Development of the olfactory and accessory lobes in the american lobster: An allometric analysis and its implications for the deutocerebral structure of decapods

Abstract

The allometric changes characterizing the growth of the deutocerebrum (midbrain) of the American lobster (Homarus americanus) are studied using computerized three-dimensional reconstructions of serial brain sections. During the embryogenesis of the midbrain, the paired accessory lobes (higher order processing areas) appear later than the paired olfactory lobes (primary olfactory centers), but the former grow faster from 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 lobes are one of the largest neuropils of the brain. However, these lobes stop growing in adult animals, whereas the brain and olfactory lobes continue to enlarge, albeit at a slow rate. The overall shape of the brain and the relative proportions and locations of the deutocerebral neuropils and associated cell clusters of various lobster ontogenetic stages are similar to those of selected adult decapods. In addition, the relation between deutocerebral organization and brain size seem parallel during lobster development and across crustacean species. Measurements of the brains of 13 species of decapods (illustrated in Sandeman et al. [1993] J. Exp. Zool. 265:112, plus Homarus) indicate the following trends: Small brains possess olfactory lobes but no accessory lobes, larger brains possess accessory lobes that are medial and small relative to the olfactory lobes, and the largest brains contain relatively voluminous posterior accessory lobes. These observations indicate that some differences in the organization of the deutocerebrum are related to absolute brain size in crustaceans and suggest that ontogenetic scaling of proportions may apply to the deutocerebral neuropils of decapods. Peramorphosis and paedomorphosis in the evolution of the decapod brain are considered.

Why Are Olfactory Systems of Different Animals So Similar?

“…In decapods, the axons of olfactory receptor neurons interact with fibers from interneurons and projection neurons in unusual cone-shaped glomeruli located in the olfactory lobe, as has been demonstrated in the Australian yabby (Cherax destructor), red swamp crayfish (Procambarus clarkii), American lobster (Homarus americanus), and Caribbean spiny lobster (P. argus) [Sandeman and Luff, 1973;Mellon and Munger, 1990;Schmidt et al, 1992a;Helluy et al, 1995]. In lobsters, the glomeruli of the olfactory lobe comprise three distinct horizontal zones or compartments that are innervated by different interneurons [Schmidt et al, 1992b;Langworthy et al, 1997;Schmidt and Ache, 1997].…”

Serotonin DepletionIn VivoInhibits the Branching of Olfactory Projection Neurons in the Lobster Deutocerebrum

“…On passing through the OGTN, the OGT projects bilaterally to the eyestalks where it innervates the lateral protocerebrum ( Fig. 1) (Helluy et al, 1995).…”

Neurogenesis in the developing crab brain: Postembryonic generation of neurons persists beyond metamorphosis

“…Studies in lobsters and crayfish have revealed similar results (Beltz et al, 1990(Beltz et al, , 1992Helluy et al, 1993). Furthermore, it has been supposed that the ingrowing central projections of new sense organs that are continually added throughout development increase the complexity of the developing crustacean brain (Laverack, 1988a(Laverack, , 1988bMellon and Alones, 1993;Helluy et al, 1995).…”

“…These authors observed a more than 600-fold increase in brain volume from hatching to 3 kg adult animals and raise the question about the mechanisms that might contribute to this growth in bulk of the brain. The continual integration of in-growing new sensory axons may be part of the answer (Laverack, 1988a(Laverack, , 1988bMellon and Alones, 1993;Helluy et al, 1995). Furthermore, the gradual maturation and refinement of the fiber system of central interneurons has been shown to contribute to this process (Helluy et al, 1993, Harzsch and Dawirs, 1995, 1996 .…”