Collaboration between Yamagata University and Kamo Aquarium – Rapid adaptive evolution of jellyfishes inhabiting marine lakes in Palau

Abstract

The Palau Project Group at the Faculty of Science, Yamagata University, Japan has collaborated with the Tsuruoka City Kamo Aquarium, Yamagata, Japan since 2003. The Palau Islands include approximately 70 marine lakes formed by the rising of ancient coral reefs. In some marine lakes that are completely isolated from outer lagoons, seawater can soak through minute pores in the limestone surrounding the lakes, causing tidal changes to occur. However, marine organisms cannot enter or exit marine lakes. Based on our previous evolutionary genetic analyses, certain fish (in Atherinidae, Apogonidae, and Gobiidae) and mussels (in Mytilidae) exhibit distinct patterns of evolution between marine lake and outer lagoon populations, and also among marine lake populations. Dawson and Jacobs (2001) reported that three distinct species of moon jellyfishes are distributed in Palau; Aurelia sp. 3 and Aurelia sp. 6 inhabit outer lagoons and Aurelia sp. 4 inhabits marine lakes. We surveyed and collected jellyfish and confirmed that the species are highly differentiated both genetically and morphologically. Based on additional comparative observations of Aurelia sp. 4 and Aurelia sp. 3, the former clearly exhibits specific morphological and ethological characters adapted to the environmental conditions that characterize calm marine lakes. Based on geological estimates of marine lake formation, such adaptive evolution in jellyfish occurred rapidly between 5,000 and 15,000 years ago.