Molecular phylogeny of marine mites (Acariformes: Halacaridae), the oldest radiation of extant secondarily marine animals

Abstract

The family Halacaridae comprises more than one thousand mostly marine or rarely freshwater species. Many are predacious, but among marine mites, some genera evolved the ability to feed on macroalgae. We inferred a time-calibrated phylogeny based on 18S rDNA, 28S rDNA, and Cytochrome oxidase I (5,143 nt aligned) and all non-monotypic halacarid subfamilies plus a representative outgroup set (72 taxa). The family Halacaridae was rendered as the sister-group of Parasitengona, diverging 321.5, 264.0-381.3 Ma and radiating 271.3, 221.7-324.2 Ma (median, HPD). Thus, marine mites represent the oldest known extant animal lineage that secondarily invaded the sea, with the marine turtles being the second oldest such lineage (crown group 212.3, 194.9-231.4 Ma). Two freshwater mite lineages, represented by Limnohalacarus (219.2, 165.9-274.6) and Porohalacarus (175.3, 118.5-233.1), were inferred mutually non-monophyletic, suggesting two independent invasions to freshwater. The conventional subfamily Rhombognathinae (macroalgae feeders) was not recovered as monophyletic, with Metarhombognathus-Rhombognathides, restricted to the Northern Hemisphere, originating 177.5, 134.8-223.3 Ma and diversifying 88.3, 32.7-152.3 Ma. This is congruent to a previous hypothesis of their northern origin prior to the opening of the Norwegian Sea (58 Ma). Our phylogeny indicates the need for reclassification of the traditional subfamilies and suggests that previous molecular results (e.g., Rhombognathus deeply nested in Copidognathinae) is an analytical artifact due to a chimeric sequence.

Keywords: Algivory; Marine-freshwater transitions; Rhombognathinae; Secondarily marine taxa; Timetree.