Evolutionary events are primarily caused by transitions to physically distinct environments, such as shifting from sea to land. Changes in niches and ecological successions cause many speciation events. Significant morphological, physiological, reproductive, and behavioral adaptations are features of these evolutionary pathways. Among the Decapods, crabs (Brachyura) are marine in origin, but they have successfully colonized most intertidal and terrestrial habitats, developing significant functional adaptations. Our study aims to evaluate functional adaptations in the gills of the crab Achelous hastatus (Brachyura, Portunidae). We retrieved samples of 20 specimens coming from the coast of the Strait of Messina (location Ganzirri), Messina, Italy (38°15′N 15°36′E). Histologically, an additional outer layer surrounding the gill lamellae, composed of muco-protein material and scattered cells, many of them haemocytes, can be noted. Using confocal microscopy, a strong reactivity of this layer to both Toll-like receptor 2 and Na+/K+ ATPase can be seen, suggesting that this layer is involved not only in the defense of the gills, which are in close contact with the external environment, but also in the exchange of ions to regulate salinity. These adaptations are consistent with the crab intertidal habitat, and similar adaptations have been found in the gills of brachiurians in mangrove forests in Brazil. Moreover, the presence of mucus may help in avoiding dehydration, and the reactivity to the antibodies tested suggests a reinforcement of the defense of the organism and ion regulation system. The data presented here provide new insights into the evolution and adaptations of marine organisms and contribute to our understanding of the internal defense and salt regulatory systems of this crab.