Coral reef health is influenced by many factors, including some that are biotic and others that are anthropogenic. This article will focus primarily on marine grazers as factors that both negatively and positively affect reef health.  

Herbivorous fish

Coral reefs rely on many species of herbivorous fishes to keep algae populations in check. Algae often compete with corals for sunlight, nutrients and space, and fish eating these competitive algae are extremely beneficial to maintaining a healthy reef. If left unchecked, some algae and seaweeds can over-grow and kill large patches coral. A recent study by Hay and Burkepile (2008) showed that the presence of herbivorous animals on a reef is not sufficient for maintaining a healthy reef, but that a proper balance between fish species is necessary to keep algal populations from killing corals.

Figure 1: Stoplight parrotfish, Sparisoma viride, are important grazers of algae (photograph: Leo Roest).

This is because different species of fish specialize in eating certain types of algae. For example, many parrotfish species specialize in eating hard, calciferous algae whereas surgeon fishes and tangs often eat softer types of algae that protect themselves by means of chemical defenses. Over the course of their ten-month study, Hay and  Burkepile found that coral areas which had multiple species of herbivorous fish foraging around them thrived compared to less fish species-rich areas. During the study, species rich areas increased in coral cover by more than 20% as compared to areas where only one fish species or no fish were present, which decreased in cover by up to 30% as a result of increased algal biomass.

"While herbivorous reef fishes may prey upon some corals, the beneficial role they play in decreasing algae growth and allowing corals to settle and grow is much more important".

Although the presence of fish seems to benefit corals, a delicate balance exists between these groups. While plant-eating fish allow established coral colonies to flourish without being consumed by resilient and aggressive populations of algae and seaweeds, coral recruitment can be negatively impacted by large numbers of herbivorous fish. Newly-established corals are often subjected to heavy grazing by herbivores and planktivores, both of which can play important roles in controlling coral settlement and ultimate survival. During a study by Mumby and coworkers (Center for Biodiversity and Conservation) in 2007 and 2008, it was shown that removal of large herbivorous fishes showed a positive short-term effect on coral recruitment in pristine reef systems. Another part of the study focused on differences in coral communities in marine reserves compared to their surrounding "fished" waters, and showed that the number of large grazers in the reserves was 15% higher than in surrounding waters resulting in decreased seaweed cover and increased coral recruitment.                                                                                

The important message here is that while herbivorous reef fishes may prey upon some corals, the beneficial role they play in decreasing algae growth and allowing corals to settle and grow is much more important. Without sufficient numbers of these important marine grazers, existing corals would be overgrown by opportunistic algae and coral larvae would have a difficult time finding a clean space without algal overgrowth where they could settle and survive.

Sea urchins

These spiny echinoderms are important beneficial grazers on coral reefs. One of the most commonly studied urchins is Diadema antillarum, the long spine sea urchin (fig.2). It is a generalist algal grazer that eats both macroalgae (e.g. Halimeda, Caulerpa, Gracilaria, etc.) and turf (filamentous) algae, essentially acting as a marine lawnmower. The importance of this urchin was clearly illustrated in Jamaica in the early 80's when a massive die-off of D. antillarum occurred, supposedly as a result of a virulent pathogen originating from the Panama Canal. Without these urchins, algae overgrew many of the coral reefs along Dominica's island shelf, drastically reducing available space for fish to breed. This resulted in a decline in native fish populations, which meant a significant loss of fishing grounds and reef-related tourist activities. Without the long spine urchin to keep algal growth in check, surveys conducted through the Institute for Tropical Marine Ecology in Dominica reported that some study sites had an algae cover of over 65%. The loss of almost 99% of individuals from a single urchin species was enough to decimate these Caribbean coral reefs and to have a large negative impact on the local economy. Prior to the loss of D. antillarum, approximately 60% of the Jamaican reefs were covered with coral; at the year 2000, corals still barely covered 10% of the reefs.

Figure 2: The long-spined sea urchin, Diadema antillarum, fouraging for algae on a coral reef (photograph: Daniel Smith).

The opposite situation is of course a problem as well. When populations of reef fishes that predate sea urchins are reduced due to overfishing, sea urchins multiply rapidly and can eat almost all the algae present on a reef; the total depletion of algae can have detrimental effects on coral reefs. Although excessive amounts of algae can overgrow reefs, a coral reef with no algae would not be able to support species diversity for very long. Algae are an extremely important component of the marine food web, providing nourishment for many different species of herbivorous fish, mollusks and invertebrates, many of which are keystone species in a reef without which a coral reef ecosystem would fall apart.

Once again, the main idea here is that sea urchins can be both beneficial and harmful to a coral reef and that they must be present in optimal densities in order to provide the greatest benefit to a reef.

"The loss of a single urchin species was enough to decimate Caribbean coral reefs and to have a large negative impact on the local economy".

Sea stars

These echinoderms play a minimal role in algal grazing, instead subsisting primarily on molluscs, crustaceans, worms, sponges and sometimes even corals. The most notorious of the corallivorous sea stars is Acanthaster planci, or the Crown-of-Thorns starfish (fig.3). It is a very large nocturnal starfish with toxic spines that can consume up to 6 m² of coral per year per individual.  They are extraoral grazers that evert their stomachs and release digestive enzymes to dissolve their prey prior to ingesting it.

Figure 3: The predatory coral grazer, the Crown-of-Thorns starfish (Acanthaster planci, photograph: Matt Wright).

Population booms of A. planci have devastated large amounts of corals in the Great Barrier Reef and other Indo-Pacific reefs and it is still not known exactly why there are so many of them. Their overabundance is probably a result of surplus nutrients from agricultural runoff causing a massive increase in the planktonic food supply for the pelagic seastar larvae as well as a steep decline in the abundance of their main predator, the Triton (Charonia tritonis), due to excessive overcollecting by humans. The disastrous effects of coral grazing by this ravenous sea star clearly illustrate the importance of maintaining a proper predator balance in a reef ecosystem. 

Crabs

Numerous species of crabs exist on the reef and function to keep all sorts of nuisance algae from overgrowing the delicate corals. The Emerald crab (Mithrax sculptus) will eat a wide variety of algae commonly found on reefs. A study by Coen (1986) showed that when M. sculptus was excluded as a coral head forager for one month, mean algal cover exceeded 75%, indicating this crab species plays an important role in reducing fouling algae in reef ecosystems. Similarly, Red Leg (Clibanarius sp.) and Neon-Blue Striped Hermit crabs (Calcinus elegans) are both often incorporated as part of the "clean-up crew" in reef tanks because they eagerly dispose of unwanted algae.

Figure 4: Hermit crabs are part of the clean-up crew of both coral reefs and home aquaria (photograph: Hans Leijnse).

Molluscs

Molluscs account for a substantial amount of the grazing that goes on in coral reef ecosystems; some common reef-dwelling molluscs include creatures such as snails, chitons, abalone, limpets and sea hares. Snails of the genus Turbo, Trochus and Astrea (fig.5) are extremely popular with aquarists because these gastropods seem to eat even the most unappealing of algae. Although these animals (gastropods in particular) come in a wide variety of shapes and sizes, there is one distinguishing characteristic they have in common: the radula. This appendage looks like a bony tongue with teeth on it and is used to scrape algae off rocks. The radula continues to grow through life, so it never becomes worn down and puts molluscs at the risk of starvation. Because of their soft bodies that are usually protected by shells, they are able to conform to the shapes of their landscape, allowing them to fit their mouths into tiny crevices and holes, making them ideal "small scale" grazers.  

Figure 5: An Astrea snail grazing on an aquarium wall (photograph: Jorick Hameter).

Mesograzers

Mesograzers are defined as invertebrate herbivores that are less than 2.5 cm (1 inch) in length; this includes juveniles of many species. Some of the most well-studied animals in this category are amphipods. Amphipods are small shrimp-like crustaceans that prefer to consume brown algae and are an important food source in marine environments. In the words of Emmett and Hay (2000), " Because of their small size, high abundance, short generation times, and consequently high rates of secondary production, mesograzers, and particularly the crustaceans (amphipods), are major conduits of primary production to higher trophic levels, and are thus critical players in near-shore trophic transfer." Thus, the mesograzers consume the algae, and they are subsequently consumed by larger predators.

"We need to remain aware of the long-term changes that could occur as a result of making poor environmental choices, and do what we can to be responsible stewards of the marine world".

Concluding remarks

The take home message which can be gleaned from this article is that without grazers, coral reefs would not be able to exist. If algae were allowed to grow unchecked, every inch of coral would be covered by some form of brown, red, gold, green and/or blue-green algae. Grazers can be vertebrate or invertebrate, long- or short-lived, large like a terminal stage stoplight parrotfish (Sparisoma viride) or tiny like an amphipod grazing on a film of brown algae, but each species has to exist in the right proportions in order to maintain a pristine coral reef. Every careless interaction that humans have with the ocean could disrupt this delicate balance and have an extremely negative impact on sustained coral reef health, whether it is overfishing of a keystone species causing the slow collapse of a beautiful coral reef ecosystem, or the agricultural runoff which rapidly induces aggressive algal blooms overgrowing the fringing reef of a coastal town. So, we need to remain aware of the long-term changes that could occur as a result of making poor environmental choices, and do what we can to be responsible stewards of the marine world.

Figure 6: A common marine amphipod,  Bathyporeia pelagica (photograph: Hans Hillewaert).

References:

Burkepile, D.E. and M.E. Hay. 2008. Herbivore species richness and feeding complementarity affect community structure and function: the case for Caribbean reefs. Proceedings of the National Academy of Sciences, USA 105:162021-16206.

Coen, L.D. 1988. Herbivory by crabs and the control of algal epibionts on Caribbean host corals. Oecologia. 75(2): 198-203.

Emmett J.D. and  M.E. Hay. 2000. Strong impacts of grazing amphipods on the organization of a benthic community. Ecological Monographs 70(2): 237-263.
Lessios, H. A. 1988. Mass Mortality of Diadema antillarum in the Caribbean: What Have We Learned? Annual Review of Ecology and Systematics. 19: 371-393.

Mumby, P.J., Dahlgren, C.P., Harborne, A.R., Kappel, C.V., Micheli, F., Brumbaugh, D.R., Holmes, K.E., Mendes, J.M., Broad, K., Sanchirico, J.N., Buch, K., Box, S., Stoffle, R.W. and A.B. Gill. 2006. Fishing, trophic cascades, and the process of grazing on coral reefs. Science 311: 98-101.

Mumby, P.J., Harborne, A.R., Williams, J., Kappel, C.V., Brumbaugh, D.R., Micheli, F., Holmes, K.E., Dahlgren, C.P., Paris, C.B and P.G. Blackwell. 2007. Trophic cascade facilitates coral recruitment in a marine reserve. Proceedings of the National Academy of Sciences 104(20): 8362-8367.

Stump, R. 1996. An investigation to describe the population dynamics of Acanthaster planci (L.) around Lizard Island, Cairns section, Great Barrier Reef Marine Park. CRC Reef Research Centre Technical Report (10), CRC research Centre, Townsville, pp 1-56.