Filter-feeding isopods (Antarcturus cf spinacoronatus) conquer a major new habitat on the underside of the Antarctic shelf ice

The colonization of a new habitat affords an important advantage to the successful pioneer species because competitors, predators and parasites are likely to be out of step. Here, we report the first retrieval and molecular characterization of a cryo-benthic community of isopod crustaceans that live at depths of 80-150m on the underside of a floating shelf-ice tongue at the Drescher Inlet (Riiser-Larsen Ice Shelf), Eastern Weddell Sea. The specimens were retrieved with a sampler mounted on a remotely operated vehicle (ROV) when video transsects were carried out. The molecular analysis of mitochondrial and nuclear genes of four specimens spanning the size range of individuals collected confirmed that (1) all belonged to a single species and that (2) this species has previously been identified in benthic communities in the Eastern Weddell Sea as Antarcturus cf. spinacoronatus (Baltzer et al 2000). The molecular phylogeny shows that the cryo-benthic A. cf. spinacoronatus are deeply nested in a family of isopods characterized by increasing complexity of morphological and behavioural adaptations to the acquisition of detrital and planktonic food particles. This demonstrates that the floating shelf-ice was likely colonized from the seafloor and not vice versa and that the filter-feeding life style of A. cf. spinacoronatus formed a predisposition playing a key role in the colonization of the new habitat. Density estimates of A. spinacoronatus under the floating shelf-ice (25 adults and 190 juveniles per square meter) are significantly higher than on the seafloor, suggesting that the transition to the new habitat devoid of any macrofaunal competition or predation provides a major advantage to the species and thus may be a geographically more widespread phenomenon. Our results are an indication that earlier reports of increased mid-water foraging of Weddell Seals (Liebsch et al. 2007) and the identification of filter-feeding isopods on seal-mounted still images from the underside of the shelf ice (Watanabe et al. 2006) are causally linked. References Baltzer, C., Held, C., & Wägele, J. W. 2000. Furcarcturus polarsterni gen. nov., sp nov., a large deep-sea arcturid isopod from the Drake Passage, with a preliminary molecular characterization. Polar Biology, 23(12), 833-839. Liebsch N., Wilson R.P., Bornemann H., Adelung D. & Plötz J. 2007. Mouthing off about fish capture: jaw movements in pinnipeds reveal the real secrets of ingestion. Deep Sea Research II 54:256-269. Watanabe, Y., Bornemann, H., Liebsch, N., Plötz, J., Sato, K., Naito, Y., & Miyazaki, N. 2006. Seal-mounted cameras detect invertebrate fauna on the underside of an Antarctic ice shelf. Marine Ecology Progress Series, 309, 297-300