Rapid evolutionary radiation of marine zooplankton in peripheral environments

Abstract

Populations of jellyfish, Mastigias sp., landlocked in tropical marine lakes during the Holocene, show extreme genetic isolation (0.74 < ST < 1.00), founder effects (genetic diversity: 0.000 < < 0.001), rapid morphological evolution, and behavioral adaptation. These results demonstrate incipient speciation in what we propose may be modern analogues of Plio-Pleistocene populations isolated in ocean basins by glacially lowered sea level and counterparts to modern marine populations isolated on archipelagos and other distant shores. Geographic isolation in novel environments, even if geologically brief, may contribute much to marine biodiversity because evolutionary rates in marine plankton can rival the most rapid speciation seen for limnetic species, such as cichlids and sticklebacks. Marine lakes present situations rare in their clarity for studying evolution in marine taxa.founder effect ͉ island ͉ rate of evolution

Genetic Diversity and Evolution of Marine Animals Isolated in Marine Lakes

“…The fauna and flora in most of the marine lakes are quite different from those in the oligotrophic lagoons that have an abundance of hard corals; some diagnostic species of fish, mollusks, jellyfish, sponges, and green algae are observed in the lakes (Dawson and Hamner, 2005).…”

“…Indeed, unique patterns of speciation and adaptation have been observed for jellyfish (Dawson and Hamner, 2005), sea anemone (Fautin and Fitt, 1991), foraminifera (Lipps and Langer, 1999;Kawagata et al, 2005), and bacteria (Venkateswaran et al, 1993) in the marine lakes; however, little is known about the evolutionary features of the nektonic and benthic animals in the lakes. Therefore, we focused on the fish and bivalve species in the marine lakes and analyzed their genetic diversity, differentiation, and population structures in comparison with those of related species, or different populations of the same species, that inhabit the barrier-reef lagoons.…”

“…Finally, with the rising sea level of the end of the Last Glacial Maximum, seawater flooded the depressions through fissures or tunnels (Hamner and Hauri, 1981;Hamner and Hamner, 1998). Geologically, the marine lakes were estimated to have formed in chronological series, namely, the deeper depressions flooded first (~12,000 years ago) and the shallower depressions flooded later (~5,000 years ago) (Dawson and Hamner, 2005). We surveyed more than 25 marine lakes in Palau and show the limnological characteristics of the 9 major marine lakes in Table 1.…”

“…It is thought that the marine lakes, which are small bodies of sea water in embayments and depressions on the limestone islands, have been gradually formed by floods resulting from rising sea levels after the Last Glacial Maximum (approx. 18,000 years ago); these waters were isolated from the surrounding barrier-reef lagoons and became sealevel marine lakes (Dawson and Hamner, 2005). Most of these lakes have been avoided by the local population due to the treacherous surface of the surrounding karst, lack of fresh water on the islands, and persistent myths that the marine lakes are haunted (Hamner and Hamner, 1998).…”

“…Even geographically well-separated populations may be connected genetically, because there are few barriers to prevent gene flow in the oceans (Mayr, 1954;Palumbi, 1994). In contrast to the open ocean environment, the marine lakes of Palau (Western Caroline Islands), which are surrounded entirely by land and isolated from the sea, provide unique local environments for genetic differentiation of marine organisms (Dawson and Hamner, 2005;Gotoh et al, 2009;Goto et al, 2011). We have focused on marine lakes as isolated marine environments and have conducted continuous evolutionary studies of marine organisms in the Palau Islands for the past 13 years.…”