Herbivory

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A Parrotfish Grazing on Algae. Photo By: Paul Humann Copyright 1999. Published in Reef Fish Behavior: Florida Caribbean Bahamas. New World Publications  Inc. Verona, Italy.
 

          Herbivory is a key component of any ecological community. Herbivores are the first step in the energy transfer from plants up through the food web. They are also known as primary consumers because while plants and algae make their own food, herbivores get food from an outside source and then consume it, and they are the first ones in the food web to do so.

            In the coral reef community there are many species of fish which fill this ecological role: roughly 25 percent of the fishes are herbivores or make plants a part of their diet (Deloach, 1999).  They play a key role in maintaining the coral reef community. Without the strong grazing pressure that herbivores exude, the competitively superior algae would overgrow the reef (Lewis, 1986). This would smother the corals and allow no place for the larvae of other marine animals to settle and grow.

            The powerful effects of herbivores on the community can be seen when they are removed. In a devastating near eradication of a key herbivore, Diadema antillarum – an urchin, due to disease, the algal community nearly overgrew the reef and caused a major shift in the reef dynamics (Harvell et al, 1999). The fishes alone were not able to keep the algae in check and the loss of the reef was devastating. In three years living coral coverage fell from 45-55 percent to 5-15 percent (Deloach, 1999). Thus, it is obvious that herbivores play a key role in the dynamics of coral reefs.

            Parrotfishes are one of the major groups of herbivores on coral reefs. There are thirteen recorded species in Bermuda alone (Thomas). They use their hard, beak-like mouths (their teeth have actually fused to form a beak, a very useful adaptation) to scrape at short tufts of filamentous algae from dead coral and rock. They generally do this by taking many short bites. They also ingest large amounts of calcium carbonate while grazing on the algae. They grind up the calcareous sediments using their flattened pharyngeal teeth, which is known as the “pharyngeal mill,” and a specialized alimentary tract extracts the food from this mixture (Deloach, 1999). Then the inorganic sediment moves through their stomach-less gut until it is discharged as sand; it has even been estimated that a single large adult parrotfish can produce a ton of sand in a year (Deloach, 1999).

            Due to the abundance of herbivores on the reef, the algae there have evolved special defenses to avert predation. Much of the algae on reefs are calcified. That is, they have evolved to contain calcium carbonate in their cells to make it difficult for herbivores to digest the algae (Schupp and Paul, 1994). A main group that employs this defense is the crustose corallines, which grow right on the substrate in a thin layer making it difficult for herbivores to get a mouthful. However, many herbivores like the parrotfish have adapted to be able to process the calcium carbonate. Even though the parrotfish do consume an abundance of calcium carbonate, it does not seem to have very adverse effects and is simply passed through and results in sand.

Photo: A  Parrotfish grazing on Algae growing in the reef matrix. Photo By: Paul Humann Copyright 1999. Published in Reef Fish Behavior: Florida Caribbean Bahamas. New World Publications  Inc. Verona, Italy.