The efficiency of aerial respiration and the overall dependence on water availability during embryonic development have never been tested in semi-terrestrial or terrestrial crabs. In the present experimental study, we show that respiration of the embryos of a semi-terrestrial intertidal crab, Armases miersii from Jamaica, is bimodal. They are able to extract oxygen from both air and water, although a lower air respiration with approximately half O2 uptake than in water from the stage III onward was observed. Comparing the embryos of ovigerous females that were impeded for 18 h per day to submerge their egg clutches in seawater (n=8 treatments), with those having unlimited access to water (n=7 controls), we observed no differences in embryonic respiration or development time, but the lack of water caused complete brood-loss in 5 females and massive developmental problems in the remaining 3 broods, with an average hatching rate of only 28% and deformations in most newly hatched larvae. Although the higher embryonic O2 uptake in water suggests a reduced embryonic ability to extract oxygen from air, we propose as an alternative hypothesis an adaptive down-regulation of the metabolic rate, which is not linked to oxygen extraction but to the water dependence of vital metabolic pathways, such as the excretion of ammonia and CO2. Since bimodal embryonic respiration has only been known from vertebrates, this is the first study demonstrating aerial respiration in brachyuran embryos. However, developmental deficiencies in water-limited egg clutches suggest that the embryos A. miersii still depend on water to avoid desiccation stress and, probably, an impediment of excretory processes.