Chitons (Mollusca, Polyplacophora) from Rapa, the southernmost island of Polynesia

The RAPA 2002 workshop permitted the first comprehensive collection of polyplacophorans from the island of Rapa (Austral Islands, South Pacific) to be made. The collection includes three genera and three species, two of the species being new to science. The isolation of Rapa may explain the uniqueness of this small polyplacophoran fauna. The two new species are: Leptochiton hiriensis n. sp. and Chiton (Rhyssoplax) rapaitiensis n. sp. The latter species, which is the most common (83% of the specimens), is a brooder. Leptochiton hiriensis n. sp. differs from congeneric species of the same size in the sculpture of the valves, which show concentric growth marks in the central area and concentrically arranged nodules in the head valve, postmucronal region of the tail valve and the lateral areas. Chiton (Rhyssoplax) rapaitiensis n. sp. is compared to the related species C. (R.) linsleyi (Burghardt, 1973), C (R.) rhynchotus (de Rochebrune, 1884), and C. (R.) maldivensis (E. A. Smith in Gardiner, 1903). In addition to its smaller size, the new species differs in tegmental sculpture and in the ornamentation of the dorsal girdle scale. Onithochiton lyellii (Sowerby, 1832), a species also known from the Gambier Islands and Pitcairn Islands, is restricted herein to the Polynesian region. In addition to the descriptions of the new species, characters relevant in identification of O. lyellii are described and illustrated for the first time in scanning electron micrographs.