On the Larval Sensory Organs and Central Nervous System of the Zoea 1 of the Asian Shore Crab Hemigrapsus sanguineus (Decapoda, Brachyura).

Many decapod crustaceans occupy different habitats during their ontogeny and undergo a niche shift during the transition from the larval to the juvenile phase. Their life cycle comprises pelagic larvae and, in most species, benthic juvenile-adult stages. Already at hatching, decapod larvae possess a wealth of organ systems, necessary to autonomously survive and develop in the plankton. They also exhibit a rich behavioral repertoire to appropriately respond to environmental stimuli such as light, gravity, hydrostatic pressure, tidal currents, temperature, salinity, chemical cues, and food concentration. Considering the impressive larval behavioral performance, neuroanatomists in the past have already studied selected aspects of the functional morphology of larval nervous systems, but this knowledge is distributed across a wide taxonomic range. Our study aims to provide a concise description of the morphology of selected sensory organs and that of the central nervous system in the Zoea 1 of the Asian shore crab Hemigrapsus sanguineus using multiple neuroanatomical methods such as three-dimensional (3D) reconstruction from histological section series, scanning electron microscopy, and immunohistochemistry combined with confocal laser-scan microscopy. The larval developmental cycle of this includes five zoea stages and one megalopa stage. In the Zoea 1, our data reveal well-developed visual and olfactory organs comprising hundreds of sensory receptor neurons and also complex primary visual and olfactory neuropils related to processing this sensory input. We observed a particularly high investment of nervous tissue in the central visual pathway. Furthermore, the larval brain comprises several characteristic secondary neuropils which, as higher order neuropils, in the adult crustacean brain are thought to integrate highly processed multimodal sensory input. These are, for example, the lateral protocerebrum (which in adult crabs will house the mushroom bodies) and the median protocerebrum that includes the central body. The discussion of our findings is framed against the knowledge of the larval behavioral performance and, considering the larval investment in higher order neuropils, the possibility of complex behavioral decisions that integrate aspects of learning and memory.