Family Calcinidae is a hermit crab family morphologically diverged from Diogenidae. Calcinus is a type genus of Calcinidae. It inhabits tropical coral reefs with a colorful body. Among them, C. vachoni was firstly collected in southern Korea. Herein, this study reports Korean C. vachoni with its morphological, ecological, and molecular characteristics. Morphological characteristics of 12 C. vachoni caught on Jeju Island were examined. Ecological characteristics of C. vachoni were reviewed briefly and cox1 DNA barcoding analysis of five Korean C. vachoni was conducted with 16 species, 5 genera, and 3 families of Micronesia and GenBank hermit crab data. Korean C. vachoni has 13 pairs of gills, a larger left cheliped than the right one, a broad triangular rostrum, a few tufts of setae on ventral margins of dactyl and propodus of the first ambulatory leg, and cream dactyl and bluish-gray propodi of pereopods identical to the original description and previous studies. The cox1 sequences of Korean C. vachoni were monophyletic with cox1 sequences of C. vachoni in GenBank. Calcinus vachoni sequences were divided into three groups regarded as cryptic species. They were correlated with geographical distance as in the previous study. The review of ecological characteristics of C. vachoni shows that it inhabits nearby coral which might have a symbiotic relationship. The habitat of C. vachoni was extended to the northwest which could be related to recent climate change.

In this study, the tropical hermit crab genus Calcinus and Family Calcinidae were first reported in Korea. For molecular identification, the cox1 sequences of Korean C. vachoni and other tropical hermit crabs in Kosrae, Micronesia were first obtained and analyzed by this study. In addition, the cryptic diversity of C. vachoni groups noted in the previous study was reviewed and expanded their geographical range. Furthermore, we suggested the association relationship between C. vachoni and corals for the first time.