Ecological, evolutionary, and functional correlates of sensilla number and glomerular density in the olfactory system of decapod crustaceans

One of the features common among olfactory systems for vertebrate and invertebrate species is the division of the primary processing area into distinct clumps of synaptic neuropil, called glomeruli. The olfactory glomeruli appear to serve as functional units of olfaction and are the location of the primary processing between chemosensory afferents and second-order neurons. Although glomeruli are found across all phyla, their numbers and size appear to be characteristic for each species, giving rise to the speculation that there is a relationship between glomerular number and function. It has been hypothesized, for example, that animals with more glomeruli may be able to resolve a wider range of odors. Crustacean species are distributed among freshwater, marine, and terrestrial habitats in arctic, temperate, and tropical climates. They also exhibit a variety of lifestyles and behaviors in which olfaction may play a dominant role. Feeding, for example, ranges from carnivorous, through subaquatic and terrestrial omnivorous scavenging, to filter feeding. Mating and territorial behaviors also are known to involve chemical signals. The current study examines glomerular numbers in the olfactory lobes of 17 crustacean species from six of the seven taxa now included in the reptantian decapods. Estimates of the glomerular numbers were obtained from the analysis of sectioned material treated immunocytochemically with an antibody against synapsin that labels proteins contained in neuronal terminals. The numbers of glomeruli found in the different species were then compared with the volume of the glomerular neuropil, numbers of olfactory sensilla, life styles, habitat, and phylogenetic affinities. The picture that emerges from these correlations is that the decapod crustaceans have exploited various strategies in the construction of their olfactory systems in which the problems of size, sensitivity, and selectivity have all interacted. We find a continuum across the groups ranging from those that favor a high convergence of receptor neurons onto a few glomeruli to those that share a small number of receptor neurons among many glomeruli. The potential functional consequences of these differences are discussed.