PROJECTOMIC RECONSTRUCTION OF THE CENTRAL COMPLEX IN THE FIDDLER CRAB AFRUCA TANGERI

The decapod crustacean Afruca tangeri is a semiterrestrial crab that anchors its behaviours to a burrow. For example, during low tide, Afruca engage in feeding excursions throughout the tidal zone where they maintain their horizontal body axis according to the direction of their burrow, even when its location is not visible. Given they have a relatively small brain, structurally similar to other arthropods, and that their exoskeleton facilitates the implementation of in vivo chronic neuronal recording devices, fiddler crabs offer a unique opportunity to investigate how external sensory information and idiothetic cues are integrated to guide spatial behaviour. Here we aim to characterise the detailed anatomical organisation of the fiddler crab central complex (CX), a conserved brain structure implicated in spatial representation and navigation.
We developed a sample preparation protocol enabling large en bloc electron microscopy to obtain a volumetric dataset encompassing the central complex, including columnar and other intrinsic neurons. Volume reconstruction reveals a structural organisation containing subdivisions of distinct neuropils, and a columnar and layered architecture with characteristic projection motifs between compartments. Comparison with well-characterised arthropod central complexes such as those of drosophila and the honeybee suggests a conserved organisational logic, while also highlighting species-specific features of neuron morphology and connectivity. The resulting anatomical framework constrains hypotheses about navigation-based computations and establishes a foundation for in vivo recordings and behavioural manipulations in relatively simple, freely roaming animals.