2 research outputs found
Determining the Drivers of Anti-Tropical Distributions Across the Fish Tree of Life
Anti-tropical distributions are those where populations of a single species, or multiple closely related taxa, are distributed outside of, and on opposing sides of, the tropics. These latitudinally disjunct distributions have been noted for over a century. Despite this long history of interest, little has been concluded regarding the actual mechanisms that drive this pattern, with several prominent hypotheses competing with one another in the literature. Here I review the proposed drivers of anti-tropicality, and subsequently test them using fishes with a variety of life history and taxonomic differences. This includes (1) a temperately restricted family with anti-tropical distributions – Cheilodactylidae, (2) a tropical reef fish family with a single temperate anti-tropical genus – Prionurus, and (3) a variety of fishes from across the fish tree of life that have populations split by the tropics. Using complete taxonomic sampling, and phylogenomic approaches coupled with fossil calibration points, I find evidence for recent equatorial divergence events in the Pleistocene and Pliocene, as well as divergence events dating to the Miocene for both Cheilodactylidae and Prionurus. Furthermore, taxonomic issues were detected, and explored within both of these groups. To disentangle the multiple hypotheses that can explain recent transitions, I used ecological niche models coupled with extant distributional data for a variety of species across the fish tree of life that exhibit intra-specific anti-tropicality. These data reveal distinct support for both glacial dispersal, and biotic exclusion from the tropics. These results are then interpreted in a comprehensive framework to determine what drives anti-tropical distributions in marine systems. Overall, multiple mechanisms seem responsible that act in concert over time to produce these distributions. Certain equatorial divergence events are recovered in time periods currently not associated with any anti-tropical hypotheses. It seems likely that stochastic crossing events may be important in the initial colonization of a new hemisphere
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The effects of sea level fluctuations on coral reef fishes : genetic differences between outer reef and lagoon inhabiting wrasses (genus Halichoeres)
textSea levels fluctuated following glacial cycles during the Pleistocene, reaching approximately 115-130m below current sea levels in the Indian and Pacific Oceans during the last glacial maximum 17,000 years before present. The effects of these sea level fluctuations on population structure have been shown in many near-shore marine taxa, revealing several common patterns. However, the underlying mechanisms behind these observed patterns are largely unknown. Drops in sea level affect the distribution of shallow marine biota, exposing the continental shelf on a global scale, and displacing coral reef habitat to steep slopes where shelf breaks are shallow. In these circumstances, we expect that species inhabiting lagoons should show reduced genetic diversity relative to species inhabiting more stable outer reefs. Here, I tested this expectation on the scale of an entire ocean-basin with four wrasses (genus Halichoeres): H. claudia (N=194, with ocean-wide distribution) and H. ornatissimus (N=346, a Hawaiian endemic) inhabit seaward reef slopes, whereas H. trimaculatus (N=239) and H. margaritaceus (N= 118) inhabit lagoons and shallow habitats throughout the Pacific. Two mitochondrial markers (cytochrome oxidase I and control region) were sequenced to resolve population structure and history of each species. Haplotype and nucleotide diversity were similar among all four species. The outer reef species showed significantly less population structure, consistent with longer pelagic larval durations and a historically stable population. Mismatch distributions and significant negative Fu’s F values indicate Pleistocene population expansion for all species, and (contrary to expectations) reduced genetic diversity in the outer slope species. These data indicate that lagoonal species may persist through the loss of habitat, but are restricted to isolated refugia during lower sea level stands, which may inflate genetic diversity during high sea levels. Outer reef slope species on the other hand have homogeneous and well-connected populations through their entire ranges regardless of sea level fluctuations. These findings contradict the hypothesis that shallow species are less genetically diverse as a consequence of glacial cycles.Marine Scienc