The olfactory pathway in the peracarid crustacean Parhyale hawaiensis (Malacostraca): new insights into the evolution of olfactory processing in Pancrustacea.

Our current understanding of the functional morphology of olfactory systems in arthropods largely relies on information obtained in hexapods. Existing analyses of the olfactory pathway in crustacean representatives have suggested that these animals share several corresponding anatomical elements with hexapod olfactory systems but that the latter likely feature a different olfactory wiring logic from receptor to olfactory glomerulus. This study sets out to further explore the diversity of arthropod olfactory systems by presenting a detailed morphological analysis of the peripheral and central olfactory pathways in an emerging model system, the peracarid crustacean Parhyale hawaiensis (Malacostraca). These animals feature all neuronal elements that characterize malacostracan crustacean's olfactory systems, and the simplicity of this animal's olfactory system provided the unique opportunity to quantify the numbers of olfactory sensilla and associated sensory neurons, olfactory interneurons and olfactory glomeruli. These data showed that the number of those neuronal elements is highly variable across individuals, contrasting with more stable numbers of neuronal elements in hexapod olfactory systems that typically are characterized by olfactory glomeruli with individual identities and constant numbers. We discuss the possible steps needed for an evolutionary transformation of a malacostracan crustacean type of olfactory system into a hexapod type.