The sheer number and variety in coral reef fish form, coloration, and behavior 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 (Cl. 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 of coral reef fish biology and ecology :
General adaptations for life on the reef
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 designs that originated in far different kinds of habitats. The three most notable and common such adaptations are discussed below.
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.
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 angelfish (pictured above).
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.
Perhaps the most striking of all the adaptations of fishes inhabiting coral reefs is the variety of brilliant and sometimes bizarre color patterns that adorn these animals.
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 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.
With the unusual variety of prey items available, it is not surprising that the adaptations found in reef-dwelling fishes include highly specialized jaws, mouths and teeth suited to particular kinds of food sources commonly found in coral reef habitats.
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 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.
The number and complexity of adaptations found in fishes inhabiting coral reefs 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.
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.
The clownfish (left) is a coral fish that has adapted to shelter among anemone tentacles. It does this by first performing a ritualistic "dance" with a potential host anemone, lightly brushing up against the tentacles to initiate the relationship.
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
Feeding types among reef fishes have been classified in various ways, but here we will consider four basic types of feeders: herbivores, planktivores, benthic carnivores, and piscivores.
These categories are not mutually exclusive. For example, many so-called "herbivores" sometimes feed on small animals, while some "piscivores" may also occasionally take larger invertebrates. Some species may feed as planktivores while young but switch to a piscivorous lifestyle as adults.
Herbivorous fishes are those that feed mainly or entirely on plant material. 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.
Planktivorous coral reef fishes are those that prey upon small animal plankton (zooplankton). There are several widely used strategies to this mode of feeding.
The term "benthic carnivores" (also sometimes referred to as benthivores) is used here to describe fishes that prey on a variety of animals living on or near the sea floor. As a group, benthic carnivores make up the majority of fish species associated with coral reef ecosystems.
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).
Piscivorous coral reef fish 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.