The “amphi”-brains of amphipods: New insights from the neuroanatomy ofParhyale hawaiensis(Dana, 1853)

ABSTRACT Background Over the last years, the amphipod crustacean Parhyale hawaiensis has developed into an attractive marine animal model for evolutionary developmental studies that offers several advantages over existing experimental organisms. It is easy to rear in laboratory conditions with embryos available year-round and amenable to numerous kinds of embryological and functional genetic manipulations. However, beyond these developmental and genetic analyses, research on the architecture of its nervous system is fragmentary. In order to provide a first neuroanatomical atlas of the brain, we investigated P. hawaiensis using immunohistochemical labelings combined with laser-scanning microscopy, X-ray microcomputed tomography, histological sectioning and 3D reconstructions. Results As in most amphipod crustaceans, the brain is dorsally bent out of the body axis with downward oriented lateral hemispheres of the protocerebrum. It comprises almost all prominent neuropils that are part of the suggested ground pattern of malacostracan crustaceans (except the lobula plate and projection neuron tract neuropil). Beyond a general uniformity of these neuropils, the brain of P. hawaiensis is characterized by a modified lamina (first order visual neuropil) and, compared to other Amphipoda, an elaborated central complex. The lamina displays a chambered appearance that, in the light of a recent analysis on photoreceptor projections in P. hawaiensis , corresponds to specialized photoreceptor terminals. The presence of a poorly differentiated hemiellipsoid body is indicated and critically discussed. Conclusions Although amphipod brains show a general uniformity, when compared with each other, there is also a certain degree of variability in architecture and size of different neuropils. In contrast to other amphipods, the brain of P. hawaiensis does not display any striking modifications or bias towards one particular sensory modality. Thus, we conclude that its brain may represent a common type of an amphipod brain.