Recording earthworm diversity on the tropical island of Martinique using DNA barcoding unveiled endemic species in bromeliad plants

In a context of accelerating biodiversity loss, some species may become extinct before taxonomists have time to collect and describe them. This is an even more critical problem for taxa with poor taxonomic knowledge, such as earthworms in the tropics. To overcome this taxonomic impediment, we investigated earthworm diversity on the tropical island of Martinique using DNA barcoding. Firstly, we sampled earthworms at 81 sites evenly distributed in two categories corresponding to natural and anthropized environments. We obtained a total of 684 sequences of a fragment of the COI gene. MOTU delimitation and morphological identification suggested that this dataset corresponded to 49 putative species, of which 16 were known exotic species. The rarefaction curve suggested that the sampling effort was not sufficient to capture total earthworm diversity in the natural environment and that many more species could be discovered. Secondly, we focused on a heterogeneous landscape around the town of Morne Rouge, where arboreal earthworms were sampled using a standardized protocol. We found 8 different species in epiphytic plants, three of which were found almost exclusively in bromeliad plants that were sampled above 1.4 m from the ground. The presence of these species, probably endemic to Martinique and new to science, was significantly correlated to the proportion of forest land cover. The most common earthworm in these habitats was Dichogaster andina, an exotic species found in bromeliads and Heliconia plants that were sampled at every height from the bottom of the tree up to 6.8 m. Its presence was correlated to the proportion of total river length in the landscape, suggesting that rivers may act as a dispersal vector. This study therefore highlights the great diversity of earthworms in Martinique, while emphasizing the main threats to this biodiversity, namely the alteration of habitats and the presence of invasive species.