Physical Defenses Sweeper Tentacles
Physical defenses in other anthozoans include sweeper tentacles, sweeper polyps, acontia, and mesenterial filaments, all packed with nematocysts. Nematocysts are generally small and of a singular type in octocorals (Hyman, 1940); some families such as the Xeniidae lack them altogether. Octocorals as a rule, do not develop aggressive structures. However, there have been isolated reports of sweeper-tentacle-like structures in some genera but they have not been investigated further. To date the only octo-coral that has been shown to definitely produce sweeper tentacles is the Caribbean encrusting gorgonian Erytbropodium carihae-orum (Sebens and Miles, 1988). When colonies of this gorgonian encounter other corals, the polyps along the edges begin to transform. All eight tentacles become elongated (up to three times their normal length of 2 cm) and lose the characteristic side pinnules. They become lighter in colour and develop a bulbous tip (acros-phere) on the end; both the tentacle and the acrosphere are packed with stinging cells. These sweeper tentacles are capable of killing stony coral tissue quite rapidly, allowing for the rapid overgrowth of the stony coral colony (Sebens and Miles, 1988). We have seen other gorgonians, Muricea spp. tor example, develop simi 1 a r structures.
As mentioned in the previous section, sclerites afford a certain level of protection for many species of soft corals (see earlier description).
Soft corals, especially encrusting forms, can quickly cover coral reef areas. In some cases toxic chemicals play a role in clearing space for growth, in others sweeper tentacles can allow for the overgrowth of stony coral colonies. Another method is illustrated by the nephthiid coral, Nepbthea brassica. It can overgrow stony coral colonies by laying down a protective polysaccharide layer in areas close to or in contact with stony coral tentacles; and then it simply overgrows the stony coral (LaBarre and Coll, 1986).
Nutrition is a poorly studied area in octocorals in sharp contrast to the work done on zooxanthellae and feeding in stony corals (see volume tr* j v one). As in stony corals, there are genera of octocorals who possess zooxanthellae and genera that do not (see chapter seven).
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