Elucidation of a New Snyderane‐Type Sesquiterpene from Okinawan Sea Hare <i>Aplysia argus</i> and Identification of Their Feeding Targets

A new snyderane-type sesquiterpene, argusene (1), and 10 known terpenes, 3,3-dimethyl-5-methylene-4-(3-methylpenta-2,4-dien-1-yl)cyclohex-1-ene (2), luzonensol (3), palisadin B (4), 12-hydroxypalisadin B (5), 12-acetoxypalisadin B (6), palisadin A (7), aplysistatin (8), pacifigorgiol (9), debromolaurinterol (10), and 3-bromobarekoxide (11), were isolated from the sea hare Aplysia argus collected from Ikei Island in Okinawa Prefecture, Japan. Their structures were established using spectroscopic methods such as infrared, nuclear magnetic resonance, and high-resolution-electrospray ionization-mass spectrometry. Since all these compounds were derived from a particular red alga, it was determined that this sea hare targets this red alga for feeding, thereby confirming a specific dietary relationship. The cytotoxicity of 1-11 against the human hepatoma cell line HepG2 was evaluated, and 1, 4, 5, 6, 7, 8, 10, and 11 decreased cell viability in a concentration-dependent manner. In addition, intracellular lipid accumulation induced by free fatty acid treatment in serum-free medium was significantly suppressed in the presence of 8 (5.0 µg/mL).