Mar Chiquita is the southernmost in a series of temperate brackish lagoons extending along the coast of the Atlantic Ocean from southern Brazil to northeastern Argentina. These shallow lagoons are fringed by dunes, salt marshes and mud flats, where dense populations of intertidal crabs thrive. This particular type of estuarine habitat (termed cangrejal, from Spanish cangrejo = crab) is dominated by the semiterrestrial and euryhaline species Neohelice (formerly Chasmagnathus) granulata. Through its intense burrowing and omnivorous feeding activities, this ecosystem engeneer influences the biogeochemical conditions, vegetation growth, habitat structure, benthic community composition, and other characteristics of its habitat. Since two decades, this crab has increasingly also been used as a model in studies of ecophysiological and life-history adaptations to transitional environments between the sea, freshwater and land. Numerous investigations have shown adaptions in its osmoregulation, energy metabolism, and other physiological, biochemical, and behavioural traits. In the early life-history stages, most adaptations are related to an export strategy. Hatching rhythms with maximum release at nocturnal ebb tides minimize larval predation by pelagic fish and ensure a rapid downstream transport to lower estuarine or coastal waters, where subsequent zoeal development takes place at higher and more stable salinities. This is later followed by re-immigration of megalopae, whose recruitiment is stimulated by adult odors and other chemical cues associated with the cangrejal habitat. These ontogenetic migrations coincide with genetically programmed changes in larval osmoregulation: hyper-regulation is strong at hatching, lower in the subsequent zoeal stages, and high again in the megalopa and juveniles. In conclusion, there are strong interactions between the key species N. granulata and its coastal lagoon environment, shaping each other like in a process of co-evolution.