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Mollusca is the second-largest animal phylum with over 100,000 species among eight distinct taxonomic classes. Across 1000 living species in the class Polyplacophora, chitons have a relatively constrained morphology but with some notable deviations. Several genera possess “shell eyes”, true eyes with a lens and retina that are embedded within the dorsal shells, which represent the most recent evolution of animal eyes. The phylogeny of major chiton clades is mostly well established, in a set of superfamily and higher-level taxa supported by various approaches including multiple gene markers, mitogenome-phylogeny and phylotranscritomic approaches as well as morphological studies. However, one critical lineage has remained unclear: Schizochiton was controversially suggested as a potential independent origin of chiton shell eyes. Here, with the draft genome sequencing of Schizochiton incisus (superfamily Schizochitonoidea) plus assembly of transcriptome data from other polyplacophorans, we present phylogenetic reconstructions using both mitochondrial genomes and phylogenomic approaches with multiple methods. Phylogenetic trees from mitogenomic data are inconsistent, reflecting larger scale confounding factors in molluscan mitogenomes. A consistent robust topology was generated with protein coding genes using different models and methods. Our results support Schizochitonoidea is a sister group to other Chitonoidea in Chitonina, in agreement with established classification. This suggests that the earliest origin of shell eyes is in Schizochitonoidea, which were also gained secondarily in other genera in Chitonoidea. Our results have generated a holistic review of the internal relationship within Polyplacophora, and a better understanding on the evolution of Polyplacophora.
A review was done on all species of the genus Talassia (family Vanikoridae Gray, 1840), which are known from an upper bathyal depth range in the Atlantic Ocean. Four new species are proposed: Talassia mexicana sp. nov. from the Gulf of Mexico, T. laevapex sp. nov. and T. flexisculpta sp. nov. from off Mauritania and T. rugosa sp. nov. from off Angola. Empty shells of the new species were found in sediment samples collected in habitats associated with deep-water corals. The new species were compared with the type species Talassia coriacea (Manzoni, 1868) and the deep-water species T. tenuisculpta (R.B. Watson, 1873), T. dagueneti (de Folin, 1873) and T. sandersoni (A.E. Verrill, 1884). Particularly the shape and sculpture of the protoconch show regional differences. Other specific characteristics are macro- and micro-sculpture of the teleoconch.