The Sea Slug Tethys fimbria Linnaeus, 1767 (Nudibranchia: Tethydidae) Expands its Distribution Northwards to the Sea of Marmara

This study reports our observation of the large sea slug Tethys fimbria Linnaeus, 1767 in the north-ern-eastern Sea of Marmara (the Princes’ Islands). We observed T. fimbria on the detrital bottom of the sea at a depth of 35 m in June 2022. This species had previously been reported during spawning in a recent study conducted in the southern part of the Çanakkale Strait (the Dardanelles Strait that connects the Marmara Sea with the Aegean Sea). Prior to 2022 there had been no reports of this conspicuous species from the Marmara Sea in the scientific literature or in photo-records among divers. Following our observation of the species in the sea around the Princes’ Islands, underwater photographers reported pictures of T. fimbria from different locations in the Marmara Sea on social media platforms. The sea slug seems to extend its distribution northwards. The potentially new arrival of the large sea slug in the Sea of Marmara, rather than a distribution shift from deox-ygenated deep Marmara basins or a climate-related northwards expansion, is discussed.