Sphaerillo boninensis Nunomura, 1990 (Crustacea, Isopoda, Oniscidea) is a junior synonym of a pantropical species, Venezillo parvus (Budde-Lund, 1885)

Abstract Re-examination of the holotype and paratype of Sphaerillo boninensis Nunomura, 1990 from Chichijima Island of the Ogasawara archipelago, which is registered as a UNESCO World Heritage Site, indicates that this species is a junior synonym of a pantropical species, Venezillo parvus (Budde-Lund, 1885).


Introduction
Sphaerillo boninensis Nunomura, 1990, described from specimens collected on Chichijima Island, Ogasawara archipelago, is a small terrestrial isopod formerly considered endemic to this archipelago, a UNESCO World Heritage site approximately 1,000 km from the mainland of Japan. However, there are questions around the taxonomy of this species. The genus that this species has been attributed to has changed many times (see the taxonomic account; Nunomura 1990Nunomura , 1999aNunomura , 2011, and the species is included in the genus Spherillo in the current taxonomic treatment (Nunomura 2015). In addition, Saito (1993) recorded specimens collected from Chichijima Island and identified as the pantropical species Venezillo parvus (Budde-Lund, 1885) by Drs Ferrara and Taiti, but this species has been overlooked in reviews of Japanese terrestrial isopod species (Nunomura 1999a(Nunomura , b, 2015Saito et al. 2000).
Venezillo parvus, originally and imperfectly described as Armadillo parvus by Budde-Lund (1885), was subsequently transferred to the genus Venezillo Verhoeff, 1928(Green et al. 1990Taiti and Ferrara 1991a). Today Venezillo accommodates more than 140 recognized species (Boyko et al. 2008). More comprehensive, subsequent descriptions of V. parvus provided more useful taxonomic characteristics than those present in the original description, i.e., in Schultz (1963) as V. evergladensis Schultz, 1963, a species now considered to be a junior synonym of V. parvus,  as Sphaerillo (?) parvus (Budde-Lund, 1885), Schmidt (2003) and Gregory (2014) as V. parvus. Figures provided by Schmidt (2003) depicted the basis of the male pereopod 7 as bearing dense setae on the anterior corner of the ventral side, a characteristic present also on the male pereopod 7 of S. boninensis (Nunomura 1990, fig. 146I). Additionally, the oblique lobe on the ventral surface of pereonite 2 reported for S. boninensis by Nunomura (1990, fig. 146B) is a diagnostic feature of the genus Venezillo (Verhoeff 1928;Vandel 1952). These similarities indicated that S. boninensis was referable to Venezillo and a close relationship between S. boninensis and V. parvus might exist.
Terrestrial isopods are now scarce in the southern part of Hahajima Island, Ogasawara archipelago, in an area invaded by the land nemertean Geonemertes pelaensis Semper, 1863 (Shinobe et al. 2017). This terrestrial predator poses a threat to native biodiversity in this region. Resolving the systematic status of S. boninensis, or at least specimens referred to it, from this area would improve our understanding of actual versus perceived threats to endemic faunas of this region, and ultimately, the conservation and management of species throughout this archipelago.
The aim of this study was to resolve relationships between S. boninensis and V. parvus. This is achieved through re-examination and redescription of appropriate type material of the former.

Sample collection
Type material was loaned from the Toyama Science Museum (collection acronym TOYA). As the previously dissected male holotype and paratype of S. boninensis were in poor condition, a description of the whole body, cephalon, antenna 1, and pleopod 5 of the female paratype was necessary. Additionally, as the male pereopods 1 and 7 are broken in male holotype, they can be only partly described. Male pereopod 2 was not found in the holotype and paratype.

Discussion
The genus Venezillo is characterized among other characters by a narrow lobe located obliquely or horizontally on the ventral surface of the pereon epimeron 2 (Verhoeff 1928; Vandel 1952). The holotype of S. boninensis has an oblique lobe on the ventral surface of the epimeron 2, for which reason it is more appropriately assigned to the genus Venezillo. In addition, in all other morphological characteristics, type materials of S. boninensis are consistent in morphology with those of V. parvus as redescribed by Schmidt (2003): the apical corner of the basis of the male pereopod 7 bears dense short setae (Fig. 3F), the endopodite of the male pleopod 1 has long setae at the tip and scale-like setae on the inner margin (Fig. 4A), and the exopodite of the male pleopod 1 is triangular (Fig. 4B). For these reasons, I regard S. boninensis to be a junior synonym of V. parvus, a species widely distributed in the tropical belt. While S. boninensis was considered endemic to the Ogasawara archipelago, and potentially threatened by an invasive nemertean predator (Shinobe et al. 2017), the pantropical V. parvus, due to its widespread distribution and invasive tendency, is not considered to be threatened at species level.