Abstract
A monophyletic chiton lineage whose members characteristically have hundreds of ocelli embedded in their upper valve surfaces are presently assigned to either Toniciinae or Acanthopleurinae. This subdivision is partly based on whether the adult girdle is nude (Toniciinae) or is covered with calcareous armor such as spines or scales (Acanthopleurinae). Their fossils date only to the Miocene so improved phylogenetic resolution would provide enhanced opportunities for investigating recent eye evolution. These chitons also include some of the most conspicuous and ecologically important chitons living in tropical intertidal habitats, especially Acanthopleura spp., and also some species in cooler waters including Tonicia spp. in southern Chile. DNA sequence comparisons strongly corroborate the ocellus clade but not its internal subdivision into conventional Toniciinae and Acanthopleurinae groupings. Instead, our results support separate New World and Old World radiations. For example, the large-bodied "Acanthopleura" echinata with its stout girdle spines and the scale-bearing Enoplochiton niger are normally classified as Acanthopleurinae. These species co-occur in Chile and are supported as sister species despite their girdle differences. Even more surprising, their close phylogenetic relationship to the nude-girdle Tonicia spp. (Toniciinae) in Chile suggests that Acanthopleurinae is presently an artificial grouping. Our success in deciphering this global biogeographic pattern has been accelerated by our recent addition of chiton samples from the Central Indo-Pacific region, representing additional (including new) species to our comparison. Our results indicate that phylogeny correlates with geography, not with the apparently labile girdle ornamentation.