Packaging and Delivery of Chemical Weapons: A Defensive Trojan Horse Stratagem in Chromodorid Nudibranchs

Abstract

BackgroundStorage of secondary metabolites with a putative defensive role occurs in the so-called mantle dermal formations (MDFs) that are located in the more exposed parts of the body of most and very likely all members of an entire family of marine mollusks, the chromodorid nudibranchs (Gastropoda: Opisthobranchia). Given that these structures usually lack a duct system, the mechanism for exudation of their contents remains unclear, as does their adaptive significance. One possible explanation could be that they are adapted so as to be preferentially attacked by predators. The nudibranchs might offer packages containing highly repugnant chemicals along with parts of their bodies to the predators, as a defensive variant of the strategic theme of the Trojan horse.Methodology and Principal FindingsWe detected, by quantitative 1H-NMR, extremely high local concentrations of secondary metabolites in the MDFs of six species belonging to five chromodorid genera. The compounds were purified by chromatographic methods and subsequently evaluated for their feeding deterrent properties, obtaining dose-response curves. We found that only distasteful compounds are accumulated in the reservoirs at concentrations that far exceed the values corresponding to maximum deterrent activity in the feeding assays. Other basic evidence, both field and experimental, has been acquired to elucidate the kind of damage that the predators can produce on both the nudibranchs' mantles and the MDFs.SignificanceAs a result of a long evolutionary process that has progressively led to the accumulation of defensive chemical weapons in localized anatomical structures, the extant chromodorid nudibranchs remain in place when molested, retracting respiratory and chemosensory organs, but offering readily accessible parts of their body to predators. When these parts are masticated or wounded by predators, breakage of the MDFs results in the release of distasteful compounds at extremely high concentration in a way that maximizes their repugnant impact.

Volatile secondary metabolites as aposematic olfactory signals and defensive weapons in aquatic environments

“…2D). The feeding deterrence activity of compounds 1-3 was evaluated against the cooccurring generalist shrimp Palaemon elegans Rathke, 1837, following an approach already used to assess the palatability of other bioactive metabolites (31), including marine furanosesquiterpenes (32,33). Like other palaemonid species (34,35), P. elegans is suitable for evaluations of this kind, which require a good view of the ingested food in the digestive system (31).…”

Chemoecological studies on marine natural products: terpene chemistry from marine mollusks

“…1) as well as four known diterpenes [10]. Terpenes 1 and 2, both of which we found to show bioactivity against P388 mouse leukemia cells, were subsequently isolated from the Chinese nudibranch Chromodoris sinensis (now known as Goniobranchus sinensis) and have been implicated in feeding deterrency [11]. Chemical evaluation of two large specimens of C. reticulata provided six new oxygenated diterpenes (3 -8) together with seventeen known diterpenes.…”

“…Given that field experiments prevent assessing the state of conservation and the purity of the compounds immediately prior to the assays in remote locations, the choice of the model predator fell on easy available shrimps with broad adaptability, allowing their survival for long time in a small volume of seawater in the chemical laboratory. P. elegans is commonly found throughout the Mediterranean region, but some assays conducted with it have been carried out on compounds isolated from nudibranchs collected in other parts of the world [11]. Even though the use of a non-local model predator can have the advantage of excluding confounding effects due to avoidance-learning [11], the general ecological validity of the assay is not yet well established, given that different consumers can vary in their response to particular extracts or purified metabolites.…”

“…P. elegans is commonly found throughout the Mediterranean region, but some assays conducted with it have been carried out on compounds isolated from nudibranchs collected in other parts of the world [11]. Even though the use of a non-local model predator can have the advantage of excluding confounding effects due to avoidance-learning [11], the general ecological validity of the assay is not yet well established, given that different consumers can vary in their response to particular extracts or purified metabolites. The availability of the model organisms near to the chemical laboratory where the bioassays have to be carried out is desirable.…”

“…#628, and was based on the experimental method published by Mollo et al [11,29]. A crude alkaloid fraction (CAF) prepared from sponge tissue using an acid-base extraction protocol, and containing a mixture of 3-alkylpiperidine alkaloids containing haliclonacyclamine A (34) as the major component, was dissolved in acetone (0.5 mL), and combined with freeze dried squid (50 mg), alginic acid (30 mg) and purified sea sand (30 mg).…”