We examined the ontogenetic change of body color patterns in the laboratory-raised juveniles of six terrestrial hermit crab species, including Birgus latro, Coenobita brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens, which commonly occur in the southern islands, Japan. The body color patterns of coenobitid juveniles were species-specific. The diagnostic features of body color patterns enable identification of juveniles of coenobitid crab species in the wild, thereby helping to understand the precise habitats of each coenobitid species.

Terrestrial hermit crabs in the family Coenobitidae (genera Coenobita and Birgus) are distributed in tropical and subtropical regions. They occupy various habitats ranging from shore to inland forests, and the two shore‐dwelling species, Coenobita rugosus and C. violascens, possess different distributional characteristics on Ishigakijima Island, Ryukyu Archipelago, Japan. Coenobita rugosus is distributed throughout the coast of the island and is abundant in beach areas, whereas C. violascens has mainly been found in river mouth areas. However, very little is known about the habitats used by the early life stages of coenobitid crabs because identifying the species of recently landed early juveniles is difficult. We tested whether the species compositions of early juveniles of coenobitids differed between beach and river mouth sites on Ishigakijima Island. We collected and identified the early stage coenobitids using PCR–RFLP techniques. A total of 576 early juveniles of five Coenobita species were collected, of which 0.7% were C. brevimanus, 7.3% were C. cavipes, 0.2% were C. purpureus, 70.1% were C. rugosus, and 21.7% were C. violascens. The early juveniles of Birgus latro were not found. The early juveniles of C. rugosus occurred at both beach and river mouth sites, and they were abundant at beach sites. The early juveniles of C. violascens were only found at river mouth sites. These findings indicate that C. rugosus and C. violascens complete their life cycles on land near the localities where they land. The early juveniles of the inland‐dwelling species, C. cavipes, were also mainly collected from river mouth sites, which suggested that juveniles of C. cavipes selected landing sites near river mouth areas and then migrated into the inland forests, passing through riverside areas. Our results highlighted the importance of river mouth areas for recruitment to adult habitats by some coenobitid species.

Salinity is an important ecological factor affecting larval survival and development of coastal and estuarine decapod crustaceans. We investigated the low salinity tolerance limits of larvae in the six terrestrial hermit crab species, the coconut crab Birgus latro, and the land hermit crabs Coenobita brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens to infer their early life history strategies. Zoeae and megalopae were exposed to six different salinity levels ranging from 5-30 ppt with intervals of 5 ppt for 24 h, and the median lethal salinity (MLS) was estimated as the salinity at which 50% of test larvae died. The MLS estimates were lowest in the first zoeae, increased during the zoeal stage, and declined in the megalopal stage in all species. Early zoeae and megalopae were euryhaline and later zoeae stenohaline, suggesting that coenobitids exhibit a larval export strategy towards the offshore (oceanic) marine waters. Interspecific variation was evident in the salinity tolerance of the first zoeae, probably reflecting the salinity conditions at the species-specific larval hatching place. In contrast, the low salinity tolerance ability of megalopae did not differ among species, suggesting that coenobitid megalopae might require similar habitats for the settlement and initiation of benthic life.

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