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Very similar are:
Trapania cf. aurata, which lacks the orange on the gills, rhinophores and at the posterior end of the foot,
Trapania vitta, which lacks the orange longitudinal band at the dorso-lateral processes.
Trapania cf. vitta, which has pur white dorso-lateral processes, and lacks the orange middorsal line at the posterior end of the foot, and
Trapania gibbera, which differs by a dark brown bar between the oral tentacles.
Trapania sp. 08, which differs by brown/black tipped rhinophores, and by translucent white gills, and lacks orange middorsal line at the posterior end of the foot,
Trapania armilla, which differs by a tuberculate body, and by a thin dark brown ring on each oral tentacle.
More information on Trapania aurata are on Bill Rudman's Sea-Slug Forum!