Chemical defense
[10] Bacteria of the genera Chromobacterium, Janthinobacterium, and Pseudoalteromonas produce a toxic secondary metabolite, violacein, to deter protozoan predation.
Another bacteria, Pseudomonas aeruginosa, aggregates into quorum sensing biofilms which may aid the coordinated release of toxins to protect against predation by protozoans.
[7] In addition, sclerotia of Aspergillus flavus contained a number of previously unknown aflavinines which were much more effective at reducing predation by the fungivorous beetle, Carpophilus hemipterus, than aflatoxins which A. flavus also produced and it has been hypothesized that ergot alkaloids, mycotoxins produced by Claviceps purpurea, may have evolved to discourage herbivory of the host plant.
[17][18] Such defensive chemicals may be stored in various tissue types of the lichen thallus, or they may accumulate on the mycobiont hyphae as extracellular crystals.
[13][15] Documented allelopathic targets include jack pine, white spruce, and garden variety tomato, cabbage, lettuce, and pepper plants.
[15][18] In 2004 the death of hundreds of elk near Rawlins, Wyoming was linked to consumption of tumbleweed shield lichen (Xanthoparmelia chlorochroa).
[19] A wealth of literature exists on the defensive chemistry of secondary metabolites produced by terrestrial plants and their antagonistic effects on pests and pathogens, likely because human society depends upon large-scale agricultural production to sustain global commerce.
[20] These compounds serve a variety of physiological and allelochemical purposes, and provide a sufficient stock for the evolution of defensive chemicals.
[21] Defensive chemicals used to avoid consumption may be broadly characterized as either toxins or substances reducing the digestive capacity of herbivores.
[6] Although the focus has been on broad-scale patterns in terrestrial plants, Paul and Fenical in 1986 demonstrated a variety of secondary metabolites in marine algae which prevented feeding or induced mortality in bacteria, fungi, echinoderms, fishes, and gastropods.
Among the most successful insect orders employing this strategy are beetles (Coleoptera), grasshoppers (Orthoptera), and moths and butterflies (Lepidoptera).
[2] For example, the fruit fly (Rhagoletis basiola) can chemically detect a nearby parasitoid (an organism that acts as both a parasite and a predator) and halt its egg-laying.
[33] Similarly, spider mites are also able to sense damaged body parts of individuals of the same species, or conspecifics, and present the same avoidance behavior as with predator cues.
[34] Furthermore, spider mites exhibit a similar behavior with egg-laying as the fruit fly and will elect to move to areas absent of predator cues before oviposition.
Spider mites will not avoid areas with other, non-predator volatiles meaning these organisms are able to chemically distinguish threats from non-threats.
[38] Aphids, small insects that can be found feeding on the sap of plants, exhibit many strategies in terms of chemical defense.
[39][40] Aphids have structures called cornicles along the posterior side of their abdomen which are used to deliver secretions containing both volatile and nonvolatile compounds.
These smears are used to fatally bind predators' mouthparts, antennas, legs, etc., meaning these compounds are typically used more for physical defense rather than chemical.
[41][40] Pea aphids (Acyrthosiphon pisum) produce a warning chemical called (E)-β-farnesene which is excreted as a volatile compound in the presence of predators or perceived threats.
[2] Additionally, pea aphids are highly attune to which predators are in their area as they can chemically identify what is posing as a threat and adjust their response accordingly.
[57] This storage mechanism is advantageous because the defensive chemicals are located near the surface of the sea slug and are readily available for any mucus secretion.
[51] The second mechanism of defensive chemical storage exhibited by sea slugs is preserving the secondary metabolites in other areas of their body.
The fact that sea slugs can effectively survive and evade predation without the use of the shell highlights the success of storing and modifying secondary metabolites as a defensive mechanism.
[50] This mechanism relies on the invertebrates releasing and sensing chemical cues throughout their aquatic environment and modifying their behavior as a result.
[59] Predators of clams, namely blue shell crabs and whelks, are able to identify their prey by sensing the chemical cues present in the filtered water.
[60] The use of chemical warnings and alarm pheromones is a mechanism used by many marine invertebrates, clams and blue shell crabs are only two examples of this defensive strategy.
[62] In the field, invertebrates such as the Atlantic decorator crab (Libinia dubia) experience significantly less predation when "clothed" in noxious seaweed than their unclothed conspecifics.
There is evidence to suggest that the ability to produce toxins evolved along with aposematic coloration, acting as a visual cue to predators to remember which species are not palatable.
This finding gives insight to the roles of proteins, the nervous system, and the mechanics of chemical defense, all of which promote future biomedical research and innovation.
Some mammals can emit foul smelling liquids from anal glands, such as the pangolin[66] and some members of families Mephitidae and Mustelidae including skunks, weasels, and polecats.
