Abstract The aeolid nudibranch Phestilla sibogae is well studied in terms of its larval nervous system and neuronal involvement in metamorphosis. Central neuro- nes in the adult have also been identified anatomically and electrophysiologically. We describe the neurotrans- mitter contents of these neurones and provide details of neuritic projections and developmental changes during growth (3 to 18 mm body length). Central ganglia from specimens of all sizes contained 100–115 serotonin-im- munoreactive neurones, some of which appeared to be homologues of cells identified in other gastropods. Tyro- sine hydroxylase immunoreactivity and aldehyde-in- duced fluorescence marked a common set of 28–30 cate- cholaminergic neurones located anteriorly in the cerebro- pleural ganglia and laterally in the pedal ganglia. Gangli- onic neuropile and nerve trunks also contained many catecholaminergic fibres. About 65–100 intensely la- belled FMRFamide-immunoreactive neurones were lo- cated symmetrically throughout the central ganglia, although one population was located only in the right pedal ganglion. Another 40–45 FMRFamide-immunore- active neurones were weakly or variably stained. Central ganglia also contained 27–29 intensely labelled pedal- peptide-immunoreactive neurones, including those that were apparently homologues of cells previously de- scribed in Tritonia diomedea, and 16–19 weakly labelled pedal-peptide-immunoreactive neurones, including giant cerebropleural neurones coexhibiting FMRFamide im- munoreactivity. Little cell addition involving any trans- mitter phenotype occurred as animals grew in body length, body growth being accommodated by growth in the size of individual cells, consistent with an approxi- mate doubling in the size of the ganglia themselves. Keywords Serotonin · Catecholamine · Dopamine · FMRFamide · Pedal peptide · Central ganglia · Gastropod · Phestilla sibogae (Mollusca) Introduction The nudibranch gastropod Phestilla sibogae has been well studied as a model for understanding several aspects of the neuronal bases of behaviour. The larvae of this tropical species also serve as convenient models of early development (Bonar and Hadfield 1974; Kempf et al. 1992, 1997) since they lay eggs all the year round and the larvae hatch within a week of oviposition. After a few days, the larvae can be induced to metamorphose in- to juvenile sea slugs by means of extracts of the sole prey of adult P. sibogae, viz., a species of coral, Porites compressa (Hadfield 1978; Hadfield and Pennington 1990). P. sibogae is therefore highly amenable to studies of neural involvement in triggering molluscan metamor- phosis (Hadfield et al. 2000; Pires et al. 1997, 2000; Pires and Hadfield 1991). In the adult, individual cells have been characterized morphologically and electro- physiologically, and their electrical activities have been correlated with specific behaviour patterns and move- ments (Willows 1985). The anterior sensory organs (rhinophores, tentacles and oral and cephalic shields) of P. sibogae have been studied intensively, because these structures are highly sensitive to specific cues emanating from their exclusive prey (Boudko et al. 1998, 1999). In addition, the time from hatching to egg-laying in P. sibogae has previously been reported to be as short as 27 days (Miller and Hadfield 1990) but recent studies suggest that the interval until maturation of the testes This research was supported by Office of Naval Research Grant no. N00014–94–1-0524 to M.G.H. and Natural Sciences and Re- search Council of Canada Grant no. OPG38863 to R.P.C. R.P. Croll ( ✉ ) Department of Physiology and Biophysics, Dalhousie University, Halifax, NS, Canada, B3H 4H7 e-mail: Roger.Croll@Dal.ca Tel.: +1-902-494-3517, Fax: +1-902-494-1685 D.Y. Boudko · M.G. Hadfield Kewalo Marine Laboratory, University of Hawaii, Honolulu, HI 96813, USA Present address: D.Y. Boudko, The Whitney Laboratory, University of Florida, St. Augustine, FL 32086 USA Cell Tissue Res (2001) 305:417–432 DOI 10.1007/s004410100394 REGULAR ARTICLE Roger P. Croll · Dmitri Y. Boudko Michael G. Hadfield Histochemical survey of transmitters in the central ganglia of the gastropod mollusc Phestilla sibogae Received: 29 November 2000 / Accepted: 13 March 2001 / Published online: 20 June 2001 © Springer-Verlag 2001