The family Smeagolidae (Mollusca: Gastropoda: Pulmonata) typically inhabits the undersides of boulders buried in gravel bottoms and crevices on beaches. These snails have a colourless, translucent, tiny slug-like body that is approximately 2–10 mm in length, and notably lack shells, eyes, and tentacles. Smeagolidae is an exceptionally rare taxon globally, with only five described species known from Australia and New Zealand in the Southern Hemisphere and only four islands of Japan in the Northern Hemisphere. The taxonomic position of this family remains uncertain owing to its unique morphology and distribution patterns. This study presents the first record of Smeagolidae in Honshu from Wakayama Prefecture, Japan. Over an 11-year period (2014–2025), distribution surveys across 88 sites in the prefecture revealed the presence of Smeagolidae, specifically in Cape Shionomisaki, Kushimoto-cho. Cape Shionomisaki became the fifth locality in the Northern Hemisphere after Narugashima Island in the Hyogo Prefecture, thereby extending the easternmost boundary of its distribution in the Northern Hemisphere. The survey documented a substantial population of Smeagolidae, totalling 582 individuals over a 2-year period, making it the largest population discovered worldwide. Despite the restricted habitat, observations throughout the study period at Cape Shionomisaki suggest that this family may have completed its life cycle in the area. Further investigations using both morphological and molecular analyses are needed to determine whether the Smeagolidae discovered in Wakayama Prefecture belong to the same species found at other sites in Japan.
Shunsuke SAEKI (Thu,) studied this question.