Calcified Seaweeds on Coral Reefs: Complex Defenses, Trophic Relationships, and Value as Habitats

Calcified green seaweeds can become abundant on reefs ; because they combine chemical and mineral ( = CaCO,) defenses with nocturnal growth that allows nutritionally valuable new tissues to be produced during the night when day-active fishes are not feeding. For herbivores with acid-mediated digestion, CaCO, alone may strongly deter feeding by buffering gut pH even when it has no effect on algal toughness. CaCO, and secondary metabolites sometimes act synergistically and strongly deter. herbivores that are not deterred by either trait in isolation. Because these plants experience reduced disturbance from large generalist consumers, they function as safe habitats for small animals that avoid consumers by association with these well-defended seaweeds. A limited number of small herbivores have specialized on these noxious plants, often morphologically mimicking the plant, and sequestering the plants' chemical defenses for their own protection. A large number of other small animals (brittlestars, polychaetes, demersal zooplankton) that do not feed directly on the plant's tissues use calcified seaweeds as refuge and nursery habitats where they escape consumers. Defensive characteristics of calcified red seaweeds are less thoroughly studied, but chemical defenses appear to be less common than in the calcified green seaweeds. However, calcified reds also provide important refuge habitats for small animals some of which serve as both herbivores and mutualists that prevent the host seaweed from being overgrown by competitors. The defenses of calcified seaweeds, thus, have important indirect effects that enhance the diversity of associated reef species by producing spatial refuges from predation.