Rhyacophila species

Ito, Tomiko, 2021, The genus Rhyacophila Stephens (Trichoptera, Rhyacophilidae) in Yakushima Island, northern Ryukyu, southwestern Japan, Zootaxa 5023 (1), pp. 44-58 : 55-57

Rhyacophila species in Yakushima Island

Rhyacophila nigrocephala and two unidentified species (R. sp. 1 and R. sp. 2) had been found on the island previously ( Kuhara & Ito 2017). In this study, four new species (including clarification for the two unidentified species), and two newly recorded species are added and bring the present total to seven species of this genus for the island. This is the highest species number of Rhyacophila species for any island in the Ryukyu Archipelago, since zero to four species of the genus are known for each of other islands ( Ito 2017a).

All new species are apparently endemic to Yakushima ( Figs 6A–6D ), and three of them, R. yakushima , R. arakawa , and R. nagata , have related species on the main islands of Japan and/or continental Asia. The remaining new species, R. miyanoura , is related to species on the main islands of Japan and species found in the more southern area, central Ryukyu of Japan. Two previously named species, Rhyacophila brevicephala and R. nigrocephala ( Figs 6E, 6F ), are common on the main islands of Japan, and Yakushima is their southernmost distribution area ( Taira & Nozaki 2021). Rhyacophila shikotsuensis ( Fig. 6G ) is found on Yakushima and from Hokkaido to Taiwan via the Ryukyu Archipelago ( Taira & Nozaki 2021). Therefore, the high number of Rhyacophila species on Yakushima might be due in part to the location of the island, which is closer to the main islands of Japan, where 60 Rhyacophila species are known ( Taira & Nozaki 2021; Torii & Kobayashi 2021), than are the other islands of the Ryukyu Archipelago ( Fig. 1A ). This diversity and the phylogenetic relationships of the island’s Rhyacophila species also reflect its position near the boundary between the East Palearctic and Oriental biogeographic regions ( Ito 2017b).

A small number of adults of Rhyacophila brevicephala and R. nigrocephala were collected from a few sites lower than 100 m a.s.l. ( Figs 6E, 6F ). On the main islands of Japan these species live in the lower reaches of rivers, often with broad riverbeds ( Hattori 2005), and therefore suitable habitats may be scarce on mountainous Yakushima. Two adults of Rhyacophila shikotsuensis were collected at 240 m and 1200 m a.s.l. sites ( Fig. 6G ), but their habitat preference was not apparent in the present study, since the collecting efforts were infrequent during their adult flight season, early winter. In contrast, many adults of the four new species were collected at a wide range of elevations, from 20 m to 1400 m a.s.l., and were relatively abundant in the upper reaches ( Figs 6A–6D ), suggesting that they may prefer relatively narrow, shallow, and fast-flowing streams in forests and might be supported by the nature of the mountains of Yakushima. The mountainous environment of the island may be another reason for the high species number of Rhyacophila in Yakushima, since mountains higher than 1000 m a.s.l. are absent elsewhere in the Ryukyu Archipelago ( Tachihara 2003).

Figures

FIGURE 1. Yakushima Island: 1A, Location of the island; 1B, collected sites (solid circles) and weather station (open square) on the island.

FIGURE 6. Adult collection sites of seven species of Rhyacophila on Yakushima. Solid circle = 1–4 adults collected; open circle = more than 5 adults collected.