Comparison of the structure and presumptive function of Type II microtrichs and associated cuticular microstructures in species of Gammarus (Amphipoda, Crustacea) from different habitats

Type II microtrichs are slender, peg-like sensilla found on the exoskeleton of amphipod crustaceans. Using scanning electron microscopy, the microtrichs and associated cuticular microstructures were compared in six species and two subspecies of Gammarus known to live in different habitats, ranging from freshwater lentic and lotic to marine intertidal. In all the species, the microtrichs were clustered in arrays on the lateral and dorsal areas of the exoskeleton, and there were similarities in their configuration: semi-circular, L-shaped, and linear. Intraspecific variation of sensillum numbers and arrangement in arrays was slight compared with interspecific differences, where axial and lateral orientations of the microtrichs are believed to be related to current detection. Associated cuticular microstructures included in-common finely pixelated, polygonal plates, modified in some species by lateral stretching, the development of short spines, a scale-like appearance, and raised triangular peaks some of which may be related to locomotion, such as burrowing. Grouping of the eight species/subspecies along phylogenetic lines showed little support based on relatedness centred around calceolus-possession or water salinity requirements. A significant microstructural difference was observed between subspecies of Gammarus lacustris.