Jellyfish swarms, tourists, and the Christ-child

One of the most remarkable sights in the Western Pacific is a perennial swa rm of 1.5 million golden medusae (Mastigias sp.) crowded into a land-locked marine lake in Palau, Micronesia. This 'Jellyfish Lake' became a popular o ff-gassing stopover for SCUBA divers and a destination in its own right for non-diving tourists in the mid-1980s. Since then, tourism in Palau has boo med, increasing 500% between 1986 and 1997. However, in December 1998, the golden-medusae disappeared. Apart from patchy occurrences between December 1998 and April 1999, the medusae have since been absent from the lake. Fiel d measurements, including temperature and salinity depth profiles, Mastigia s medusae population sizes, and the distribution of scyphistomae, in 'Jelly fish Lake' between 1979 and 1999 were integrated with laboratory-based expe riments on the effects of salinity, temperature, sunscreen and zooxanthella e enrichment on Mastigias scyphistomae or medusae. These studies indicated that the disappearance of medusae was due to physical changes in lake struc ture, including a substantial increase in temperature, initiated by the 199 7-98 El Nino. Here, we describe these studies, the changes in Jellyfish Lak e and their probable influence on the Mastigias. We further elucidate the c hanges in Jellyfish Lake by reference to coincident changes in three other 'jellyfish lakes' in Palau: Big Jellyfish Lake, Clear Lake and Goby Lake.