Brain morphology in the peracarid crustacean Neomysis integer (Leach, 1814) with an emphasis on sexual dimorphism of the olfactory pathway.

Our current understanding of brain organization in malacostracan crustaceans is strongly biased towards representatives of the Decapoda ("ten legged" crustaceans) such as crayfish, crabs, clawed lobsters and spiny lobsters. However, to understand aspects of brain evolution in crustaceans, a broader taxonomic sampling is essential. The peracarid crustaceans are a species-rich group that embraces representatives of, e.g. the Isopoda, Amphipoda and Mysida ("opossum shrimps"), taxa whose neuroanatomy has not been carefully examined. The current study sets out to analyze brain morphology of the mysid Neomysis integer (Leach, 1814; Peracarida, Mysida) using immunohistochemistry against the presynaptic protein synapsin and the neuropeptides RFamide, SIFamide and allatostatin combined with three-dimensional reconstruction of elements of the central olfactory pathway. Furthermore, we studied the inventory of sensilla on the first pair of antennae using cuticular autofluorescence. Anterograde filling with neuronal tracers allowed visualisation the central projections of the sensilla on the first pair of antennae. This species is known to display a sexual dimorphism in both the peripheral and central olfactory pathway. We focussed our analysis on this aspect because in contrast to Hexapoda, reports on a sexual dimorphism of the olfactory system are extremely rare in malacostracan crustaceans. We provide a detailed description of the sensilla associated with a male-specific structure on the pair of first antenna the "lobus masculinus". Furthermore, we analyzed the projection patterns of theses sensilla into the "male-specific neuropil" in the deutocerebrum and critically discuss our results in comparison to examples of sexual dimorphism in the chemosensory pathways in other malacostracan crustaceans and hexapods.