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Coral Reef Ecosystems

An ecosystem is a natural network of interactions between living organisms, both plants and animals, with each other and with the natural environment in which they live. The nature of an ecosystem depends on many factors of the physical environment, such as living space (underwater or on land), sources of energy, temperature, chemistry, water, climate, and weather to name a few. The living organisms are affected by both physical factors as well as interactions with associated organisms, such as nutrients or food sources, population density, predation, competition, and many types of symbiosis.

Coral reef ecosystems are restricted to the sea floor and are one of the richest natural ecosystems in high organism abundance, species diversity (number of kinds of organisms), and role of interspecies interactions (symbioses). Coral reefs exist mainly in shallow, tropical waters, but can also be found at great depths in cold water. One particular feature of coral reefs is the role of colonial organisms, particularly corals (Phylum Cnidaria) that build thick and extensive frameworks of intergrown calcium carbonate skeletons. This framework provides a three-dimensionally complex structure in which many other species of invertebrate animals, marine plants (algae, seas grass), and vertebrates (fish, turtles, even birds) find homes and food. At the heart of the frame-building process is a unique symbiosis between single-celled microscopic green algae (zooxanthellae) that live within the body tissues of many host coral species. With exposure to penetrating sunlight, these algae live by photosynthesis that produces a great deal more food (carbohydrates) than the algae need for their basic survival, growth, and reproduction. The excess productivity is transferred within the coral to form the main energy source for the coral, enabling greatly increased growth and formation of the calcium carbonate skeleton.

Corals that lack this symbiosis with zooxanthellae cannot form reef structures, but they can exist in reef communities and also at great depths where light does not penetrate, or in colder waters that are not favorable for the symbiotic algae. The largest and most diverse coral reef ecosystems (such as the Great Barrier Reef or mid-oceanic atolls) actually thrive in warm (>18oC, shallow tropical seas (< about 60 m depth) where nutrients like nitrogen, phosphorus and suspended food like phytoplankton and zooplankton are sparse (so-called oligotrophic waters). Coral reef waters are generally very clear (low turbidity) and have normal marine salinity (~35o/oo), but reefs can exist even in turbid waters, and experience salinity fluctuations from hyposaline (from influx of rivers or rainfall) to hypersaline (from evaporation in very shallow zones).

Corals

Corals are the principal animals that construct the reef framework that is the basis for a coral reef. Corals are not very familiar to most people because they live on the sea bottom usually in the tropics and belong to a large group of invertebrates (Cnidaria) that most people do not encounter. The most familiar cnidarians are “jellyfish” and sea anemones, but also include “soft corals”and “hard or stony corals”. All cnidarians have unique stinging cells with contain nematocysts in the tentacles. When injected into prey animals, nematocysts immobilize the prey that is taken into the mouth by the tentacles. These same nematocysts can cause painful, even lethal injuries by jellyfish, such as the Portuguese “Man-o-War” and “fire corals” known to swimmers and divers. The simplest coral has a soft-tissue body (polyp) much like a sea anemone that attaches to the sea bottom by a basal disk and has a cylindrical form topped by a circular oral disk with a ring of tentacles surrounding the mouth. Unlike anemones, the coral polyp deposits a calcium carbonate skeleton beneath its basal disk that can form a cuplike corallite. Corals can have a single polyp (solitary) or multiple, closely spaced corallites (compound), sometimes forming very large, boulderlike or massive structures (colonies) that grow together to form a reef framework.

True colonial corals called hermatypic have soft tissues that connect adjoining polyps so that nutrients and wastes can be transferred within the colony. Corals that form small clumps of corallites usually do not have interconnecting tissues and are not truly colonial; hence they do not have reef-building capability (ahermatypic). The skeletons of corals have a bewildering variety of growth forms: disk-like, massive, dome-shaped, spherical, columnar, branching, sheet-like, plate-like – all characteristic of particular groups and species.

Coral polyps are radially symmetrical with a 6-fold pattern (“hexacorals” or Scleractinia) of tentacles and mesenteries radiating from the mouth. The corallites reflect the 6-fold symmetry in arrangement of radiating septa. Some extinct corals, Rugosa, have 4-fold symmetry and Tabulata have honey-comb-like corallites with weak septa and basal plates called tabulae.

Scleractinia (Stony corals)

Octocorallia

Octocorals, commonly called “soft corals”, “sea whips, or “sea fans”, are a very diverse and abundant group of cnidarians that are found on coral reefs and elsewhere in the sea. Octocorals have polyps with 8 tentacles instead of 6 as found in stony corals, forming colonies arranged along single or branching strands or columns, but also as thick, “fleshy” sheets. Some colonies have a calcified axis or a rigid network of interconnected tubes (“organ pipe coral”). Octocorals are often very abundant on reefs and can occupy considerable surface area with their “leathery” holdfasts. Many species have symbiotic algae like stony corals. Octocorals mostly lack a rigid skeleton but all contain microscopic calcareous spicules that add to the buildup of reef sediments.

Echinoderms

Echinoderms are marine invertebrates constituting the Phylum Echinodermata that are diverse and abundant as reef-dwelling marine animals. Echinoderms include five major groups (classes): sea stars (Asteroidea), sea urchins (plus heart urchins and sand dollars – Echinoidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea), and feather stars (Crinoidea), and all occur on coral reefs. The body plan of all echinoderms has five-part (pentameral) symmetry, best known in the 5-armed seas stars and brittle stars. Echinoderms form calcium carbonate skeletons in the form of plates, spines, and other complex parts such as arm “ossicles” of sea stars and crinoids.

Echinoderms play many different ecological roles in the reef ecosystem: predation (sea stars), herbivory (sea urchins), suspension feeding (crinoids, brittle stars), and detritus-feeding (holothuroids). Echinoderms have been associated with reefs of many types as far back in Earth history as about 500 million years.

Crinoids

Living crinoids exist in two body forms: the “sea lily” that attaches to the sea bottom by a “stalk” composed of disk-like plates called columnals and a root or holdfast and the stalkless “feather star”. The “crown” at the top of the stalk consists of the body with the mouth directed upward and surrounded by five “arms” that usually branch and bear shorter branches called pinnules. The arms and pinnules carry a gutter-like “food groove” that is lined with tiny tentacles (“tube feet”) that are used for capture of suspended food particles and also respiration by means of absorption of dissolved oxygen through a thin tissue wall and excretion of dissolved CO2.

The feather star has a tiny stalk as a juvenile but breaks free to become a free-moving adult. Feather stars move around by using the arms to crawl or swim by gracefully lashing in coordinated up and down strokes. Coral reefs in the western Pacific can have as many as 50 species of feather stars perched on corals, soft corals, or sponges. Stalked crinoids lived on ancient reefs in shallow water, but today are restricted to depths below 100 m.

Macroinvertebrates

Invertebrates – animals without backbones – exist on coral reefs in tremendous abundance and variety. There are about 35 major groups of animals that are called phyla (singular phylum), and about 30 occur on coral reefs. All but the Chordata are invertebrates.

Macroinvertebrates are those large enough to be seen without the aid of a magnifier of some kind, and these can be readily seen by divers on coral reefs. I will mention some of the most common.

Sponges

Next to colonial corals, sponges (Phylum Porifera) can be the largest invertebrates on reefs. In all, there are 5000 – 10000 species of living sponges; most are strictly marine and a great number live on reefs. Because most sponges are soft-bodied, they do not contribute to the calcareous framework of a reef, but they do contribute considerable mass. Some extinct sponges formed hard coral-like skeletons that did build reefs as old as almost 500 million years. Sponges live as filter feeders, removing tiny food particles as small as bacteria from water they pump through their bodies where they extract food particles by means of specialized cells called “collar cells” or choanocytes. Food particles can be living single-celled organisms or bits of organic detritus. By removing organic detritus, sponges act as a water recycling or sanitary system for reef waters. Many other organisms live within sponge tissues, such as arthropods and worms, and others utilize sponges as a perch or substratum, like feather stars (crinoids) or brittle stars or basketstars (ophiuroids). Many kinds of reef fish actually feed on sponges, angelfish for example.

Sponges mainly in the Indo-Pacific region have symbiotic microalgae living within their tissue that are photosynthetic, so that the entire sponge with its algae is a net producer of oxygen (like plants!) Sponges have a wide variety of shapes and sizes, from mound-like masses to tree-like branches or plates, to carpet-like encrustations. An important group of reef sponges (clionids) has the capacity to etch the calcium carbonate substratum of a reef by secreting acids as they grow, contributing to bioerosion of the reef and weakening of the rigid framework.

Tunicates

Tunicates (seas squirts) are a phylum (Urochordata) resembling sponges that is very diverse and colorful on reefs. Although sponges are regarded as non-colonial animals, tunicates exist as solitary individuals or in colonies. The lump-shaped form or encrusting habit of tunicates can often lead to confusion with sponges, but tunicates have two body openings (one incurrent, the other excurrent) compared to sponges that are covered with tiny incurrent pores and a single or multiple very large excurrent openings or oscula). Tunicates also live as filter feeders.

Mollusks

Mollusks are one of the most common macroinvertebrates living on reefs. Mollusks include snails (gastropods), bivalves (clams, oysters, scallops), squid, cuttlefish and octopi (cephalopods), chitons (having 8 and sea slugs (opisthobranch gastropods or nudibranchs), as well as some free-swimming gastropods (pteropods). Many thousands of species of mollusks live on reefs, filling a wide range of ecological niches. They live as filter feeders (bivalves), predators (snails, squid, octopus), grazers (snails) vegetarians (conchs) and even as parasites. The giant clam (Tridacna) forms a large bivalved shell (> 1 m long) through the “assistance” from symbiotic algae (zooxanthellae like those in corals) embedded in the soft tissue of the mantle and exposed to light. A major extinct group of bivalves (rudistids) also constructed large, massive shells that were major reef-builders during the Cretaceous Period, probably also thanks to symbiotic algae.

Arthropods

Arthropods are very diverse and abundant on reefs, occupying many ecological niches. Arthropods (“joint-legged” include many familiar groups like crabs, lobsters, shrimp, and barnacles. Arthropods are known for their mobility, either walking or swimming. Many arthropods are omnivorous, consuming both plant and animal food, ranging in size from minute suspended items to large prey. Arthropods are also very common as commensals, symbionts, or parasites in highly modified body forms.

Worms

Worms (several phyla: annelids, sipunculids, nemerteans, nematodes) are well represented in coral reef communities. Many annelids live as filter feeders, such as colorful “Christmas tree” worms with spiral feeding tentacles, also “feather-duster” tubeworms, and spaghetti-like tentacles in others. Annelids can be free-living predators (coral-eating fireworms), sediment feeders, or commensals and parasites. Some annelids contribute to reef framework construction by building calcareous tubes, while others like sipunculids contribute to bioerosion as borers into coral skeletons.

Reef Fish

About one-third of all marine fish species occur on coral reefs, totaling over 4000 species. Reef fish range in size from tiny, hidden types like gobies to very large sharks, whale-sharks, manta rays and sting rays. Fish are highly conspicuous on reefs, because of their size, activity, and brilliant coloration. Fish are present in all reef environments, from the deepest reefs to very shallow reef flats, tide pools, back reef sea grass beds, and coastal mangroves. Most fascinating perhaps are the myriad ecological roles played by fish in the coral reef ecosystem. The extremely high diversity and complexity of fish behavior can be best appreciated by considering the variety of feeding activities found in reef fish.

Fish are prominent throughout the food web or food chain of a coral reef. As consumers, fish feed on the smallest of food particles, such as bacteria, single-celled animals and algae, and even tiny bits of organic matter in the water mass or sediments of a reef. Species such as damselfish are herbivores (vegetarians) that nibble on microbial or leafy algae covering the limestone surface of a reef framework. Others like surgeonfish graze on leafy algae or sea grass blades. Parrotfish, that grow to sizes over 1 m in length, use their rasp-like teeth to scrape the hard limestone surface of the framework for filamentous blue-green algae that are embedded in the substrate. In the process they ingest scrapings of calcium carbonate and excrete the ground-up reef rock as sand. This activity contributes greatly to the bioerosion of the reef framework and formation of sand.

Probably the greatest diversity of reef fish is found among those living as predators of other reef organisms. Because of their great mobility, fish predators can feed over vast ranges of territory, and many live in schools. Food available in the reef water mass includes everything from organic detritus, phyto- and zooplankton, and larger prey items like larval and juvenile fish to fully grown fish. Some mobile species like rays, even whale sharks, can be considered predatory but specialize on engulfing great volumes of water from which they sieve and concentrate suspended plankton (filter or suspension feeders). Some free-swimming predators simply pick off small, individual prey items, one at a time. Garden eels emerge from burrows in the sand, waiting to pick off food items drifting by in the current. Many reef predators are specialists, well adapted for feeding on all kinds of reef dwellers including coral, sponges (angelfish), worms, or mollusks living on or within the reef structure (benthivores). Some benthivores like pufferfish blow out a jet of water to excavate the sand pockets on a reef where burrowing prey like worms and crustaceans are living. Other specialists with strong jaws and crushing teeth (stingrays, pufferfish, triggerfish) can attack shelled prey like mollusks or sea urchins and are termed “durophages”. Some reef predators live as “ambush predators”, awaiting close approach of prey that they grab by rapid mouth gaping (scorpion-fish, toad-fish) or by pouncing on prey (lizardfish, lionfish). At higher levels of the food chain, fast-swimming predators like jacks, barracuda, and sharks are after large fish prey.

All of the foregoing fish feeding types rely mostly on daylight for locating their food sources by sight. At night, the entire reef “stage” is repopulated by species adapted to darkness in a remarkable changeover at dusk that is reversed at dawn. In daytime, fish abundance and diversity is far greater on a reef than at night, and time-lapse recordings reveal a genuine “blizzard” of fish! In order to avoid visual predators, potential prey fish take up protected, sheltered locations against the backdrop of a reef wall or beneath overhangs. Under cover of darkness, many vulnerable fish venture farther away from the protection of the reef to do their own foraging. Nevertheless, many nocturnal predators like moray eels are specialized at finding prey by chemosensation. By night many macroinvertebrates like crinoids (feather stars) and basketstars that are cryptic or hidden within the framework by day emerge for suspension feeding or hunting (octopi, lobsters, crabs). It is quite clear that the striking differences in the presence and activity of reef fish and their potential prey are largely based on “risk-benefit” factors in the struggle for survival on a coral reef.

Finally, the vast diversity of reef fish that live in highly specialized associations (partnerships or symbioses) with other reef species deserve mention. Prominent examples are the clownish that live comfortably within the stinging tentacles of a host anemone, or the small clingfish that attach to feather star arms. The clownfish-anemone symbiosis is a true mutualism (each species benefiting). By day, elongated trumpetfish hover, head-down, awaiting unsuspecting prey passing beneath them, but at night can be found hovering within the protection of sea whip branches in a kind of mimicry, suggesting that even under cover of darkness, there are predators lurking.

Turtles

Divers on coral reefs frequently see sea turtles, both hawksbill and green turtles on reefs worldwide – despite the fact that their numbers were drastically reduced ever since human fishing began to alter “natural” reef communities. The smaller hawksbills actually feed on sponges that are the preferred diet of few other reef animals except for angelfish, but also consume seaweed (the macroalga Sargassum) that also grow on reef surfaces. Green turtles feed on the sea grass (Thalassia) that forms vast beds closely associated with coral reefs, leading to the term “turtle grass”, and also graze the “algal turf” that grows on rock surfaces of reefs. Coral reef ecologists recognize that these turtles play a role very similar to herbivorous fishes on reefs that has (or formerly had) a significant influence on coral reef community structure.