Larval types and a summary of postembryonic development within the pycnogonids

The pycnogonid literature consists mainly of genus and species descriptions occasionally interspersed with observations on their biology and natural history. Few papers are devoted to detailed studies of pycnogonid biology, and there are few general summaries available. Papers which specifically cover pycnogonid postembryonic development usually deal with observations of larval development in a single species, and other references to pycnogonid larvae are widely scattered in the literature. The current paper is a significant improvement over earlier papers on pycnogonid postembryonic development since it brings together and summarizes the previous pycnogonid literature on this topic. In addition to summarizing the existing literature, three new pycnogonid postembryonic developmental lines are described and named: Protonymphon or Typical Protonymphon, Encysted Larva, and Atypical Protonymphon. A fourth line, the Attaching Larva, was previously described and a putative fifth line, the Gabelsflach larva, is more likely a crustacean than a pycnogonid. Walking leg development, including the order of addition of leg segments for the Typical Protonymphon and the Attaching Larva, is also included and the relevance of this topic for furthering our understanding of pycnogonid higher level relationships is examined. Much of the widely scattered literature on pycnogonid development has been summarized in a series of tables which organize this literature by host organism, life cycle, larval type, comparison of development of leg segmentation, and distribution of larval type by family. Synonyms and complete name citations of many of the species discussed in this paper are also included because many of the pycnogonid species used in earlier descriptions of post-embryonic development have undergone (in some cases radical) name changes to the point where many of them are unrecognizable to the modem pycnogonid biologist.