We evaluated the thermal adaptations of embryos of 6 terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita): B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens. Embryos of each species were cultured in vitro at 6 different temperatures (18 to 34 degrees C) in artificial seawater to avoid air desiccation; the lower threshold temperatures for embryonic development were estimated using heat summation theory equations. Additionally, partial effective cumulative temperatures (> lower threshold temperature) until hatching were determined for ovigerous females of each species cultured in containers. The relationships between the embryonic growth index values (relative area of the embryonic body vs. total embryo surface) and effective cumulative temperatures were expressed using cubic equations. Lower threshold temperature was estimated to be 12.7 to 14.5 degrees C. The effective cumulative temperature and egg incubation period estimates from the appearance of the embryonic body to hatching were higher in B. latro and C. brevimanus, followed by C. rugosus, C. cavipes, and C. violascens, and lower in C. purpureus, suggesting that C. brevimanus may retain an ancestral thermal adaptation trait for embryos, as in B. latro, which is considered the most ancestral species in the coenobitid phylogeny. Egg size varied among species but did not affect the thermal adaptations of embryos. The lower effective cumulative temperature and shorter egg incubation period may be advantageous to producing broods during the shorter summer breeding season in C. purpureus, which has the northern-most geographical distribution.