An adult female of the Jewel-Box clam crab Gemmotheres chamae (Roberts, 1975) was collected from a shrimp trawl during a biological exploration off Campeche coast, Mexico. This finding represents its first record in the Gulf of Mexico and the second locality for this species along the Atlantic coast of America, the first being from off North Carolina, U.S.A. As an adult, G. chamae has a soft, thin carapace, and a subconical protuberance on article 1 of the antennae (with the nephridiopore of the antennal gland), so it is considered to belong to the Pinnotherinae sensu stricto. G. chamae and Nannotheres moorei Manning and Felder, 1996 (Atlantic) are the only members of the American Pinnotherinae sensu stricto that have the maxilliped 3 with a 2-segmented palp. The asymmetry of pereiopod 3 is confirmed, as well as that of the pereiopod 4, but the right legs are the longest. All these features are diagnostic for G. chamae.
Distribution; symbiotic Pinnotheridae; bivalve Chama; Western Atlantic; Campeche