On the swarming of Creseis acicula (Rang 1828) (Mollusca, Gastropoda) in the Western Arabian Gulf

Abstract

An incidental swarming event of a zooplankton species took place in the anchorage area of Ras Tanura (Western Arabian Gulf), between June 10th and 13th 2020, causing serious problems to the anchored vessels. Examination of the samples collected from the clogged water pump of a vessel showed that the swarming was caused by the gastropod species Creseis acicula. Incidentally, it happens to be the first documented report on planktonic Gastropoda swarming in the Saudi waters of the Arabian Gulf. The swarming lasted only 2-3 days. Examination of the satellite images before, during, and after the swarming event indicated some changes in the color patterns of Ras Tanura surface waters, which could be linked to the swarm event. Due to bad weather conditions during and after the swarm period, it was not possible to collect seawater samples from the anchorage area and hence it was not possible to confirm whether this event could be linked with an algal or cyanobacterial bloom, as previously reported in another area of the Indian Ocean. Although most of the studies that reported C. acicula swarming indicated that this phenomenon is a natural one rather than being caused by external factors, some general precautionary/mitigation measures were proposed. Further studies and field observations are needed to better understand the causes of sea slug swarming and to avoid the issues it can cause to vessels, as it happened in the Western Arabian Gulf.