Coral Reef Fishes: Adaptations and Diversity

Coral reef fishes are perhaps the most obvious, colorful, and fascinating of all the myriad creatures that inhabit the coral reef biome.

The sheer number and variety in form, coloration, and behavior of these fishes provides endless appeal to scuba divers, snorkelers, and underwater photographers like no other members of the reef community.

This group of animals includes members of two different classes of vertebrates: the bony fishes (Class Osteichthyes) and the "cartilaginous fishes" (sharks and rays; Class Chondricthyes). The vast majority of fishes dwelling on coral reefs are bony fishes.

In the Greater Caribbean region some 500-600 species of fishes associated with coral reef ecosystems are recognized, whereas the Indo-Pacific region as a whole houses perhaps 8-10 times that number. The reasons for the extraordinarily high diversity seen among coral reef fishes are not well understood, and remain a topic of considerable debate among reef fish ecologists.

While entire books can (and have) been written about these animals, here we provide an introduction to two major aspects of the biology and ecology of coral reef fishes:

• General adaptations for life on the reef

• Feeding strategies

General Adaptations For Life On The Reef

Coral reefs are uniquely complex and colorful marine environments, with a physical structure radically different than that of the open waters that comprise 99% of the world's oceans. Therefore, it is not surprising that many of the fishes that live in these ecosystems have developed a number of anatomical and other specialized adaptations for life in such environments.

The basic body plan of fishes twhat dwell in reef habitats differs in a number of crucial ways from the basic fish body design that originated in far different kinds of habitats. The three most notable and common such adaptations are discussed below.

Body Shape

The typical body shape of reef dwelling fishes differs substantially from that of most open water fishes. The latter are generally built primarily for sheer speed, and have evolved appropriate torpedo-like shapes that offer low frictional resistance (drag) to movement through water.

The queen angelfish exemplifies the modern coral reef fish body plan. © Fotolia.com

In the complex coral reef environment however, a premium is placed upon maneuverability rather than sheer speed. Thus, many reef dwelling fishes have evolved a body plan that maximizes their ability to make rapid turns and stop quickly, highly useful traits for an animal attempting to avoid predators in physically complex habitats.

By quickly dodging into fissures in the reef, swiftly circling around coral heads, or coming to a sudden halt next to a solid object (like a hard coral colony), prey can more readily avoid predators that lack such abilities.

The essence of this design scheme is a deep and laterally compressed body (shaped like a pancake), exemplified by the surgeonfish (pictured left), and it is the "trademark" of coral reef fish adaptations.

A less obvious but critical aspect of this altered body plan includes a shift (compared to open water fishes) in the placement and orientation of the pectoral and pelvic fins.

These changes to the main steering fins of reef fishes act in concert with the flattened body shape to maximize maneuverability, including the ability to make sharp turns and sudden stops.

The only way to truly appreciate the cumulative effects of the referred adaptations in body architecture is to actually witness the ability of these fishes to escape attacking predators by swiftly and skillfully using the cover afforded by coral reefs.

Adaptive Coloration

Perhaps the most striking of coral reef fish adaptations is the variety of brilliant and sometimes bizarre color patterns that adorn them. In many cases these color patterns contrast starkly with the usual color patterns of open water fishes which typically are monochromatic or silvery, as befits the backgrounds against which they are normally seen.

The lionfish, a venomous coral reef fish, displays vivid warning coloration. Courtsy NOAA

The reasons for the unusual color patterns seen in coral reef dwelling fishes have been debated for some time. Many reef fish color patterns are intended to make the bearer less conspicuous to predators or potential prey. Coloration may also be used for species recognition, to facilitate mating success.

Sometimes, territorial reef fishes or those possessing venomous spines or flesh display "warning patterns" (see photo, left) that enhance recognition by likely enemies or competitors.

Reef fishes are not born with the knowledge that one kind of fish makes a fine meal while another kind will mean trouble if attacked, ingested, or even approached too closely.

However, after experiencing the unpleasant results of an encounter or two with such "protected" species, other fishes quickly learn to avoid them.

Common adaptive coloration in nocturnally active fishes generally consists of uniform reddish hues, which appears very dark under low light.

Feeding Structures

With the unusual variety of prey items available, it is not surprising that coral reef fish adaptations include highly specialized jaws, mouths and teeth suited to particular kinds of food sources commonly found in coral reef habitats.

Parrotfish beaks are specialized feeding structures. © Fotolia.com

Not surprisingly, eating and digesting plant material requires specialized feeding structures and digestive systems that differ considerably those of meat eaters (carnivores).

For example, the parrotfishes (Scaridae; pictured left) have evolved beak-like mouths perfectly suited for scraping algae from hard coral surfaces.

In contrast, butterflyfishes have evolved forcep-like mouths armed with numerous fine teeth well suited to nipping exposed coral polyps.

Still, many other common reef dwelling fishes, such as snappers (Lutjanidae) retain a more generalized feeding structure plan that enables them to utilize a wide variety of prey items, including smaller fishes and invertebrate animals.

Other Adaptations

The number and complexity of coral reef fish adaptations is far too great to even begin to approach on a single web page; indeed, that topic alone could fill a sizeable entire web site. Here, by way of concrete example, we present just one such case of an adaptative response of these fishes to deter predation - in this case, both chemical and behavioral.

Anemone provides shelter for clownfish. © Fotolia

Anemones are common coral reef animals, equipped with a formidable defense mechanism - tentacles bearing stinging cells called nematocysts. Some coral reef dwelling anemones form close symbiotic relationships with other animals, most notably certain fishes as well as some shrimps and others.

Clownfish (Amphiprion spp., pictired left) are a group of coral fishes that have adapted to shelter among anemone tentacles. They do so by first performing a ritualistic "dance" with a potential host anemone, lightly brushing up against the tentacles until a relationship is established.

Special chemical contained within the mucus layer enveloping the skin of the clownfish have the capacity to block anemone nematocyst stings. Because the anemone host normally feeds on fishes, the clownfish thereby gains protection from predators. In exchange, the clownfish cleans its host from parasites.

Coral Reef Fish Feeding Types

A common and useful (from an ecological perspective) way of analyzing the highly diverse fish assemblages inhabiting coral reefs is by categorizing the many species into a few basic "guilds" (groups of species that use common resources in similar ways), based upon the different types of food resources used by each group.

Feeding types of coral reef fishes have been classified in various ways, but we will herein discuss four basic types: herbivores, planktivores, benthic carnivores, and piscivores.

The four basic feeding strategies described below are not necessarily mutually exclusive; some "herbivores" may sometimes feed on small animals, while some "piscivores" may also take larger invertebrates. Some species may feed as planktivores or benthic carnivores while young, but switch to a piscivorous lifestyle as adults. Larval reef fishes that have yet to settle on reef habitat often have a very dirfferent feeding mode than they will after achieving reef residency.

Herbivores

Herbivorous coral reef fishes are those that feed mainly or entirely on plant material found within coral reef ecosystems. Most of the biomass of herbivorous fishes found within coral reef ecosystems is distributed among only a few major groups in both the Indo-Pacific and Greater Caribbean regions. These are discussed below.

Parrotfishes are the largest and most colorful of herbivorous coral reef fishes. Their name stems from the brilliant hues borne by many species and the beak-like mouths that characterize the group. The cutting edge of the beak is used to scrape algae from coral surfaces. At night, parrotfish secrete a protective mucous envelope that surrounds their bodies as they sleep within reef crevices. The "cocoon" is believed to deter detection by predators.

Surgeonfishes and the closely related rabbitfishes (see below) are the mid-size models of herbivorous coral reef fishes. The name "surgeonfish" stems from the retractable scalpel-like spines at the base of each side of the tail. These are formidable defensive weapons. Surgeonfishes browse the reef in search of suitable algae during the day, but retreat to reef crevices after dark. Some species feed in mainly in schools, while others roam the reef to feed as solitary individuals.

Rabbitfishes are named for their blunt, elongated snouts. They are common members of coral reef fish assemblages throughout the Indo-Pacific region from Hawaii to the Red Sea, but are entirely absent from coral reefs of the Greater Caribbean region. Although rabbitfishes they lack the retractable tail spines of the surgeonfishes, they are still well-armed. Numerous strong spines and dorsal and anal fins connected to venom glands are capable of delivering a painful toxin.

Dameslfishes: Unlike the parrotfishes, surgeonfishes, and rabbitfishes that are wide-ranging nomadic browsers, small herbivorous damselfishes are more sedentary territorial "farmers" that maintain and vigorously guard small patches of algae on the reef. They seldom stray more than a few feet from their homes and will aggressively attack any fish, regardless of size, who has the audacity to intrude upon their territories. Even large parrotfishes retreat from such attacks.

Planktivores

Planktivorous coral reef fishes are those that prey upon small animal plankton (zooplankton). There are several widely used strategies to this mode of feeding.

Open Water Plankton Feeders: The daytime open-water plankton feeding "fraternity" of coral reef fishes consists of a hodgepodge of species of diverse heritage, including specialized genera and species of damselfishes (see Chromis, left), wrasses, snappers, sea basses, and surgeonfishes. These fishes have adapted to an atypical lifestyle and bear little superficial resemblance to most other members of their respective groups.

Benthic Planktivores: A diverse assemblage of small daytime plankton feeders do so while remaining close to the reef or other nearby benthic habitats. In such places, plankton concentrations have already been diminished by other predators, but this drawback is compensated by quick access to the safety of nearby reefs or tunnels. Among this group are the jawfishes (pictured left), which feed by day near their tunnels on passing plankton.

Nocturnal Planktivores: Opportunities to feed in different ways are seldom wasted by coral reef fishes. As darkness falls, the "day shift" of planktivores (see above) takes shelter within the reef, and are replaced by a "night shift" consistings of different species with adaptations appropriate to low light conditions. The masters of this nocturnal feeding strategy are the small cardinalfishes (Apogonidae) and larger squirrelfishes and soldierfishes (Holocentridae, pictured left).

Benthic Carnivores

The term "benthic carnivores" (also sometimes referred to as benthivores) is used here to describe fishes that prey on a variety of invertebrate animals (and sometimes small fishes) living on or near the reef substrate or nearby sea floor habitats. As a group, benthic carnivores make up the majority of fish species associated with coral reef ecosystems.

Most reef benthivores feed by day, and for good reason. During daylight hours, most of their invertebrate prey are motionless and well concealed. Thus, keen eyesight and precise attack movements - abilities only possible under daylight conditions - are a basic necessity for locating and capturing such prey. Such daytime hunters are generally inactive and hidden in the reef after dark.

Below, we present but a few representatives of this highly diverse group of hunters.

Butterflyfishes are among the most ubiquitous and colorful of daytime reef benthivores. Their forcep-like mouths armed wih fine comblike teeth serve them well in browsing on exposed coral polyp tentacles and other tiny reef invertebrates. Butterflyfishes generally sport flamboyant colors - most often, shades of bright yellow. Their flat, pancake-like bodies enable these fish to make very rapid turns to escape larger predators.

Triggerfishes (pictured left) are members of an entire order (Tetraodontiformes) of coral reef fishes that has evolved to become specialized masters of this hunting strategy. Also included in this group are the filefishes, trunkfishes, and puffers. All have the keen eyesight, precise movements, and specialized mouths needed to excel at this lifestyle, as well as formidable defenses (e.g., stout spines, toxins) that prevent them from becoming easy prey themselves as they hunt.

Trunkfishes: Few benthic carnivores have the capacity to hunt in safety by day out in the featurelss sand and seagrass expanses far from the reef. The heavily armored trunkfishes are an exception. These fearless predators are often seen hunting for shelled benthic invertebrate prey far from nearby reefs during daylight hours. Their size and covering of thick bony plates (rather than ordinary fish scales) provide safety from all but the largest of piscivores.

Many species of Grunts shelter on the reef by day and venture out into sand plains and seagrass meadows distant from their "home" reef to feed under the cover of darkness. On Caribbean reefs, some species perform a nightly migration to foraging areas up to half mile from "home". There, they disperse as solitary hunters to feed throughout the night on a variety of invertebrate prey. Just before dawn, they reassemble and follow underwater trails back to the home reef.

Some other common daylight benthic carnivores that hunt and feed on or near the reef include the blennies (Blennidae), gobies (Gobiedae), wrasses (Labridae), and goatfishes (Mullidae).

Piscivores

Piscivorous coral reef fishes are those that prey mainly or entirely upon other fishes. There are three different basic hunting strategies employed by such predators, each requiring different physical adaptations and behaviors. These may be defined as pursuit, stalking, and ambush.

Pursuing Predators are those that rely on sheer speed to run down prey in open water. They are capable of sustained high speed swimming, and bursts of extremely fast attack speeds. Examples include the jacks (Carangidae, pictured left), mackerels, and many sharks. These fishes typically have streamlined torpedo-shaped bodies, sickle-shaped tail fins, and very narrow caudal peduncles (the body area just forward of the tail fin) equipped with lateral keel-like projections.

Stalking Predators utilize stealth to approach their prey before striking. Common coral reef fishes employing this strategy include barracudas, needlefishes, and trumpetfishes. All have slender, elongated bodies that present a minimal profile to the prey when seen head-on, just prior to attack. Symmetrical posterior fins function like the vanes of an arrow to ensure an accurate strike. Oversized caudal fins are used to quickly accelerate from a hovering position.

Ambush Predators: rely on disguise and stillness to hunt. Their coloration and shapes make them virtually invisible as they wait motionless for unsuspecting victims to wander within striking range. Attacks are generally made at very close range, often less than a body length. Most have very large upturned mouths that can inhale their luckless victims in a single swift gulp. Common ambush piscivores include frogfishes, lizardfishes (pictured left), flatfishes, groupers, and scorpionfishes.