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The ragged sea hare Bursatella leachii Blainville, 1817 is a heterobranch mollusc with a circumtropical distribution, that since the mid-1900s colonized the Mediterranean Sea and is now considered as one of the alien taxa most widespread in the basin. Here, we backdate its confirmed presence in the Gulf of Naples and Italy as a whole based on an overlooked sample preserved in the natural history collections of Stazione Zoologica Anton Dohrn (Naples, Italy). The finding reported in the present paper confirms the importance of natural history museums in documenting biogeographic distribution patterns. It also casts doubts on the often assumed pattern of colonization of the Mediterranean basin (combining a Lessepsian origin and natural secondary spread), as it contributes to disrupt the nearly perfect Lessepsian-like spreading highlighted in recent articles.