Patelloida parva, Paran & Sasaki & Asakura & Nakano, 2025

Distribution of Patelloida parva n. sp. is influenced by the Kuroshio current and tropical waters

Sasaki (1999) documented the presence of P. parva n. sp. in Okinawa and Amami-Oshima Islands, as well as Kagoshima, Kochi, and Wakayama Prefectures. Unlike the widely distributed P. lanx in the Pacific Ocean, Sea of Japan, and Seto Inland Seas , P. parva n. sp. is confined to the Pacific coasts. Our study reveals its preference for capes and coasts near the open ocean, contrasting with P. lanx , which also inhabits bays and inland shores.

All P. parva n. sp. samples sequenced in this study were collected from Kashiwajima, Kochi Prefecture, and Kushimoto, Wakayama Prefecture —coastal areas significantly influenced by the Kuroshio current. Both locations are impacted by the Bungo Channel and Kii Channel, with Kashiwajima closer to the entrance to Sukumo Bay than Kushimoto to the entrance of Kii Channel. Kashiwajima, Kochi, with more than 800 reported species, and Kushimoto, Wakayama, experience a diverse coastal fish fauna due to the Kuroshio current’s influence.

The selected type locality for P. parva n. sp., Ogokuda beach in Cape Shionomisaki, Kushimoto ( Fig. S4 View Fig ), boasts the highest shellfish diversity on Honshu Island ( CBD 2015). The northern limit of P. parva n. sp. along southern Wakayama aligns with the existing northern limit of coral reefs ( Nakabayashi et al. 2019). The species’ occurrence in Japan is likely influenced by warm waters from the Kuroshio current, possibly originating from the tropical waters of the Indo-west Pacific. Its absence from the Tsushima current region suggests a preference for higher salinity and lower nutrient concentration, distinctive differences between of the Kuroshio and Tsushima currents. However, further investigation is necessary to comprehend the species abundance, ecology, and distribution in its habitat range.

The possible tropical origin of this new species could be linked to the phylogenetic distance of Patelloida parva n. sp. closer to the P. saccharinoides , a tropical species, compared to the Patelloida saccharina species complex in Japan. This hypothesis corroborates previous descriptions that the morphology of our undescribed species is similar to Patelloida saccharinoides ( Sasaki 1999) . Divergence time was not estimated as the divergence rate for the 16S rRNA region in gastropods remains to be elucidated, but several inferences can be made. Tropical marine gastropods in Japan were reported to have expanded their habitat ranges northward during the Late Neogene (~5–2 Ma) when sea surface temperatures were warmer ( Tomida and Kitao 2002; Yokoyama 1911). During this period, the Kuroshio current was also believed to have existed in some form ( Tomida et al. 2013), facilitating the migration from the tropics. Formal confirmation of the occurrence of this species in the Ryukyu Islands could also strengthen the hypothesis of a tropical origin. Overall, these evolutionary processes may have influenced the demographic history and present-day structure of Patelloida parva n. sp.