COMPARATIVE BRAIN ONTOGENY OF THE CRAYFISH AND CLAWED LOBSTER - IMPLICATIONS OF DIRECT AND LARVAL DEVELOPMENT

The freshwater crayfish Cherax destructor and the lobster Homarus amer icanus have many similarities including life style, body form, and neu ral organization. However, the ontogenic history is very different in the two species. The development of Cherax is short and direct whereas the development of Homarus comprises three pelagic larval stages and takes more than twice as long from extrusion to benthic stages at cons tant temperature. In order to determine the progression of maturation of the nervous system in each species and the potential implications o f pelagic forms on brain structure, the timing of appearance of 22 gen eral and neural developmental events clearly identifiable in both spec ies was compared. The onset of serotonin antigenicity in the different parts of the brain was chosen as one marker of neural development. Du ring the first month of embryogenesis the timing of morphological, phy siological, and neural events is similar in the two species. Morpholog ical development is then accelerated in the crayfish near hatching tim e and over the two postembryonic stages before the advent of the indep endent benthic stage. Such heterochronic processes can at least partly account for the different developmental patterns in the two decapods. Among the characters showing similar timing in the two species is the formation of glomeruli (presumptive zones of synaptic contact) in the olfactory lobes of the deutocerebrum, although this event is embryoni c in Homarus but postembryonic in Cherax. In contrast, glomerular form ation in the accessory lobes is heterochronic: in both species, the gl omeruli of the accessory lobes are acquired postembryonically, that is , 3 to 4 months earlier in Cherax than in Homarus. These data suggest that the development of the glomeruli in the olfactory lobes may depen d primarily on internal developmental signals, whereas the triggering of glomerular formation in the accessory lobes may depend on external cues. The fact that, in Homarus, only the postlarval stages show matur e accessory glomeruli may be a reflection of the functional requiremen ts of benthic life. (C) 1993 Wiley-Liss, Inc.