Pteropod assemblages in the oceanic waters of the southern Gulf of Mexico (June 2015)

The pteropod community structure in the oceanic waters of the southern Gulf of Mexico (sGoM) was analyzed in June 2015. For the first time, we used an integrative morphological and molecular approach for pteropod identification in the sGoM. A total of 32 pteropod taxa were morphologically identified from the oceanic waters of which 12 were successfully sequenced, at least for one of the loci implemented (COI and 28S). The molecular data support the taxonomic identity of half of them. However, despite the morphological differences in the other six taxa, the molecular evidence displayed low genetic variability, resulting in a resolution to the genus level. During the sampling campaign, the oceanographic conditions were very homogeneous in all the sampled areas, with high surface temperatures (SST >27.5 °C) and low chlorophyll (CHL <0.5) concentrations. Nevertheless, CHL presented a positive and significant correlation with total pteropod abundance, herbivores abundance and carnivores abundance, suggesting a bottom-up control of the pteropod community. The ecological analysis showed that four species accounted for 63% of the pteropod abundance: Heliconoides inflatus, Creseis conica, Limacina trochiformis and Creseis acicula. The cluster analysis showed the presence of four assemblages, named after their distribution and species composition. The oceanic assemblage presented a wide distribution over the sampled area, characterized by a set of oceanic species such as H. inflatus, Styliola subula and Limacina lesueurii, with no apparent correlation with the measured environmental variables. The coastal oceanic assemblage presented the highest pteropod abundances; it was characterized by the dominance of the neritic species C. conica and L. trochiformis, and significant positive correlation with CHL concentration. The other two assemblages were distributed in the vicinity of an anticyclonic eddy with high stratification and low CHL values in the northwest, and presented a mixed dominance among the oceanic species H. inflatus, C. acicula and S. subula. The oceanic assemblage is suggested as the basal pteropod community in the sGoM. It is modified by the deflection of the coastal current in the southeast that transports nutrients and biota into the oceanic domain and produce relative enrichment of the CHL concentration. The presence of a Loop Current anticyclonic eddy in the northwest, increases the stratification and accentuates the oligotrophic conditions, leading to a different pteropod community well adapted to those oligotrophic conditions.