Associational chemical defense

Interestingly, the release of PUFAs without subsequent action of a lipoxygenase can act as a wound-activated defense diatom in rich fresh water biofilms. This reaction could be directly associated with a chemical defense against the grazer Thamnocephalus platyurus [78]. 

While secondary metabolites of plants and animals have been the subject of many chemical investigations, their associations and roles in their host organism are at times controversial this is particularly so, when insufficient observations exist. Nevertheless, natural products provide fruitful areas of research [69]. There is little doubt that chemical defense against predators is an important aspect of survival. In the marine environment, early observations of nudi-branch- sponge relationships were reported and those relating to isocyano compounds are summarized in Table 6. 

These associational defenses are examples of a mechanism that is to date underexplored in tropical regions and that probably represents one of the most important cascading effects of chemically defended macroalgae on community structure. Thus, chemically defended macroalgae can act as local biodiversity centers, by increasing the coexistence of species under their chemical defense umbrellas. 

Lima LMS, Alor R, Uriostegui R, Murray SN, Pereira RC. Within-plant variation in palatability and chemical defenses in the green seaweed Avrainvillea elliottii. Bot Mar (submitted for publication) Littler MM, Taylor PR, Littler DS (1986) Plant defense associations in the marine environment. Coral Reefs 5 63-71... 

One may wonder why chemical defenses differ from plant to plant, or why carbon dioxide attracts certain creatures but not others. How does a special chemical and the interactions associated... 

Hay, M.E. (1992) The role of seaweed chemical defenses in the evolution of feeding specialization and in the mediation of complex interactions. In Paul, V.J., ed., Ecok ical Roles of Marine Natural Products. Comstock Publishing Associates, Ithaca and London, pp. 93-118. 

Numerous catechols and hydroquinones in both glycoside-masked and -unmasked forms are useful metabolites in plant chemical defense. Many such metabolites are present in concentrations that can prove detrimental due to oxygenation of the tissue accompanying wounding of the plant in the infection process or in other direct physical injury. Some agents are also synthesized subsequent to enzyme induction in association with infection to mediate chemical defense, as in the broad class of defensive substances known as phytoalexins.12 Some of these induced substances are oxidizable polyphenols, while others are not (Figure 8.1). 

Soft corals have been shown to release terpene chemicals into the water column.210 Their interactions with other invertebrates,211 or with algae122-212-213 have been monitored. Some algal species cause localized tissue necrosis, or reduce terpene levels, in adjacent soft corals 122 213 however, stressing soft corals by transplanting them to new sites stimulates terpene synthesis.199 Some species of algae, notably the chemically protected Halimeda spp., grow in abundance at the base of soft coral colonies, but the soft coral chemical defense does not play a role in maintaining this association.214... 

The relationship described above might lead one to suspect that the reason that the hyperiid amphipod Plyperiella dilatata abducts and carries a chemically defended sea butterfly (Clione antarctica) is that it lacks the ability to synthesize or produce its own chemical defenses. Indeed, Hay et al.39 have also shown that amphipods may associate with chemically defended algae to provide defense against fish predators, rather than produce defensive compounds themselves. 

However, recently there has been evidence that the production of chemical defenses by benthic marine amphipods is indeed possible, as Norton and Stallings155 report on the distribution and abundance of three aposematic, chemically defended gammarid amphipods in the northwest Pacific Ocean. There is no reason to suspect that planktonic amphipods cannot similarly evolve their own chemical defenses. Hyperiid amphipods, which are known to associate extensively with gelatinous zooplankton, are perhaps uniquely situated to exploit the chemical defenses of other planktonic organisms. 

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