An icon species of Sacoglossan sea slug, Cyerce nigricans. Thus is a species of sacoglossan sea slug, a shell-less marine opisthobranch gastropod mollusk in the family Caliphyllidae. The Cyerce slugs are commonly referred to as butterfly slugs or butterfly nudibranchs. Island Xiaoliuqiu, Little Liuqiu (Chinese: 小琉球), located southwest in Taiwan. Taiwan's only large coral island.