Polar emergence and the influence of increased sea-ice extent on the Cenozoic biogeography of pectinid molluscs in Antarctic coastal areas

Abstract

Abstract From the Paleocene to the Pliocene, Chlamys-like genera of scallops (pectinid bivalve molluscs) inhabited coastal areas around the Antarctic continent. Today, all of the Chlamys-like genera are extinct around Antarctica and restricted to lower latitude habitats along the surrounding continents. The only remaining scallop in the high Antarctic zone is the large (greater than 108 mm in height), endemic, thin-shelled Adamussium colbecki. Adamussium has a circumpolar distribution with its highest abundances (greater than 50 m−2 and 2 kg m−2) in Antarctic coastal areas, in contrast to the offshore habitats where its ancestors had predominated since the Oligocene. Shell morphology and substrate data indicate that the Chlamys-like faunas were largely exposed to high-energy conditions associated with open water in the coastal zone, while the Adamussium faunas have been associated with low-energy hydrodynamic conditions. Modern and mid-Holocene distributions in the Antarctic coastal zone further indicate that Adamussium is largely restricted to protected embayments and habitats with extensive sea-ice coverage. In addition, a decadal mark-and-recapture experiment demonstrates that Adamussium has extremely slow growth and a century-scale lifespan, which complement the life-history traits that are considered to be characteristic of species inhabiting predictable and constant environments in the deep sea. Stabilization of the water column by increased sea-ice extent—together with stenothermal conditions, prolonged darkness and limited primary production—would have enhanced the habitat similarities between polar coastal environments and the deep-sea after the Plio-Pleistocene climate threshold. Subsequently, the deep Antarctic continental shelf would have facilitated faunal shifts between deep-sea and coastal habitats as with the polar emergence of the Adamussium fauna. Such biogeographic interchanges across high-latitude continental shelves since the end of the Cenozoic would have contributed significantly to the latitudinal diversity gradients that have been noted from both deep-sea and shallow-water marine faunas.