Rediscovery and molecular phylogenetic placement of the endemic diving beetle Allodessus skottsbergi (Coleoptera: Dytiscidae) in Rano Kau crater of Rapa Nui (Easter Island)

The diving beetle genus Allodessus Guignot, 1953 comprises five described species distributed across the Pacific. However, its systematics have long been obscured by uncertain species boundaries and incomplete molecular sampling. One of its members, Allodessus skottsbergi (Zimmerman, 1920), is endemic to Rapa Nui (Easter Island, Chile) and had not been recorded since its description nearly a century ago, prompting assumptions of extinction. Here, we document the rediscovery of A. skottsbergi in ephemeral ponds within the Rano Kau volcano crater and provide the first molecular data for the species. Using low-coverage whole-genome sequencing, we recovered its mitochondrial genome and key nuclear markers, which were integrated into a phylogenetic analysis of Allodessus . Contrary to earlier hypotheses, A. skottsbergi is not the sister lineage to the remaining species; instead, A. megacephalus forms the basal lineage to Allodessus bistrigatus plus A. skottsbergi . Analysis of the mitochondrial cox1 gene revealed negligible genetic divergence among A. bistrigatus, A. oliveri , and A. thienemanni , and morphological re-examination did not identify consistent diagnostic differences, prompting the formal synonymisation of A. oliveri and A. thienemanni under A. bistrigatus . In contrast, A. skottsbergi is supported as a distinct species by its unique morphology, geographic isolation, and genetic divergence. Its rediscovery provides new insight into the evolution and biogeography of Allodessus and underscores the critical conservation importance of temporary pond habitats on Rapa Nui.