Defense chemicals, examples

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. 

An illuminating example of a community consequence of plant defensive metabolites, and of a grazer s adaptation to exploit its host plant, is the ability of some herbivores to sequester plant-derived secondary metabolites for their own defenses (e.g., Avila 1995 Marin and Ros 2004). In terrestrial systems, several specialist insects can sequester defensive chemicals from their host plants (e.g., Nishida 2002), but in aquatic plant-herbivore systems with a prevalence of generalist herbivores this is less common. However, sea hares (Anaspidea) and some other herbivorous opisthobranch molluscs are known to be able to sequester defenses from their algal foods (Avila 1995 Marin and Ros 2004). For the opisthobranch species that are known to obtain metabolites from their food, algae were the... 

The problem is much more difficult when the defensive compounds are distributed throughout the insect body and no clues are available as to which type of compounds are present. In this case, a H NMR spectrum on the insect total extract will usually not be helpful, and a reliable bioassay is needed to follow the biological activity through the fractionation process. Repellency bioassays using ants [9] or spiders [10] have been successfully used for this purpose. Chemotaxonomy can also be very helpful, as taxonomically related insects tend to produce the same kind of defensive chemicals. Thus, once the latter have been identified for a few species, the study of other species belonging to the same group is usually much simplified. A good example is provided by coc-... 

Other physiological effects of defensive chemicals have not been observed or studied for plant-herbivore interactions. For example, studies of chemically defended sessile invertebrates have shown that some compounds have emetic properties against fish predators, and that fishes can learn quickly to avoid these chemicals after regurgitating ingested food.206,258 Macroalgal compounds could potentially act in similar ways, but such effects have never been documented. 

Bathydoris hodgsoni can be found in the deep waters of the Weddell Sea. This nudibranch elaborates the drimane sesquiterpene hodgsonal (Structure 7.78) and sequesters it in its mantle tissues.50 164The nudibranch Tritoniella belli collected from McMurdo Sound is the only documented example of an Antarctic mollusc sequestering defensive chemistry from its diet.44 Among the glyceride esters (Structures 7.79-7.81) isolated from this species, chimyl alcohol (Structure 7.79) can also be found in one of its prey items, the stoloniferan coral Clavularia frankliniana. Sequestration of these defensive chemicals is opportunistic, however, and other, still undescribed metabolites are more commonly associated with defense in this mollusc.49... 

On the other hand, microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. In addition, many herbivores have potent mechanisms to detoxify xenobiotics, which allows the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids obviously function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. 

There are many examples which unequivocally support the theory that most of the defense chemicals of nudibranchs have had their origin in their dietary sources. In one study (63) the dorid imdihiBindn Chromodoris lachii was found to contain latrunculin A [7 4] anddandrolasin [75] in a ratio that supported the dietary origin of these metabolites even though the sponge on which these nudibranch feeds was not present at that location. Both latrunculin A and dandrolasin have previously... 

Mollusks commonly found in the intertidal zone include bivalves and gastropods. Bivalves are soft-bodied animals that are protected by two strong shells. Their bodies are covered by a delicate tissue, the mande, which secretes the shell and defensive chemicals. Most are filter feeders that trap small bits of food in the mucus on their gills. Clams, mussels, and oysters are examples of coastal bivalves. 

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