Description of planktonic polychaete larvae from Terre Adélie and the Ross Sea (Antarctica)

An analysis was carried out on 51 samples from 32 stations, obtained during the I.C.O.T.A. cruise between 18 January 1996 and 7 February 1996 in the Pointe Geologie Archipelago, Terre Adelie, and on samples obtained by 2 sediment traps deployed in the Ross Sea, within the framework of the Italian C.L.I.M.A. Project. These samples showed the unexpected presence of numerous larvae of benthic marine invertebrates. In addition, larvae from the Ross Sea were examined. Seven phyla with complex life-cycles and at least 18 species of benthic polychaetes were represented. Morphological descriptions by scanning electron microscopy of the most frequent larvae and identification of larval types are provided. Descriptions from the literature of larvae collected in the Antarctic Peninsula were used for comparison. Morphological descriptions do not always lead to identification at the species level; however, these descriptions are indispensable for establishing the geographic distribution of larvae. It has been demonstrated that indirect identification methods relying on the premise that the benthic fauna is entirely known and that sampled larvae belong only to species already known through their benthic adult stage may lead to obvious absurdities. As an ecological consequence, Thorson's rule, according to which indirect development should not be represented and direct development should be dominant at high latitudes, is not supported by our data. A short comparison of the local reproductive pattern with a subantarctic area, the Kerguelen Archipelago, strongly suggests that the absence of planktonic development in this area cannot be explained by Thorson's rule. This rule has been invalidated in more austral zones; the prevalent reproductive pattern in the Kerguelen Archipelago is linked to the very marked advective strength of water flow.