Hyperiid amphipods (Crustacea: Peracarida) in relation to a cold-core ring in the Gulf of Mexico

The species composition, distribution, and abundance of the hyperiid amphipods collected in March 1993 across a Gulf of Mexico cold-core ring (CCR) were analyzed. Day and night samples were collected by oblique tows (100 m to surface) with a plankton net. Hyperiids were represented by 56 species, 21 of which have not been recorded previously in gulf waters. The local oceanic community differs from that reported from adjacent neritic and neritic-oceanic areas of the Northwestern Tropical Atlantic. Overall, hyperiids were more abundant within (59% of total catch in org./1000 m³) than outside the CCR (41%). All inside CCR stations were sampled at night. Night outside vs. night inside CCR hyperiid faunas showed important differences in terms of species richness, composition and density. Cluster analysis indicated that day sta. 5 on the edge but outside the CCR was more similar to those stations inside the CCR (nighttime samples) than to the other daytime samples. Moreover, all the stations outside the CCR were clustered together independently of their day or night origin. It is suggested that the differences found were more related to differential conditions related to the CCR than to diel vertical migration. The analysis of three congeneric pairs with inverse CCR-related abundance and with known or inferred migratory patterns strengthened the idea that these pairs are probably separated by thermal preferences; also, their vertical migratory patterns seem to be abnormal inside the CCR. A relatively higher concentration of immature stages inside the CCR supports the idea that the enriched CCR waters constitute areas of increased production. Furthermore, this higher productivity enhances the chances of hyperiids to find their hosts, the gelatinous zooplankters, which are also more abundant inside the CCR.