Distribution patterns in Antarctic and Subantarctic echinoderms

Late Holocene echinoderm assemblages can serve as paleoenvironmental tracers in an Antarctic fjord

High Latitude fjords can serve as sediment trap, bearing different type of proxies, from geochemical to micropaleontological ones, making them exceptional tools for paleoenvironmental reconstruction. However, some unconventional proxies can be present and can be used to depict a comprehensive and exhaustive interpretation of past changes. Here, studying a sediment core in Edisto Inlet (Ross Sea, Antarctica) we used irregular echinoid spines and ophiuroids (Ophionotus victoriae) ossicles to trace environmental changes throughout the last 3.6 kyrs BP. Irregular echinoids can serve as proxy for the organic matter content, while O. victoriae ossicles can be used as proxy for steady sea-ice cycle along with organic deposition events. O. victoriae release a high number of ossicles, making estimation about the population quite challenging; still, presence data, can be easily collected. By applying Generative Additive Models to the stratigraphical distribution of these data, we detected an environmental phase that was previously unnoticed by other traditional proxies: the Ophiuroid Optimum (2-1.5 kyrs BP). In conclusion, here we demonstrate how echinoderm presence can be used as a valuable source of information, while proving the potential of modelling binary data to detect long-term trend in Holocene stratigraphical records.

A newly discovered radiation of endoparasitic gastropods and their coevolution with asteroid hosts in Antarctica

BACKGROUND

Marine invertebrates are abundant and diverse on the continental shelf in Antarctica, but little is known about their parasitic counterparts. Endoparasites are especially understudied because they often possess highly modified body plans that pose problems for their identification. Asterophila, a genus of endoparasitic gastropod in the family Eulimidae, forms cysts in the arms and central discs of asteroid sea stars. There are currently four known species in this genus, one of which has been described from the Antarctic Peninsula (A. perknasteri). This study employs molecular and morphological data to investigate the diversity of Asterophila in Antarctica and explore cophylogenetic patterns between host and parasite.

RESULTS

A maximum-likelihood phylogeny of Asterophila and subsequent species-delimitation analysis uncovered nine well-supported putative species, eight of which are new to science. Most Asterophila species were found on a single host species, but four species were found on multiple hosts from one or two closely related genera, showing phylogenetic conservatism of host use. Both distance-based and event-based cophylogenetic analyses uncovered a strong signal of coevolution in this system, but most associations were explained by non-cospeciation events.

DISCUSSION

The prevalence of duplication and host-switching events in Asterophila and its asteroid hosts suggests that synchronous evolution may be rare even in obligate endoparasitic systems. The apparent restricted distribution of Asterophila from around the Scotia Arc may be an artefact of concentrated sampling in the area and a low obvious prevalence of infection. Given the richness of parasites on a global scale, their role in promoting host diversification, and the threat of their loss through coextinction, future work should continue to investigate parasite diversity and coevolution in vulnerable ecosystems.

Is reproductive strategy a key factor in understanding the evolutionary history of Southern Ocean Asteroidea (Echinodermata)?

Life traits such as reproductive strategy can be determining factors of species evolutionary history and explain the resulting diversity patterns. This can be investigated using phylogeographic analyses of genetic units. In this work, the genetic structure of five asteroid genera with contrasting reproductive strategies (brooding: Diplasterias, Notasterias and Lysasterias versus broadcasting: Psilaster and Bathybiaster) was investigated in the Southern Ocean. Over 1,400 mtDNA cytochrome C oxidase subunit I (COI) sequences were analysed using five species delineation methods (ABGD, ASAP, mPTP, sGMYC and mGMYC), two phylogenetic reconstructions (ML and BA), and molecular clock calibrations, in order to examine the weight of reproductive strategy in the observed differences among phylogeographic patterns. We hypothesised that brooding species would show higher levels of genetic diversity and species richness along with a clearer geographic structuring than broadcasting species. In contrast, genetic diversity and species richness were not found to be significantly different between brooders and broadcasters, but broadcasters are less spatially structured than brooders supporting our initial hypothesis and suggesting more complex evolutionary histories associated to this reproductive strategy. Broadcasters' phylogeography can be explained by different scenarios including deep-sea colonisation routes, bipolarity or cosmopolitanism, and sub-Antarctic emergence for the genus Bathybiaster; Antarctic- New Zealand faunal exchanges across the Polar Front for the genus Psilaster. Brooders' phylogeography could support the previously formulated hypothesis of a past trans-Antarctic seaway established between the Ross and the Weddell seas during the Plio-Pleistocene. Our results also show, for the first time, that the Weddell Sea is populated by a mixed asteroid fauna originating from both the East and West Antarctic.