Diversity of Atlantidae mollusks (Gastropoda: Pterotracheoidea) from the southern region of the California current off Baja California peninsula, Mexico

The gastropod family Atlantidae is a group of holoplanktonic mollusks comprising 23 species in three genera. Most atlantids have tropical and subtropical affinity, but two thirds of the known species are also distributed in transitional and/or temperate regions. The region off the Baja California Peninsula (BCP) is a faunistic transitional area in which a mix of subarctic, temperate, subtropical, and tropical communities occurs. Since the last extensive study in 1967, only eight species of Atlantidae have been recorded. In this study, the atlantid species composition off the BCP was determined based on three oceanographic surveys in fall 2015, winter, and spring 2016. A total of 18 species were recorded. The findings of Atlanta brunnea J.E. Gray, 1850, A. echinogyra Richter, 1972, A. frontieri Richter, 1993, A. gibbosa Souleyet, 1852, A. helicinoidea J.E. Gray, 1850, A. plana Richter, 1972, A. tokiokai van der Spoel & Troost, 1972 and Oxygyrus inflatus Benson, 1835 represent northward expansion of their known distributional range, while A. fragilis Richter, 1993 and A. rosea Gray, 1850 are recorded for the first time in the American Pacific, thus increasing to 20 the number of species present in the Northeastern Pacific region. These records allow the recognition of the Pacific coast off the BCP as an area of high species diversity of atlantids in the Mexican Pacific.