The Sea Spider Genus Hannonia Hoek, 1881 (Pycnogonida): A Review and Two New Species

Hannonia is a particularly rare and poorly studied genus, which includes four species distributed along the shores of Africa and Europe. Here, two new species are described from the southern shores of Madagascar, Hannonia arnaudae sp. nov. and Hannonia laurae sp. nov. sampled during the ATIMO VATAE expedition (Muséum national d'Histoire Naturelle survey, 2010). DNA barcoding data were successfully obtained for the holotype of H. arnaudae sp. nov. Besides, additional material sampled by Bernard Thomassin at Nosy-Vé and Tuléar in July 1972 are reported: these specimens present strong affinities to the newly described H. arnaudae sp. nov., but consistently exhibit slight morphological deviations from the holotype description. In absence of DNA barcoding data for this material, it was not possible to estimate whether these variations were significant, preventing their formal identification to the species level. A checklist and a new identification key for the genus are also provided, together with additional morphological information for Hannonia stocki Munilla, 1993.All Hannonia species except Hannonia typica Hoek, 1881 were sampled in only one locality, making their geographic and bathymetric distribution range unpredictable. The genus is potentially present on the western shores of the African continent, highlighting the need for further investigation in these regions where sea spiders are rarely surveyed. The affinities of Hannonia with the 11 currently accepted pycnogonid families need to be investigated through phylogenetic studies to be properly understood. Morphological comparisons suggest possible affinities between this genus and two other Southern Africa pycnogonid genera of uncertain taxonomic placement, Boehmia, and Queubus, highlighting further the importance of integrating African pycnogonids to future research on sea spider diversity and evolution.