Caridean shrimps comprise the only decapod infraorder in the markedly impoverished high Antarctic decapod fauna. Their presence, with about a dozen species only, was previously assigned to their capability of down-regulating high haemolymph magnesium ([Mg2+]HL) concentrations at low temperatures. This regulation is missing or insufficient in reptant decapods, affecting in particular their scope of activity. This deficiency has been hypothesised as one important factor explaining their extinction during Antarctic cooling in the Tertiary, as evidenced by the fossil record.The present work demonstrates an ecological adaptation in the early life history of decapods to ecophysiological constraints in the cold, primarily a mismatch between reproductive traits and a marked seasonality of primary production and low temperatures increasing with latitude. It is shown that on a macroecological scale, natant decapods show remarkable plasticity in changing their early life history patterns with latitude and decreasing water temperature. These changes include, for example, an increase in egg size, hatching of large and advanced larvae and a reduction of fecundity with latitude. Energy saving traits to abbreviate larval developments such as a strong suppression of the number of instars, low metabolic losses in very thin exuviae during fewer moults, and a loss of highly variable developmental pathways compared to lower latitides, are observed already in the Subantarctic realm of southernmost America. Early larvae generally present a high resistance to starvation, which in combination with an abbreviated larval development allows for a better synchronisation with peaks in food availability. However, caridean larvae rely on primary production, and the evolutionary step towards complete endotrophic larval development, possibly due to phylogenetic constraint, is almost unknown in marine shrimps.The need for food in planktotrophic and extended larval development of ancestor reptants, and the lacking resistance of larvae to starvation as well as their low tendency of suppressing the number of larval instars, is herein supposed to have selected against Reptantia at high latitudes. However, it is shown that lithodid crabs, which evolved fairly recently about 13 to 25 Ma ago, did develop early life history adaptations to cold environments. Two lithodid crab species from the Subantarctic Magellan region (Lithodes santolla, Paralomis granulosa) developed complete food independent lecithotrophic and strongly abbreviated development of demersally living larvae, and a high tolerance to low temperatures typical of the Antarctic. Under present climate conditions, these anomuran crabs may represent a reptant group that might be able to recuperate the polar environments as a habitat for reptants, as already indicated by a few recent records of lithodids off the Antarctic continental shelf.