Seasonal Occurrence of Gastropterids (Gastropoda: Cephalaspidea) and Their Habitat Selection in a Subtropical Back-reef on Okinawajima Island (Ryukyu Archipelago, Japan).

Daisuke Tanamura and Euichi Hirose (2017) Gastropterids are generally small sea slugs with vivid colors. In a shallow back-reef at Zanpa (Okinawajima Island, Ryukyu Archipelago), ve gastropterid species were observed during route censuses (0-2.6 m in depth, 400 m in length) conducted from November 2011 to June 2014 (89 times, 32 months): Sagaminopteron ornatum, Siphopteron brunneomarginatum, Si. citrinum, Si. flavum, and Si. tigrinum. Among them, Si. tigrinum was observed only in 2013, but the other four species were observed every spring during the survey. When the route was divided into four zones based on depth and dominant substrata, each species was mainly found in particular zones. In laboratory experiments for substrate selection, Si. flavum significantly preferred natural sand and rubble to flat glass-bottoms, glass beads, dried sand, or bleached rubble, suggesting that substrate material is a potential key for habitat selection in this species. Although a larger number of individuals preferred natural sand from their principle habitat to natural sand from a deeper zone where this species was rarely observed, no significant difference was found in the preference by binomial test.