Location: Coral reefs around the world

Habitat Use in Coral Reef Fishes

What creatures do you envision when you think about peacock flounders, lizardfish, trumpetfish, porcupinefish, puffers, scorpionfish, or goatfish? Do names like these conjure up images of alien-like life forms? Interestingly enough, the probable images in your head may not be far from the truth. What about cavernous corals, bulbous corals, or sea whips, sea fans, and gorgonians? These names sound like descriptions of scenery on a science fiction planet or a landscape you would see only in a George Lucas film.

Some researchers describe coral reefs like Swiss cheese, which provides a fairly accurate depiction of the structural complexity of coral reef habitats. Naturally, with complex habitats, one would envision complex assemblages of creatures to exist within them, comprised of peculiar animals similar to those listed above. Coral reefs are no exception to the logical interpretation of complex habitats supporting complex communities. Coral reefs are commonly referred to as the "rain forests" of the sea, simply because they support extremely high levels of biodiversity relative to other parts of the oceans. In reality, coral reefs are far more diverse at higher levels of taxa than is any other ecosystem in the world. This case study provides a brief survey of the interesting fishes, adaptive habitat use, and foraging strategies that have evolved within the complex habitats of coral reefs (see Tables 1-3).

Home Range Size and Habitat Use

Just as there are many different fishes on coral reefs, ranging in size from the smallest fishes of just a few centimeters (blennies and gobies), all the way to large fishes of 1 meter or more in length (groupers and snappers), their requirements for home range size and habitat use are just as varied. A home range can be defined as the area in which an animal performs its typical activity cycles (i.e., the total area required for foraging, sleeping, or shelter-seeking from predation). There is a general trend in many reef species to form long-term attachments to a particular site. This result is not surprising, as familiarity with a section of the reef offers many advantages, such as intimate knowledge of feeding, sleeping, and shelter sites. There are extremely specialized fishes that have greatly restricted home ranges and spend their active as well as inactive periods in actual contact with the reef. As an extreme example of a restricted home range, one species of blenny, Acanthemblemaria spinosa, typically inhabits abandoned calcareous worm tubes and rarely ever leaves its hole! The smallest gobies and blennies appear to be able to obtain sufficient food within a few centimeters of their home shelter sites. Similarly, Indo-Pacific clown fishes (family Pomacentidrae, genus Amphiprion), which are also small in size, have intimate relationships with anemones and benefit from food trapped in the tentacles of their host anemone. Many of these clown fish actually live within the tentacles of the anemone and do not get stung. The general tendency for reef fishes is for a closer association with the substrate to be accompanied by reduction in body size. This means that smaller fishes spend more of their time close to or in contact with the bottom. As animal size increases, the need for larger home range area also increases. The larger the fish, the more likely it is to roam over larger areas while foraging. There are several reported home range sizes in the literature: blennies that are 60-70 mm in length have home ranges of 0.5m²; whereas parrotfish that are 280-300 mm in length have home ranges of 500m².

Due to the high biodiversity and abundances of fishes on coral reefs, one might question how they all coexist without extreme competition for food and space, or high rates of predation. Several mechanisms are operating in these systems to allow persistence of high biodiversity and high numbers of individual species. Some of these mechanisms include: 1) extensive overlap in the home ranges of these animals; 2) a continuous influx of new recruits from the ocean (most reef fish have pelagic larvae that are dispersed to new reefs after existing in the ocean for a period of time; thus, in one generation, larvae may colonize areas that have no adults of their own species, which may increase local diversity through new colonists); and 3) strategies that create intriguing use of reef space, or prevent all individuals from actively using the reef at the same point in time. Firstly, with respect to home range overlap, past and present research focuses on the mechanisms that affect space partitioning on the reef and competitive interactions among reef residents. Secondly, supply-side ecology, or recruitment research, is also another extremely active area of research for coral reef ecologists. Thirdly, interesting strategies that have evolved in this high diversity system to facilitate continued coexistence will be discussed in the following section.

Unique Adaptations of Reef Fishes to Use their Surroundings to their Benefit

Exploiting the Patchiness of Coral Reef Environments

Coral reefs are extremely patchy with respect to habitat composition. In other words, they have high habitat heterogeneity. Coral does not grow uniformly on the ocean floor, and there may be a high concentration of several species in one area, and open sand in another area. Reef inhabitants respond appropriately by patterning their movements with respect to structural relief. The main structure of coral reefs is comprised of numerous species of corals that grow upward on top of previous layers of calcium carbonate coral skeletons. Each coral species has a unique shape, and even within a species, this shape can be altered by environmental conditions (e.g., water current speed and direction). Over thousands of years, these environments have inevitably become more complex as layer upon layer is created. On top of the reef surface, other biotic organisms add to the complexity, including such unique shapes as cup sponges, filamentous algae, and gorgonians (soft corals that include animal colonies known as sea rods, sea fans, and sea whips). The structural complexity of reefs provides havens for numerous creatures. Most reef animals find refuge in these crevices when they are injured or distressed; obviously, sheltering in reef crevices is adaptive for prey animals that are threatened by the many predators on the surface of the reef.

Temporal Variation in Activity

One of the primary mechanisms that allows space sharing on the reef is differential times of hunting and feeding (see Tables 1-3 for examples). Many nocturnal fishes hover around or in the reef under ledges or in crevices during daylight hours when they are not hunting or foraging for food. At night, these nocturnal species typically aggregate above the reef and then migrate away from the reefs to seagrass beds or sand flats to forage. Nocturnal fishes typically have large eyes so they can see well in low light conditions. Good examples of nocturnal fishes include members of the squirrelfish family (Holocentridae) or the cardinalfish family (Apogonidae). Diurnal species, in contrast, are active during the day and either move about close to the reef, or actively feed on plankton in the water column above the reef. At night, these diurnal species seek out shelter in reef crevices. Some species have adapted their own strategy for protection at night because shelters to hide in are often limited on the reef. For example, wrasses (family Labridae) dive into the sand about half an hour before sunset, and sleep there until sunrise the next morning. Parrotfishes (family Scaridae) often secrete mucous cocoons around themselves at night, similar to a sleeping bag. These cocoons effectively block the odor of the parrotfish while it is sleeping and prevent it from being smelled by nocturnal predators.

Table 1: Diurnal Coral Reef Fish Species

Representative Example	Common Group Name	Family Name	Type of Forager	Typical Habitat Use	Interesting Notes
	Angelfishes and butterflyfishes	Pomacanthidae Chaetodontidae	Actively bite reef surface	Roam the reef in pairs or alone	Typically pair bond for their life span
	Balloonfishes or pufferfishes	Tetraodontidae	Most crush urchins	Hover above the reef or in crevices	Can inflate for protection
	Blennies	Blenniidae Clinidae	Bite reef surface	Limited home range, usually on one coral head for entire life span	Very speciose family
	Damselfishes	Pomacentridae	Bite reef surface, or are planktonic	Territorial or school above the reef to eat plankton	Some are "algae gardeners"; they remove unpalatable algae from their territories
	Gobies	Gobiidae	Bite reef surface, or clean ecto-parasites from other fishes	Extremely limited home range, often within one coral head	Maintain cleaning stations for larger fishes; highly speciose
	Parrotfishes	Scaridae	Bite coral	Roam in schools	Make a lot of noise as they eat!; sleep in mucous cocoons at night
no picture	Scorpionfishes	Scorpaenidae	Ambush	Solitary, motionless on the reef surface	Extremely cryptic
	Surgeonfishes	Acanthuridae	Herbivores	Roam in large schools	Roaming in schools gives them access to palatable algae in damselfish territories
	Triggerfishes	Balistidae	Crush urchins	Hover above the reef	Strong jaws Enable them to crush urchins and molluscs
	Wrasses	Labridae	Opportunistic	Mostly swim close to the reef surface and possess home ranges	Exploit other species by consuming eggs (damselfish) or follow goatfish; bury in sand at night

Table 2: Coral Reef Fish Species that are Active during the Day and Night

Representative Example	Common Group Name	Family Name	Type of Forager	Typical Habitat Use	Interesting Notes
no picture	Cornetfishes	Fistulariidae	Ambush	Hover or move slowly just above the reef	Curve into an "S-shape" (from above) right before they strike
no picture	Goatfishes	Mullidae	Use barbels to detect hidden invertebrates	Solitary or in aggre-gates; roam the sand flats and reef surface	Other fishes follow goatfishes to exploit their ability to detect hidden prey
no picture	Lizardfishes	Synodontidae	Ambush	Buried in sand, or motionless	Cryptic coloration
	Moray Eels	Muraenidae	Opportunist	Crevices	Somewhat cryptic; move like snakes
	Trumpetfishes	Aulostomidae	Ambush	Move slowly just above the reef	"Suck" prey into their long snouts

Table 3: Nocturnal Species of Coral Reef Fishes

Representative Example	Common Group Name	Family Name	Type of Forager	Typical Habitat Use	Interesting Notes
	Cardinalfishes	Apogonidae	Most are planktonic	In or under coral heads	Most have large eyes; most species are transparent
	Sea basses & Groupers	Serranidae	Migrate off the reef at night	Under ledges or in crevices	Some individuals can get very large; overfishing has reduced the average size of these fishes
	Snappers	Lutjanidae	Migrate off the reef at night	Hhover in schools close to reef surface	Follow same migratory routes to foraging grounds

Cryptic Use of Habitat

Although most coral reef fish species use color change for such activities as mating, defense from predators, or to show their aggression toward competitors, there are some fishes that are masters of color change and camouflage. Peacock flounders, for example, are flat fish that glide along the bottom, and are able to change their color to blend in with their surroundings. While traveling over sand flats, they can be a pasty white color; while over reef habitats they enhance the spots on their dorsal surface by displaying brown and green colors in specialized cells known as chromatophores. Lizardfishes are also masters of camouflage and often bury themselves in the sand with only their eyes above sand. Scorpionfishes are camouflaged both by color and texture as they grow decorative "eyebrows" that closely resemble filamentous algal species that grow on the reef. A highly diverse assemblage of species will often include unusual adaptations that developed through the process of evolution. It is believed that heterogeneous environments enable many species to coexist, but in order to exploit unused resources, species must evolve unusual and sometimes elaborate adaptations.

Ontogenetic Shifts in Habitat Selection

As previously mentioned, coral reef fishes have a pelagic larval phase. Larvae generally disperse away from their natal reefs and exist in the open ocean for a period of time before settling on a reef in a new location. Researchers have been investigating the cues that fish larvae may use as indicators of suitable habitat for settlement. The presence of adults of the same species in a habitat can sometimes serve as an indication of a potentially good habitat for individuals looking for a place to settle. It follows that if adults of a species exist in a particular habitat, then the habitat must be suitable for juveniles of this species. However, there are two things that can act against newly settling larvae. Juveniles settling into habitats with adults of the same species may be subjected to competition because they have similar requirements for resources. Furthermore, adult habitats may be suitable for adults, but may offer little protection for juveniles from certain predators that live on that area of the reef. In some species, juveniles are rarely found in the same habitat as adults, possibly because of the aforementioned reasons. When the juveniles of a certain species occupy different habitats than adults of the same species, these fish are defined as going through ontogenetic habitat selection. As the juveniles grow, their needs may shift, or they may be more likely to survive in different habitats over time. Good examples of species that have ontogenetic shifts in habitat are grunts (family Haemulidae) and snappers (family Lutjanidae). These fish species settle as juveniles into seagrass and mangrove habitats that are adjacent to coral reefs. When these fishes reach a certain size, they migrate to the coral reefs where they take up daytime residence under ledges or in crevices and migrate at night to foraging grounds. Although the ontogenetic habitats of some of the grunts and snapper species have been investigated, we currently lack knowledge of habitat use of many other important fishery species.

Traditional Site Use for Sleeping and Mating, Cleaning Stations, and Migratory Routes

The use of traditional sites and migratory routes may be adaptive as organisms maximize familiarity with their surroundings, which may facilitate, among other things, escape from predators. The use of traditional sites also provides more definitive allocation of space use on a reef; fishes do not have to waste a lot of time each day or night searching for new sites for sleeping, foraging, mating, or getting cleaned. Many individual reef fishes return to a traditional sleeping location every night or day, depending on whether they are diurnally or nocturnally active. Wrasses bury themselves in the same location in the sand or hide in the same crevice every night! The flexibility of these behaviors is unknown at present. Traditional sites have also been observed for spawning in certain fishes. Large aggregates of a species of fish form at traditional spawning sites during certain seasons or times, and then the fishes disperse after spawning has occurred. This is not unique to coral reef fishes; it is common in many temperate marine fishes as well. Unfortunately, humans have taken advantage of spawning aggregates in many commercial reef species, as fishes are easily caught in high numbers during spawning events. On a more positive note, some Caribbean countries have recently adopted legislation to protect these aggregates of spawning fishes.

Another interesting use of space on coral reefs that exemplifies the traditional space use is the existence of "cleaner" stations, where small fish remove ectoparasites from large fish. Gobies (family Gobiidae), and often small wrasse (family Labridae) maintain these cleaner stations at particular locations on the reef. Larger fish swim up to the location and indicate either through fin movements or color change that cleaning should commence. Fish that are frequently spotted at cleaning stations include porcupinefish (family Diodontidae), groupers (family Serranidae), larger wrasses (family Labridae), and larger parrotfishes (family Scaridae).

Finally, some species of reef fish use traditional migratory routes on their way to foraging grounds away from the reef. For example, snappers (family Lutjanidae) are nocturnal foragers that aggregate during sunset and migrate to foraging grounds across sand flats and into seagrass habitats. Studies have shown that these migratory routes are traditional and are passed on from individual to individual or from one generation to the next. These routes are an interesting adaptation to life on the reef that may assist in aiding fish migration on and off the reef efficiently and safely, and allow them to acquire additional food resources that are not as accessible on the reef itself.

Management Implications

While reef scientists and managers recognize the need to conserve habitat diversity in order to preserve a diverse fauna with wide-ranging resource needs, we have had difficulty in deciding the most effective means of achieving this goal. Efforts have been made to create or restore reef habitats that have been destroyed by human activities or natural events such as storms. Restoration and creation of artificial habitats have been attempted for mangrove, seagrass, and coral reef habitats, but few studies have adequately demonstrated successful restoration.

Good management strategies will require a better understanding of: 1) the ontogenetic requirements of important reef fish species, because food and habitat needs may change as a species grows; 2) the spatial dynamics of reef fishes with respect to space sharing and behavioral interactions of different species; 3) how fluctuations in population numbers of one species may affect other species; and 4) how habitat degradation may impact the integrity of the reef ecosystem. It is important that we do not underestimate the value of coral reefs and take their splendor for granted.

Key Principles

1. Habitat use in coral reef fishes
2. Foraging strategies and interesting adaptations
3. Importance of preserving coral reef habitats

Ethical Considerations

1. How might the ecological information concerning ontogenetic shifts in habitat selection be applied toward more effective management strategies?
2. What are some characteristics of coral reefs and their inhabitants that should be considered prior to deciding the locations of marine protected areas?
3. Many coral reefs are situated in developing countries. The local fishermen in some of these locations cannot even afford to consume their own catch, except for the poorest quality (i.e., damaged) fishes and invertebrates. Instead, their catch is typically exported to wealthier countries. Furthermore, some fishing methods are destructive to coral reef habitats or over-exploitative of the fish population and not sustainable. Suggest some solutions to more effectively manage coral reefs/fisheries to improve economic value and sustainability for local residents.

Suggested References for Further Reading

Clarke, R. D. 1977. Habitat distribution and species diversity of chaetodontid and pomacentrid fishes near Bimini, Bahamas. Marine Biology. 40: 277-289.

Sale, P. F. 1991. The ecology of fishes on coral reefs. Academic Press, Inc.

Internet Links

Coral reefs

Fish identification

Illustrations

Tables 1-3: Habitat use, foraging strategies, and some interesting points about the most common coral reef fish families. Some families have not been included in these tables: e.g., porgies, boxfishes, filefishes, jacks, hawfishes, sharks, barracuda, needlefishes, and seahorses.

Authors

K. Martha M. Jones
Department of Biological Sciences
University of Windsor
Windsor, Ontario N9E 3G5
Canada

Dr. Iain J. McGaw
Department of Biological Sciences
University of Nevada, Las Vegas
Las Vegas, Nevada 89154
U.S.A.