DEVELOPMENT OF THE OLFACTORY AND ACCESSORY LOBES IN THE AMERICAN LOBSTER - AN ALLOMETRIC ANALYSIS AND ITS IMPLICATIONS FOR THE DEUTOCEREBRAL STRUCTURE OF DECAPODS

The allometric changes characterizing the growth of the deutocerebrum (midbrain) of the American lobster (Homarus americanus) are studied us ing computerized three-dimensional reconstructions of serial brain sec tions. During the embryogenesis of the midbrain, the paired accessory lobes (higher order processing areas) appear later than the paired olf actory lobes (primary olfactory centers), but the former grow faster f rom their emergence until metamorphosis. The accessory lobes, as they enlarge, shift progressively from a medial to a posterior position in the lateral deutocerebrum. In early juvenile stages the accessory lobe s are one of the largest neuropils of the brain. However, these lobes stop growing in adult animals, whereas the brain and olfactory lobes c ontinue to enlarge, albeit at a slow rate. The overall shape of the br ain and the relative proportions and locations of the deutocerebral ne uropils and associated cell clusters of various lobster ontogenetic st ages are similar to those of selected adult decapods. In addition, the relation between deutocerebral organization and brain size seem paral lel during lobster development and across crustacean species. Measurem ents of the brains of 13 species of decapods (illustrated in Sandeman et al. [1993] J. Exp. Zool. 265:112, plus Homarus) indicate the follow ing trends: Small brains possess olfactory lobes but no accessory lobe s, larger brains possess accessory lobes that are medial and small rel ative to the olfactory lobes, and the largest brains contain relativel y voluminous posterior accessory lobes. These observations indicate th at some differences in the organization of the deutocerebrum are relat ed to absolute brain size in crustaceans and suggest that ontogenetic scaling of proportions may apply to the deutocerebral neuropils of dec apods. Peramorphosis and paedomorphosis in the evolution of the decapo d brain are considered. (C) 1995 Wiley-Liss, Inc.