Chiton (Rhyssoplax) rapaitiensis, Schwabe & Lozouet, 2006

Schwabe, Enrico & Lozouet, Pierre, 2006, Chitons (Mollusca, Polyplacophora) from Rapa, the southernmost island of Polynesia, Zoosystema 28 (3), pp. 617-633 : 630-631

A moderately high number (274) of specimens was collected of the three species reported herein ( Table 1). Table 1 shows that Onithochiton lyellii inhabits the intertidal zone only, whereas the other two species were found down to 24- 30 m. The predominant species is Chiton (Rhyssoplax) rapaitiensis n. sp. It was found at nearly all stations where Polyplacophora were found, but the number of stations investigated in total is about 4 times higher. Locally it was found syntopically with Leptochiton hiriensis n. sp., which seems rather uncommon. Chiton (Rhyssoplax) rapaitiensis n. sp. is shown to be a brooding species and represents the second known species in this subgenus with this reproduction type. The two new species are known only from Rapa Island and are considered to be endemic. Onithochiton lyellii is for the time being considered to be restricted to Polynesia. Creese (1986) has shown that the endemic New Zealand species Onithochiton neglectus broods its eggs within the mantle cavity. The settlement of the trochophores occurs within a few hours and does not allow broad dispersal, as the presence of a suitable substratum (in general all kinds of hard substratum) is essential. If one infers the same larval development for Onithochiton lyellii and excludes a non-feeding, free-swimming larval stage, it explains why this species is restricted to Polynesia. The fact that two of the three species of Polyplacophora collected are new confirms the unique nature of the marine fauna of Rapa, suspected from previous knowledge of other molluscs. Thus, preliminary observations of the protoconchs of caenogastropods collected at Rapa show that a relatively high proportion of the species has non-planktotrophic development, indicating low dispersal abilities. Among these a large part probably consists of endemic species ( Lozouet et al. 2004). In view of the isolated location of Rapa, the endemicity found in the Polyplacophora is not surprising. At Easter

Island (Rapa Nui), where a high level of endemism is reported (42%; Rehder 1980), Dell’Angelo et al. (2004) reported three species of Polyplacophora, namely Plaxiphora mercatoris Leloup, 1936 , Rapanuia disalvoi Dell’Angelo, Raines & Bonfitto, 2004, and Weedingia cf. mooreana Kaas, 1988 , of which the first two are also considered endemic. This contrasts with the low level of endemism estimated in the marine molluscs of the Pitcairn Islands (about 2%) where only one polyplacophoran ( Onithochiton lyellii ) is known ( Preece 1995, as Onithochiton cf. societatis Thiele, 1910 ).

Compared to the Gastropoda or Bivalvia, the Polyplacophora is a very minor class in terms of species richness, which makes it difficult to detect any clear trends in diversity. Even in some well-documented mollusc faunas the proportion of Polyplacophora is always extremely low. At Rapa ( Lozouet et al. 2005) Polyplacophora accounted for 0.6% of species diversity; 0.6% in New Caledonia at Koumac ( Bouchet et al. 2002), 0.3% at Enewetak Atoll ( Kay & Scott 1987), 0.4% at Hawaii and 0.9% at Okinawa (Kay & Johnson 1987). Obviously, the Polyplacopha is not an appropriate group for estimating species richness gradients in the West Pacific. Indeed, more information is needed on the species richness of Polyplacophora within the Polynesian region alone (a study is in progress by the first author) before any attempt at evaluating such trends could be made.