Abstract
Two new species of the family Alpheidae: Alpheus margaritaesp. n. and Leptalpheus melendezensissp. n. are described from Santa María-La Reforma, coastal lagoon, SE Gulf of California. Alpheus margaritaesp. n. is closely related to A. antepaenultimus and A. mazatlanicus from the Eastern Pacific and to A. chacei from the Western Atlantic, but can be differentiated from these by a combination of characters, especially the morphology of the scaphocerite and the first pereopods. Leptalpheus melendezensissp. n. resembles L. mexicanus but can be easily differentiated because L. melendezensissp. n. has the anterior margin of the carapace broadly rounded and has only one spine on the mesial margin of ischium in the major cheliped, versus an acute rostrum and an unarmed major cheliped. Additionally, a phylogenetic analysis was used to explore the relationships of these two new taxa. These results show that Alpheus margaritaesp. n. and Leptalpheus melendezensissp. n. are indeed related to the species against which we are comparing them, and demonstrate that they can be considered as different species. Additional specimens of Leptalpheus cf. mexicanus, Ambidexter panamensis and A. swifti are recorded for the first time in the Santa María-La Reforma coastal lagoon.