Cratenin, a Rare Oxylipin Marking Kleptopredation in Aeolid Nudibranchs

: Marine mollusks of the order Nudibranchia produce a wide array of secondary metabolites that play key roles in predator-prey interactions and often exhibit remarkable bioactivity. In this study, a chemical investigation of the Mediterranean aeolid nudibranchs Cratena peregrina and Paraflabellina ischitana led to the isolation and characterization of a novel oxylipin, designated cratenin (1). This unique metabolite shows an unusual alkylated monosubstituted tetrahydrofuran (THF) moiety, a structural feature rarely encountered among marine natural products. The planar structure of 1 was fully elucidated by high-resolution mass spectrometry (HRMS) and comprehensive 1D and 2D NMR spectroscopy, while the absolute configuration of the substituted THF ring was rigorously established by chemical degradation, derivatization, and NMR-based stereochemical comparison with known diasteromeric derivatives of (tetrahydrofuran-2-yl)methanol. The co-occurrence of cratenin (1) in the hydrozoan Eudendrium racemosum, a known prey of C. peregrina and other aeolid nudibranchs, strongly suggests its role as a semiochemical which mediates predator-prey interactions. The proposed biosynthetic origin of cratenin (1) from algal docosahexaenoic acid (DHA) further corroborates the hypothesis of dietary acquisition and provides compelling molecular evidence for kleptopredation, a sophisticated foraging behavior where nudibranchs consume prey that has recently ingested phytoplankton. In this view, this study reveals a clear metabolic and ecological link connecting phytoplankton, hydrozoans, and nudibranchs, underscoring the pivotal role of lipid-derived natural products in shaping chemical communication and interphyletic trophic interactions involving opisthobranchs.