Postembryonic development of Nymphon australe Hodgson, 1902 (Pycnogonida, Nymphonidae) from Antarctica

Abstract

Developmental modes in pycnogonids are variable, even between congeneric species. Most of our previous knowledge of pycnogonid life history stages is based on morphological descriptions of the larval and those postlarval instars still retained on male ovigers, whereas the free-living instars are largely unknown. In this paper, a combination of morphological and molecular tools is used in order to link those postlarval instars retained on male ovigers to free-living specimens representing putatively later stages in the postembryonic development of a sea spider species. Male specimens of Nymphon australe carrying postembryonic instars were collected from localities off the South Shetland Islands, the Weddell Sea, the Drake Passage, the Bransfield Strait, and the Ross Sea. Three subsequent free-living instars were collected from Drake Passage. Nymphon australe hatch as a postlarval instar with two-articled palps and limb buds of the two first-walking legs. Postlarval instars 1 and 2 (both carried on the ovigers of males), postlarval instars 3 and 4, and a juvenile (all three free living) are all described and illustrated. A molecular phylogenetic based on the cox1 gene links all free-living stages to N. australe populations from the Antarctic Peninsula. The postlarval development of N. australe is discussed here and compared with those known from other pycnogonid species with similar development pathways.