Comparative Neuroanatomy of Hydrothermal Vent Shrimps: Ecological Differentiation Among Four Alvinocaridid Species.

Alvinocaridid shrimps colonize hydrothermal vent fields along the Mid-Atlantic Ridge (MAR), where they experience extreme environmental conditions including high hydrostatic pressure, steep thermal and chemical gradients, and dim light. While various aspects of the biology of the dominant vent shrimp Rimicaris exoculata, such as general anatomy, microhabitat, symbiotic relationships, or trophic networks, have been extensively studied, little is known about how brain organization varies among alvinocaridid species occupying different ecological niches. Here, we investigated whether vent shrimps share common neuroanatomical features and whether differences in brain and vascular organization reflect ecological diversification among species. Using histology, immunohistochemistry, µCT scans, and 3D reconstruction, we compared the brain and the neurovascular system (cor frontale) in four MAR species with distinct ecologies (R. exoculata, Rimicaris chacei, Mirocaris fortunata, Alvinocaris markensis) and in the coastal relative Palaemon elegans. Our results reveal strong commonalities in brain organization among vent shrimps. All MAR species display a compact brain architecture with hypertrophied mushroom bodies (MBs), suggesting enhanced multisensory integration and memory capacities. Corresponding to the low-light environment, the visual neuropils (lamina, medulla, lobula) are smaller than those of the coastal species P. elegans. In all species living near hydrothermal vents, the retina exhibits modifications; it extends along the dorsal part of the cephalothorax, and its length varies according to the species, being the longest in the genus Rimicaris. The lobula satellite neuropil is absent in all alvinocaridids, suggesting a limited capacity to track moving visual signals. The olfactory lobes exhibit a common glomerular organization but differ among species in quantitative parameters, such as volume and number of glomeruli. The neurovascular system shows marked interspecific variation; the cor frontale differs in size and volume among hydrothermal vent species but retains a conserved subdivision of central arteries surrounding a longitudinal muscle bundle. We conclude that the observed variations in the volume of neuropils and arrangement of the vascular system likely reflect the ecological divergence related to microhabitat conditions and trophic strategies near hydrothermal emissions.