND BARBARA S. BELTZ Department of Biological Sciences, Wellesley College, Wellesley, Massachusetts 02481 ABSTRACT Output from the olfactory lobe (primary olfactory center) of eumalacostracan crustaceans is transmitted to the medulla terminalis (MT) and hemiellipsoid body (HB) in the lateral protocerebrum (higher order center) by a large population of projection neurons. In eureptan- tian crustaceans (lobsters, crayfish, and crabs), these projection neurons also form the output pathway from an additional neuropil, the accessory lobe (higher order center), which appears to have arisen de novo in these animals. In a previous study of lobsters and crayfish we showed that whereas projection neurons innervating the olfactory lobe project primarily to the MT, those innervating the accessory lobe project exclusively to the HB (Sullivan and Beltz [2001a] J. Comp. Neurol. 441:9 –22). In the present study, we used focal dye injections to examine the olfactory projection neuron pathways of representatives of four eumalacostracan taxa (Stomatopoda, Dendrobranchiata, Caridea, and Stenopodidea) that diverged from the eureptantian line prior to the appearance of the accessory lobe. These experiments were undertaken both to examine the evolution of the olfactory pathway in the Eumalacostraca and to provide insights into the changes in this pathway that accompanied the appearance of the accessory lobe. The innervation patterns of the olfactory projection neurons of the species examined were found to differ markedly, varying from that observed in the most basal taxon examined (Stomatopoda), in which the neurons primarily project to the MT, to those observed in the two highest taxa examined (Caridea and Stenopodidea), in which they primarily target the HB. These results suggest that substantial changes in the relative importance of the MT and HB within the olfactory pathway have occurred during the evolution of the Eumalaco- straca. J. Comp. Neurol. 470:25–38, 2004. © 2004 Wiley-Liss, Inc. Indexing terms: accessory lobe; olfactory pathway; olfactory projection neuron Olfactory cues serve important functions in the biology of eumalacostracan crustaceans, playing central roles in feeding, mating, and territorial behaviors. Correspond- ingly, the primary olfactory neuropil, the olfactory lobe, is one of the most prominent neuropils in the brains of these animals. The lateral protocerebrum of eumalacostracan crustaceans has long been recognized as another impor- tant brain center in the olfactory pathway, as it is the target of the output tract from the olfactory lobe (Fig. 1; Hanstro ¨m, 1924, 1925, 1931, 1947; Blaustein et al., 1988; Sandeman et al., 1993; Sullivan and Beltz 2001a, b). The importance of the lateral protocerebrum in the olfactory pathway has also been underlined by ablation experi- ments suggesting that this region is involved in discrimi- nating food from nonfood items and in the control of feed- ing behaviors (Maynard and Dingle, 1963; Maynard and Yager, 1968; Maynard and Sallee, 1970; Hazlett, 1971). In addition to having strong connections with the olfactory lobe, the lateral protocerebrum also has connections with the optic neuropils (Hanstro ¨m, 1925; Mellon, 2000; Sulli- van and Beltz, 2001b) and is therefore thought to be an associative brain center. The lateral protocerebrum is located proximal to the optic neuropils (lamina ganglionaris, medulla externa, and medulla interna) and is comprised of two main re- gions: the medulla terminalis and the hemiellipsoid body Grant sponsor: National Science Foundation; Grant number: IBN 0091092. *Correspondence to: Jeremy Sullivan, Department of Biological Sci- ences, Wellesley College, Wellesley, MA 02481. E-mail: jsulliv2@wellesley.edu Received 8 September 2003; Revised 21 October 2003; 24 October 2003 DOI 10.1002/cne.11026 Published online the week of January 19, 2004 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 470:25–38 (2004) © 2004 WILEY-LISS, INC.