2020 年 15 巻 2 号 p. 156-167
Beach strandings of 31 Keesingia gigas Gershwin, 2014, occurred in Exmouth Gulf, north-western Australia, between 16 and 25 March 2016, with 17 strandings recorded in just two days. Between 28 March and 31 May 2017 there were another 54 reports of K. gigas, many of multiple individuals, mostly on the Ningaloo coast west of Exmouth. These events provided an opportunity to examine this large but rarely observed jellyfish, as the only previous specimen collected was the holotype, off Shark Bay in 2012. Of 19 specimens collected and/or photographed on the beach in 2016, overall medusa length ranged from 190 to 345 mm, except for two 80 mm individuals. All but one of the smallest lacked tentacles, consistent with the original description of the species, photographs and field observations recorded in this study. The tentacles observed on the smallest animal examined may represent ontogenetic variability. Clusters of nematocysts were observed on the velarial margin of the bell. Three confirmed envenomations of adult people in 2016 did not result in them developing symptoms of Irukandji syndrome, however among six confirmed envenomations in 2017, two stings to adults resulted in some systemic Irukandji syndrome symptoms. In 2016, most sightings of K. gigas were in Exmouth Gulf, while in 2017 almost all were on the ocean side of North West Cape, indicating distribution was not solely restricted to Exmouth Gulf. Prevailing winds in the five days leading up to strandings in 2016 also suggest the jellyfish had been transported from the ocean north of Exmouth and not from within Exmouth Gulf. Similarly in 2017, prevailing winds and waves were consistent with currents moving jellyfish from north to south, with the time course of observations beginning in the northern part of Ningaloo in March and finishing in the southern section in May. All known sightings of K. gigas have been made in the March to May period in five separate years. Public awareness campaigns by the Shire of Exmouth in 2016 and Department of Biodiversity, Conservation and Attractions in 2017 were successful in raising awareness of the potential threat to swimmers and beach users and also proved effective in obtaining reports of the distribution of K. gigas.
