Reproductive morphology of the deep-sea protobranch bivalves Yoldiella ecaudata, Yoldiella sabrina, and Yoldiella valettei (Yoldiidae) from the Southern Ocean

The protobranch bivalves of the Southern Ocean are poorly understood ecologically, despite their high abundances in soft sediments from the shelf to the deep sea. The subclass has a long evolutionary history predating the formation of the polar front, and knowledge of their reproductive biology is key to understanding better their successful radiation into the Southern Ocean, and within deep-sea basins. In this study, we for the first time investigate the reproductive morphology of three deep-water protobranchs; Yoldiella ecaudata from 500 m in the Amundsen Sea; Y. sabrina from between 200 and 4,730 m in the Amundsen Sea, Scotia Sea, and South Atlantic; and Y. valettei from 1,000 m in the Scotia Sea. All three species demonstrate evidence of lecithotrophic larval development with maximum oocyte size of 130.4, 187.9, and 120.6 µm in Y. ecaudata, Y. sabrina, and Y. valettei, respectively, further supported by prodissoconch I measurements. There is evidence for simultaneous hermaphroditism in Y. valettei. Asynchronous oocyte development within specimens of Y. ecaudata and Y. valettei is described, and also between populations of Y. sabrina separated by depth. The reproductive characteristics, comparable to those of North Atlantic deep-sea protobranch species, are discussed in the context of the cold thermally stable conditions prevailing on the deep-Antarctic continental shelf and deep sea. The requirement for reclassification of this complex subclass is also discussed in relation to observed soft anatomy and shell characteristics