The mantle is bright red in colour.The body is oval-shaped.Thordisa nieseni was found on red sponges. These sea slug is oval-shaped. Rhinophores are lamellate with 12 to 13 lamellae.The rhinophores and gill are red with white at the tips. The mantle is covered by two distinct sizes of papillae (larger and short).The oral tentacles are short.
Sea slugs genus Thordisa feeds on sponges.Unfortunatelly, we have no Information about the diet of Thordisa nieseni.