Thuridilla gracilis (Risbec, 1928) [Elysia]

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Thuridilla gracilis is characterized by a dark, almost black, background, fine white longitudinal lines and the foot, head and rhinophores edged with bright orange. In some specimens the parapodia can also be bordered with bright orange and in some specimens elongate white or bright blue spots are present along the outside of the parapodia just down from the edge. Thuridilla bayeri is clearly a synonym and so is Thuridilla ratna.
More information on Thuridilla gracilis are on Bill Rudman's Sea-Slug Forum!