Behavioral and Exumbrellar Integumental Defenses in Jellyfish: The Hydromedusan Spirocodon saltatrix Releases Oxidized Derivatives of Unsaturated Fatty Acids Potentially to Avoid Predation and Bacterial Biofouling.

Cnidarian jellyfish-the medusa phase of medusozoans-are gaining recognition as important prey for a variety of predators in marine trophic webs. However, their traits as prey remain poorly understood. In this study, we examined the defensive traits of jellyfish, focusing specifically on cnidocyte-independent chemical defenses that serve as strategies against consumers, pathogens, and biofouling. To investigate these anti-consumer chemical defenses, we selected three sympatric jellyfish species: the hydromedusan Spirocodon saltatrix and the scyphomedusan moon jellyfish Aurelia coerulea as prey, and the scyphomedusan Japanese sea nettle Chrysaora pacifica as the predator. Using these species, we conducted feeding experiments, ablation of potential defensive organs, and analyses of compounds released by the prey. We found that: (1) in feeding experiments, S. saltatrix individuals exhibited an involution behavior, in which they strongly contracted and retracted their tentacles inside the umbrella when captured by C. pacifica, thereby escaping, whereas A. coerulea individuals were consumed without escaping; (2) a series of surgical ablation experiments indicated that the umbrella integument of S. saltatrix possesses a defensive function; (3) gas chromatography-mass spectrometry (GC-MS) analyses showed that S. saltatrix releases C8 to C9 alcohols and aldehydes when its integument is damaged; (4) behavioral experiments demonstrated that these compounds deter feeding by the predatory jellyfish; (5) these compounds inhibited biofilm formation by the sympatric bacterium Ectopseudomonas sp. isolated from sediment, indicating their antifouling role, though their effects on planktonic biofouling remain to be tested; and (6) the experimentally determined effective concentrations were higher than the bulk measured concentrations. Therefore, we hypothesize that the natural microscale concentrations of these compounds at the injured integument are sufficiently high to match the effective thresholds, suggesting that they function as defensive agents. Thus, our study demonstrates that S. saltatrix utilizes involution behavior and its integument to avoid predation. To our knowledge, this is the first report of exumbrellar integumental defense in a cnidarian jellyfish with suggestive evidence of chemical defense using oxidized derivatives of unsaturated fatty acids.