Holoplanktonic molluscs (Gastropoda: Pterotracheoidea, Thecosomata and Gymnosomata) from the southern Mexican Pacific

This is the first study of species composition of holoplanktonic molluscs of the Pacific coast of southern Mexico, focusing mainly on the Gulf of Tehuantepec. From two oceanographic surveys during summer 2007 and 2010, 40 zooplankton samples were obtained. A total of 40 species contained within 17 genera and 8 families was found. The results show 15 range extensions, including Atlanta californiensis and Carinaria japonica which were previously considered as endemic to the Transition Zone waters of the California Current, and Limacina helicina which was thought to only inhabit polar and subpolar waters. Atlanta gibbosa and Creseis chierchiae f. constricta are two new records for the American Pacific. The use of holoplanktonic molluscs as indicators of water masses is a common practice; however, these reported range extensions limit the validity of their use as indicators, at least within the environmental parameters established in previous studies. Although the 40 species found represent only 43% of the species recorded for the Pacific Ocean, it is noted that this species richness only represents summer conditions and an increase could be expected if sampling was extended to winter.