Effects of hyposalinity on survival and settlement of moon jellyfish (Aurelia aurita) planulae

Presettlement processes likely influence the geographical distribution and abundance of scyphozoan jellyfish medusae. In East Asian coastal waters, Aurelia aurita s.l. spawning coincides with the summer monsoon season, and extreme rainfall events subject planulae to hyposaline conditions, presumably jeopardizing their survival and settlement. In this study, laboratory experiments were conducted to determine how prolonged exposure to hyposalinity (25, 20, and 15, in addition to control salinity 32) affects swimming speed, survivorship and settlement of planula larvae, and subsequent development of metamorphosed polyps. Nearly all planulae (≥ 95%) in 32, 25, and 20 were geonegative during the first 4 h of exposure to respective salinities, but the majority of planulae (70%) in 15 were geopositive. Although no mortality was induced, hyposaline conditions affected larval behavior and the sequence of post-metamorphosis events. Salinity of 15 significantly increased planktonic larval duration and settled polyps had morphological deformities. At a salinity of 20, settled polyps had delayed tentacle development compared to control and 25 treatments. These anomalous results demonstrate that the response of planulae to environmental stress is more complex than previously assumed and may depend on the ability of the settled polyps to develop viable feeding tentacles. Salinity is proposed to be a principal factor influencing planulae dispersion and distribution in temperate monsoon regions.