As discussed elsewhere (Chapters 7 and 28), herbivory is an important process on coral reefs. An adequate level of herbivory is necessary for maintenance of the balance between reef-building corals and macroalgae, and hence many of the typical characteristics of a coral reef. Any visit to an intact coral reef will yield countless observations of animals moving across the substrate and cropping the vegetative growth of marine plants. Key herbivores include gastropod molluscs, chitons, echinoids, crabs and fish (Fig. 7.6). These organisms may be active at different times of the day or light level (e.g. dawn, dusk, night) and tidal heights. Many gastropods (e.g. abalone) and chitons set out from their hiding places at night to graze on benthic microalgae while many grazing fish (e.g. Scarids, Siganids and Acanthurids) are active only by day and will wait until the tide floods the reef crest before moving in to graze on these rich and highly productive reef areas. Grazing activity is often evident from scrape marks made by the tough beaks of grazing fish (Fig. 7.6B), or by the presence of feeding scars from grazing molluscs (Fig. 7.6C). Some grazing activity can be very subtle. Recent studies have revealed that porcellanid crabs (see Chapter 23) are significant grazers on coral reefs, cropping the ben-thic microalgae from the substrate using their chelae that are like tiny scissors. In this case, there is little visual evidence of the grazing activities of these organisms despite their importance in terms of the mass of material removed each day from a coral reef.
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