As soft-bodied organisms, marine mollusks need to rely on chemical defense (Figure 7). Often marine snails were found to contain tetrodotoxin 34 and saxitoxin 4, which are most likely taken up from food such as algae or marine sponges, accumulated and often secreted together with mucus onto the skin. In a similar way, the sea slug Elysia subornata uses caulerpenyne 5 for that it acquires from feeding on Caulerpa its defense. Also, the sea hare Aplysia depilans is able to feed on Caulerpa but this snail is defended with fatty acid-derived macrolactones such as aplyolide D 36. Sea hares (Aplysia) react to dangers with the release of ink (aplysioviolin 35) that can contain deterrents, for example, H2O2.
The variation of toxins that can be isolated from Aplysia snails suggests that the snails tend to select the
most potent antifeedants from their food and store them for their own defense.
Similarly, octopuses contain ink sacs to blur the sight of attackers but can be also highly toxic such as the blue-ringed octopus (Hapalochlaena lunulata) that uses tetro-doxin 34 for both hunting and defense.
Many starfish are not only able to recover arms but many are also chemically defended using surface-active saponins such as thornasteroside 37 from Acanthasterplanci similar to terrestrial plants.
Similarly, the nudibranches Hypselodoris godeffroyana and Chromodoris maridadilus take up the sesquiterpenoid nakafurans, for example, nakafuran-9 38, from their prey, the marine sponge Dysidea fragilis, which proved to be ichthyotoxic. Nevertheless, some toxins are synthesized by the snails themselves allowing independence from food sources. The snail Dendrodoris grandiflora produces polygodial 39, which again is a highly reactive a,^-unsa-turated dialdehyde that is both deterrent and highly toxic.
Similar to many other marine organisms, ascidians (Eudistoma sp.) make use of relatively simple brominated aromatic compounds such as moroka'iamine 40 and eudistomine G 42 that are highly effective to deter barnacle larvae and thus prevent antifouling.
Cyclic peptides such as didemnin B 43 are likely to be the active principle of the Caribbean tunicate (Trididemnum solidum) against feeding of fish. In bioassays, didemnin B 43 proved to deter feeding of the wrasse Thalassoma bifasciatum.
Jellyfish poison their enemies with proteins that lead to membrane disintegration and destroy cellular ion potentials. Sea anemones make use of protein toxins that also interfere with the upkeep of membrane potentials. Interestingly, the toxins specifically act on marine herbivores such as crabs (LD50 0.002 mgkg-1).
OH HO H Tetrodotoxin 34
Aplyolide D 36
Aplyolide D 36
Eudistomine G 42
H33C o oh H
H33C o oh H
HC O NH
Didemnin B 43
Figure 7 (Continued)
The marine anemones Palythoa are protected by paly-toxin 44, a highly complex polyketide that interacts with the Na+,K+-ATPase causing membrane depolarization and cell death. With an LD50 of <100ngkg~ (mouse), it belongs to the most toxic compounds known. Palytoxin is also found as a defensive compound in dinoflagellates, corals, and some fish. Most likely, palytoxin is not produced by Palythoa itself but is an associated symbiont.
Marine worms, such as Lumbriconeris heteropoda, produce the structurally unique neurotoxin nereistoxin 41 that interferes with the acetylcholine receptor. The compound is consequently able to serve the worm for defense and was used as a lead structure to develop the valuable insecticide cartap.
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