A postscript antipredator chemical defenses in animals

animals also defend themselves

It should not be imagined that antipred-ator chemical defenses are restricted to plants. A variety of constitutive animal chemical defenses were described in Chapter 3 (see Section 3.7.4), including plant defensive chemicals sequestered by herbivores from their food plants (see Section 3.7.4). Chemical defenses may be particularly important in modular animals, such as sponges, which lack the ability to escape from their predators. Despite their high nutritional value and lack of physical defenses, most marine sponges appear to be little affected by predators (Kubanek et al., 2002). In recent years, several triterpene glycosides have been extracted from sponges, including from Ectyoplasia ferox in the Caribbean. In a field study, crude extracts of refined triterpene glycosides from this sponge were presented in artificial food substrates to natural assemblages of reef fishes in the Bahamas. Strong antipredatory affects were detected when compared to control substrates (Figure 9.7). It is of interest that the triterpene glycosides also adversely affected competitors of the sponge, including 'fouling' organisms that overgrow them (bacteria, invertebrates and algae) and other sponges (an example of allelopathy - see Section 8.3.2). All these enemies were apparently deterred by surface contact with the chemicals rather than by water-borne effects (Kubanek et al., 2002).

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Figure 9.7 Results of field assays assessing antipredatory effects of compounds from the sponge Ectyoplasia ferox against natural assemblages of reef fish in the Bahamas. Means (+ SE) are shown for percentages of artificial food substrates eaten in controls (containing no sponge extracts) in comparison with: (a) substrates containing a crude sponge extract (t-test, P = 0.036) and (b) substrates containing triterpene glycosides from the sponge (P = 0.011). (After Kubanek et al., 2002.)

predation may occur at a demographically unimportant stage

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