Four times more species of sea spiders (Arthropoda: Pycnogonida) in Martinique Island (Lesser Antilles)

The marine biodiversity of the tropical northwestern Atlantic (TNWA) has been explored by many great naturalist expeditions. After more than one century of marine exploration, how well do we know its biodiversity? As a poorly studied taxon, sea spiders (Arthropoda: Pycnogonida Latreille, 1810) are excellent candidates to address this issue. Here, we report the results from the Madibenthos Survey conducted by the Muséum national d’Histoire naturelle (Paris) on the coasts of Martinique during Fall 2016, where sea spiders were collected during 138 sampling events by different methods. A total of 67 pycnogonid species, including 13 species probably new to science, were distinguished during this survey, whereas only 20 species were previously known from Martinique. Relying on an extensive and intensive sampling, the collection of pycnogonids from the Madibenthos Survey is one of the richest for TNWA. DNA barcoding based on 172 new CO1 sequences helped to discriminate species with inconspicuous diagnostic characters and pointed out 11 additional possible cryptic species. Richness estimators and species rarity indicate that the diversity of Martinique still remains underestimated. Results suggest that Martinique is not a hot spot for sea spiders and that similar levels of biodiversity might be expected in most other islands of the TNWA if similar sampling methods were applied. Finally, we show that the Caribbean and Atlantic coasts of Martinique harbor different sea spider faunas.