The natural products which marine and aquatic organisms use for their defense are often structurally unique and of extreme toxicity. In contrast to most terrestrial organisms - apart from some higher fungi (see Fungal Defense Strategies) and some microorganisms (see Marine and Aquatic Defense Strategies) - many marine organisms use halogenated, in particular brominated, natural products for their defense. Clearly this strategy evolved due to the relatively high amount of free halogen ions in seawater. It is striking that in comparison to most terrestrial organisms (see Animal Defense Strategies) marine and aquatic organisms use structurally highly complex secondary metabolites often derived from the polyketide pathway. Since similar compounds are only known to be produced by microorganisms, microbial symbionts are often suspected as the producers of marine secondary products. However, the marine organisms that make use of such toxins still need to have evolved resistance mechanisms.

To experimentally prove this symbiont hypothesis has been very difficult because often many marine microbial symbionts cannot be cultivated in isolation from their host or environment, indicating that the symbiosis often is intense and likely to be ancient (see Marine and Aquatic Defense Strategies). Besides these differences to terrestrial organisms, many general aspects of aquatic defense strategies parallel principles found for terrestrial organisms, such as the function of oxylipins as defense compounds and chemical signals, the use of reactive dia-ldehydes or a,/3-unsaturated aldehydic compounds as Michael acceptors, the use of neurotoxins, and the storage of inactive pretoxins that are activated immediately upon wounding (see Animal Defense Strategies, Plant Defense Strategies, and Fungal Defense Strategies).

See also: Animal Defense Strategies; Fungal Defense Strategies; Marine and Aquatic Defense Strategies; Plant Defense Strategies.

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