Description of a new genus and two new species of Darwinulidae (Crustacea, Ostracoda), from Christmas Island (Indian Ocean) with some considerations on the morphological evolution of ancient asexuals

Darwinulidae is believed to be one of the few metazoan taxa in which fully asexual reproduction might have persisted for millions of years. Although rare males in a single darwinulid species have recently been found, they may be non-functional atavisms. The representatives of this family are characterized by a slow evolutionary rate, resulting in a conservative morphology in the different lineages over long time frames and across wide geographic ranges. Differences between species and genera, although often based on small details of valve morphology and chaetotaxy, are nevertheless well-recognizable. Five recent genera (Darwinula, Alicenula, Vestalenula, Penthesilenula and Microdarwinula) and about 35 living species, including also those left in open nomenclature, are included in this family. Previous phylogenetic analyses using both morphological characters and molecular data confirmed that the five genera are good phyletic units. Here, we report on the results of a study on darwinulid ostracods from Christmas Island (Indian Ocean). The taxonomic investigation led to the description of a new genus and two new species, namely Isabenula humphreysi gen. nov. sp. nov. and Vestalenula sp. E, this latter being left in open nomenclature, because only one individual was available. A new phylogeny of the family, using 30 morphological characters of 23 described species, is here presented. A key to the genera of Recent Darwinulidae, based on the morphology of valves and soft parts in adult females, is also provided. The discovery of Isabenula gen. nov., which shares intermediate morphological features with the genera Vestalenula and Penthesilenula, opens new opportunities to critically reconsider previous hypotheses on tempo and mode of evolution in the family Darwinulidae and on the phylogenetic relationships between its representatives.