Marine sponges (Porifera) represent a preferential food source for fish, worms, and marine snails. Therefore, sponges have evolved a variety of constitutive defense compounds (Figure 4). Some of these are closely related to defense compounds from plants. For instance, several sponge-derived terpenes which proved toxic against barnacles are characterized by the presence of isocyanide, isocyanate, and thioisocyanate functions. Methylisocyanate, methylisothio-cyanate, and dimethylsulfide were detected in their volatile spectrum. However, some isocyanates also serve specialized predators for their defense. The nudibranch Phyllidia variscosa takes up the isocyano-sesquiterpene 9-isocyanoneopupukeanane 17 that serves as an antifee-dant against fish from its prey Hymeniacidon sp. sponges. Likewise, ichthyotoxic sesquiterpenes were isolated from the sponge Cateriospongia foliascens and the sponge Spongia idia. Tricyclic terpenes, such as suvanine 18, and tetra-cyclic terpenes, such as the bishomomethylscalarane derivative 19 containing a 1,4-dicarbonyl structure, proved to be toxic against fish. 1,4-Dicarbonyl compounds react with basic amino groups, for instance, those of lysyl residues to produce pyrrol derivatives and consequently destroy the function of proteins.
Similar to plants, sponges use alkaloids for defense. The sponge Petrosia seriata produces a group of bis-qui-nolizidone alkaloids which are ichthyotoxic. The same sponge generates an unusual 16-membered alkaloid
Anatoxin a 14
Figure 2 Antifouling agent of sea grass.
Anatoxin a 14
HN N Microcystin LR
Cryptophycin 1 16
Figure 3 Examples for toxins of cyanobacteria.
O O CHOO
Figure 4 Chemical defense of sponges.
named petrosine 20, to defend against its predator, the fish Lebistes reticulates.
Compounds with brominated aromatic and heterocyclic structure elements are representatives of a large group of ichthyotoxins produced by sponges. Brominated dimeric phenol ethers such as 21 from Disidea sp. have been shown to be produced by bacterial symbionts. The antifeedant properties of brominated pyrrol alkaloids, for example, oroidin 22 from the marine Agelas sponges, can be explained by their effect on the cellular calcium homeostasis.
Heterocyclic bromine derivatives exhibit not only antifeedant properties against fish but also prevent in form of bromopyrrol derivatives the settlement of tuni-cates and thus defend successfully the living space of sponges. Other antifouling compounds against barnacle larvae such as Balanus improvisus are brominated cyclo-peptides, for example, barettin 23 from the marine sponge Geodia barretti.
Surface-active saponins such as formoside 24 from Erylus formosus are ichthyotoxic and thus deter reef fish from feeding (see Plant Defense Strategies). Similarly, ionophores also exhibit strong antifeedant properties. For example, the sponge-derived polypyridinium alkaloid amphitoxin 25 disintegrates the cell membrane structure followed by release of degrading enzymes into the predator organism.
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