Chapter 10.7. Phylogeographic patterns of the Southern Ocean crinoids
Résumé
Crinoids are well represented in the Southern Ocean. They locally may con-stitute one of the major mega-epibenthic components and therefore play a crucial role in the functioning of some ecosystems. Because some species are very abundant and well distributed over the whole Southern Ocean (Elé-aume 2006; Hemery 2011; Eléaume et al. Chapter 5.25, this volume), they have been recognised as model organisms for studying the spatial variance of genetic diversity in the Southern Ocean.The main hypothesis that explains the patterns of distribution of Antarctic benthic genetic diversity is linked to the cyclical advance of ice sheets on the Antarctic continental shelf. In this hypothesis, the Antarctic shelf ecosystems have undergone cyclical disturbance events during the last 35 My (see review in Turner et al. 2009), including as many as 38 sedimentary cycles of ice sheet advance during the last 5 My (Naish et al. 2009). It is thought that ice advance has regularly eradicated the benthic shelf fauna. Thatje et al. (2005, 2008) suggested that the benthic fauna now flourishing on the shelf arose from ref-uges either in areas left free of ice on the shelf (e.g. polynyas or areas not im-pacted by ice advance), on the adjacent slopes and deep-sea floor, or on the shelves of sub-Antarctic islands. Allcock & Strugnell (2012) summarised the expected molecular patterns for each of these hypotheses. As a result, widely distributed populations were fragmented into smaller populations that have diverged, and sometimes developed barriers to reproduction. This may have been followed by range expansion and, in the case of broadcast spawners, rapid recolonisation of habitats left free of ice, and secondary contact of refu-gial populations. Benthic taxa lacking a dispersal phase are often structured in haplogroups segregated in narrow geographical ranges (Rogers 2007). Clarke & Crame (1992, 2010) proposed that the cyclical nature of the vicari-ance events that may have resulted in genetic divergence and sometimes in allopatric speciation enhanced the Antarctic “biodiversity pump” that resulted from “the regular pulses of migration in and out of Antarcticadriven by climate variability” (Clarke & Crame 1992, p. 299). Cycles of ice extension and retreat are often seen as catastrophic disturbance events leading to large-scale en-vironmental instability, leading in turn to large-scale benthic eradication. How-ever, the megabenthic fauna on the Antarctic shelf is thriving and seems to have rapidly recovered from the cyclical disturbance events, the latest being the Last Glacial Maximum (LGM) some 20,000 years ago. Variation of food availability (which may be linked to ice concentration but also to other factors such as current velocity), interspecific competition for space and food, pelagic larval duration or larval mortality, may also have contributed to the observed patterns, and should receive greater attention in the future. Here we explore, using the Cytochrome c Oxydase subunit I (COI) mitochondrial DNA, the spa-tial variance of the genetic diversity of the most common Southern Ocean crinoid species, which include broadcast spawners as well as brooders, and examine congruence with predictions of the biodiversity pump hypothesis.
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