Planktonic cnidarians in a cold-core ring in the Gulf of México

Abstract

THE SPECIES COMPOSITION AND ABUNDANCE OF MEDUSAE AND SIPHONOPHORES COLLECTED IN MARCH 1993 ACROSS A GULF OF MEXICO COLD-CORE RING (CCR) WERE ANALYSED. SAMPLES WERE COLLECTED IN DAY AND NIGHT TIME BY OBLIQUE TOWS (100 M TO SURFACE) WITH A PLANKTON NET. MEDUSAE WERE REPRESENTED BY 12 SPECIES, WHICH BY COMBINED NUMBERS WERE ALMOST EQUALLY ABUNDANT WITHIN AND OUTSIDE THE CCR. HOWEVER, THE TWO SPECIES THAT TOGETHER COMPRISED > 66% OF ALL MEDUSAE INSIDE THE CCR (NAUSITHOE PUNCTATA AND LIRIOPE TETRAPYLLA) WERE RARE OUTSIDETHE CCR AND SPECIES MOST COMMON OUTSIDE (COMBINED > 66% OF ALL MEDUSAE OUTSIDE) WERE EITHER ABSENT OR RARE INSIDE THE CCR. SIPHONOPHORES WERE REPRESENTED BY 25 SPECIES; THE SEVEN MOST ABUNDANT WITHIN THE CCR WERE ALSO THOSE MOST ABUNDANT OUTSIDE. BECAUSE OF DAY/NIGHT MIGRATIONAL PATTERNS, CONSISTENTLY GREATER TOTAL NUMBERS OF SIPHONOPHORES WERE FOUND AT NIGHT IN THE UPPER 100M LAYER. CLUSTER ANALYSIS CONFIRMED THESE DIFFERENCES IN SIPHONOPHORE COMPOSITION AND ABUNDANCE BETWEEN DAYLIGHT AND NIGHT SAMPLES BUT ALSO SHOWED DIFFERENCESBETWEEN NIGHT SAMPLES AND OUTSIDE THE CCR. ALTHOUGH THEY WERE UNCOMMON WITHIN THE CCR, TWO RARE SPECIES OF LENSIA AND TWO OF GERATOCYMBA WERE FOUND ONLY IN COLLECTIONS INSIDE THIS FEATURE. TWO OTHER SPECIES OF LENSIA, A THIRD SPECIES OF CERATOCYMBA, AND FOUR OTHER OVERALL UNCOMMON SIPHONOPHORES WERE FOUND ONLY OUTSIDE THE CCR. THUS, DIFFERENCES AT THE SPECIES LEVEL INDICATE THAT PELAGIC CNIDARIANS ARE NOT UNIFORMLY DISTRIBUTED IN THIS ZONE OF THE GULF, BUT RATHER THEY VARY IN RESPONSE TO MESOSCALE CHANGES IN THEIR PHYSICAL ENVIRONMENT.