Swarming of holoplanktonic gastropod (Creseis acicula Rang, 1828) in Andaman Island during southwest monsoon, possible reasons and its global swarming records

The major objective of the study is to identify causative factors of pteropod swarms caused by Creseis acicula Rang, 1828 (Class: Gastropoda, Order: Pteropoda) and compare it with the swarms reported from different regions of the world. The study was carried out in Port Blair Bay, Andaman Islands. Sampling for biological-chemical parameters was carried out from eight sampling stations during southwest monsoon (August). Swarming of C. acicula was observed in the outer region of Port Blair Bay. The pteropod population was found maximum in Aberdeen Bay with 1765 Nos./m³ during the low-tide. Pteropod was the dominant zooplankton group in two stations. The water column was well mixed with temperature ranging from 28.1 to 28.7°C with a vertical gradient of 0.0–0.3°C and salinity ranging from 30.27 to 32.93 PSU with a vertical gradient of 0.1–0.3 PSU. In the zooplankton sample, almost all the pteropod shells were empty and there was no trace of organisms that escaped/released from the shell. From the observation of this study and other global studies, we hypothesize that this large number of C. acicula may be due to the aggregation of shells from nearby areas by oceanographic processes.