JOURNAL OF EXPERIMENTAL ZOOLOGY 290:49–60 (2001) © 2001 WILEY-LISS, INC. Embryonic and Postembryonic Neurogenesis in the Ventral Nerve Cord of the Freshwater Crayfish Cherax destructor JEREMY M. SULLIVAN* AND DAVID L. MACMILLAN Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia ABSTRACT Previous studies of neurogenic activity in the thoracic neuromeres of indirect de- veloping crustaceans indicated that the temporal patterns of neurogenesis can be correlated with the appearance of the thoracic appendages during larval and metamorphic development. To test further the idea that the temporal patterns of neurogenesis in crustaceans are related to their life histories, we examined neurogenesis in the ventral nerve cord of a direct developing crustacean, the freshwater crayfish Cherax destructor, whose life history contains neither larval stages nor metamorphoses. Neurogenesis was examined using the in vivo incorporation of bromodeoxyuridine into DNA. During late embryonic development the thoracic neuromeres of the crayfish contain arrays of mitotically active neuroblasts similar to those previously described in the spider crab and lobster. The arrays in the crayfish abdomen are, however, greatly reduced compared with those of the thorax. On hatching, both the thoracic and abdominal appendages of C. destructor are capable of movement. The pleopods, however, do not beat rhythmically until the second postem- bryonic stage whereas the pereiopods are not used in coordinated walking movements until the third stage. An examination of the time course of neurogenesis in the ventral nerve cord revealed that neurogenic activity in each neuromere ceases during or before the moult to the developmen- tal stage in which its segmental appendage is first used in coordinated movements. These findings indicate that the patterns of neurogenesis in crustaceans are indeed related to the maturation of the segmental appendages and, in particular, to the maturation of motor behaviours. J. Exp. Zool. 290:49–60, 2001. © 2001 Wiley-Liss, Inc. *Correspondence to: Dr. Jeremy M. Sullivan, Department of Bio- logical Sciences, Wellesley College, Wellesley, MA 02181-8283. E-mail: jsulliva@wellesley.edu Received 8 May 2000; Accepted 16 October 2000 Central neurons in the ventral nerve cords of decapod crustaceans and insects are produced by segmentally repeated arrays of large neuronal pre- cursor cells, known as neuroblasts. Neuroblasts arise during early embryonic development and generate specific lineages of progeny before de- generating. Although it is unclear whether the neuroblasts of insects and crustaceans are homolo- gous (Scholtz, ’92; Whitington, ’96; Gerberding, ’97; Whitington and Bacon, ’97; Duman-Scheel and Patel, ’99) they share many similar characteris- tics. The neuroblasts of both groups divide asym- metrically and repeatedly to produce series of smaller ganglion mother cells. Ganglion mother cells, in turn, divide symmetrically to produce two post-mitotic neurons. Although the spatial and temporal patterns of neurogenesis in the devel- oping ventral nerve cords of insects have been studied extensively (reviews: Truman, ’92; Shep- herd, ’94; Levine et al., ’95; Doe and Skeath, ’96; Campos-Ortega, ’97) much less is known about neuronal proliferation in the crustacean ventral nerve cord. Our understanding of the patterns of neuro- genesis in the ventral nerve cord of malacostra- can crustaceans is based largely on developmental studies of the thoracic neuromeres of two indirect developing decapod crustaceans: the spider crab Hyas araneus and the American lobster Homarus americanus (Harzsch and Dawirs, ’94; Harzsch et al., ’98). During mid-embryonic development the thoracic neuromeres of the spider crab and lob- ster contain similar segmental arrays of mitoti- cally active neuroblasts (Harzsch et al., ’98). The time course of neurogenesis during subsequent de- velopment, however, differs markedly between the two species. These differences in the temporal pat- terns of neurogenesis appear to be related to dif- ferences between the life histories of the species and, in particular, the maturation of their tho- racic appendages.