Hidden beneath the ocean waters, coral reefs teem with life. Fish, corals, lobsters, clams, seahorses, sponges, and sea turtles are only a few of the thousands of creatures that rely on reefs for their survival. Coral reefs are also living museums and reflect thousands of years of history. Many coral reefs have been alive and thriving for centuries. Some reefs are even older than old-growth forests.
Millions of species live in and around coral reefs. In Fiji, emperor shrimp ride on the backs of slow-moving leopard sea cucumbers eating scum of their host. In addition to providing food and transport the sea cucumber offers defense. When it is disturbed it spills its toxic guts out. The sea cucumber also acts as a gathering place for the shrimp where they sometimes meet mates.
Today, these important habitats are threatened by a range of human activities. Many of the world’s reefs have already been destroyed or severely damaged by water pollution, overfishing and destructive fishing practices, disease, global climate change, and ship groundings.
Many reef sea creatures feed of coral mucus or creatures that feed on coral mucus Some jellyfish get their food from algae living in their tissues the same way coral polyps get their food from algae.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures ; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Wikipedia article Wikipedia ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ;Global Coral Reef Monitoring Network gcrmn.net
Reefs are incredibly rich ecosystems, supporting thousands of other kinds of life living together in a complex web of food and resource competition. The limestone structure produced by stony corals provide a perfect home of myriads of tiny bottom dwelling animals as well as being a safe haven for a dazzling array of reef fish you see swimming above the reefs. Reefs are vital for providing shelter, breeding areas and protection from predators, especially in the early stages of development, for a number of different species. Without a reef, there would be few fish and only a sandy bottom.
jacks and a reef shark Only the top quarter inch of the reef itself is alive. Snapping shrimp and guard crabs defend their coral territory from predators such as starfish Small fish and other herbivores graze on seaweed, which competes with coral for seafloor space. Seaweeds grow on the skeletons of the dead coral together with sponges and other animals. Clams and barnacles bore holes into the limestones and filter plankton. Sea lilies and brittlestars, bristle worms and shell-less mollusks climb through the network of branches and graze on the algae.
Parrotfish and triggerfish eat the coral to get the polyps inside. Small schools of damselfish hang out by antler coral feeding on plankton and organic particles and ready to dash for safety among the branches, Predators such as reef sharks, groupers, rock cod, lionfish, scorpionfish, stonefish and moray eels lurk around caves and crevasses waiting to gobble up any creature that lets down its guard or shows distress.
Describing a later afternoon visit to a reef off the Bahamas, Abigail Tucker wrote in Smithsonian magazine: Sixty feet below the surface, a three-foot barracuda gives us the hairy eyeball.... We see hog snapper and upside-down jellyfish and a striped sea cucumber. Magnificent sponges resemble egg cups, golf balls and chess pieces. Most flabbergasting are the colors: There are sorbet corals, emerald plates of algae, touches of lavender, banana and rose. Fish dash past in peach and platinum....As darkness starts to fall...the reef’s rainbow fades. The water looks to be filling with gray smoke. We’ve lost the reds and the oranges. You can still see yellow, then that disappears, then you lose green. Soon all you’re left with is blue.” Almost all bioluminescent creatures manufacture blue light: Its short wavelengths penetrate farthest in seawater. Some of the animals grow more active as darkness falls. Deep in the chambers of the now-ashen reef, hungry fish stir.
Reefs are among the worlds most diverse habitats. They are home to about 35,000 to 60,000 species (of the world's 274,000 known marine species) and a third of the 12,000 kinds of marine fishes. They are also incredible rich in phyla, with creatures as diverse as sea lilies, sponges, crabs, eels and sea anemones all living there. Some patches of reef boast over a 100 species of fish and more than a thousand other kinds of plants and animals.
A single reef wall often contains a wider variety of life forms — species from more phyla, or major groups — than an entire continent. The rich biodiversity of the reefs is due to abundance of light warmth provided by the tropical sun and an abundance of oxygen saturated into the water by crashing waves.
While they cover less than 1 percent of the ocean floor, coral reefs support more than 25 percent of marine biodiversity, including turtles, fish and lobsters — making them fertile ground for global fishing industries. But the biodiversity of the world's reef system is nowhere near that of the world's rainforests. This is because the species in the ocean can spread out all over the world to some degree, while mountain ranges, deserts and water barriers keep land species from spreading. Moray eels for example are found in almost all the world's reef's while different species of snakes are usually localized to a specific region.
Stephanie Pappas wrote in Live Science: Coral reefs are home to thousands of species, so it's no surprise that some are very strange. Coral itself is pretty weird; after all, reefs are built by coral polyps, relatives of jellyfish that extract calcium carbonate from the water to construct protective homes shaped like brains, fans and plants. Even weirder, most coral polyps wouldn't survive without a symbiotic relationship with an alga called zooxanthella, which lives inside polyps and provides energy via photosynthesis in return for shelter and carbon dioxide.[Source: Stephanie Pappas, Live Science, April 20, 2022]
Perhaps the weirdest animals found in reefs and off the coasts of tropical Pacific islands, however, are the sacoglossans. Sacoglossan translates to "sap-sucking," said Jeanette Davis, a marine microbiologist, science communicator and author of the children's book "Jada's Journey Under the Sea" (Mynd Matters Publishing, 2022). Sacoglossans are more often known as "solar-powered sea slugs," Davis told Live Science. These colorful slugs feed on algae, stealing some of the algal chloroplasts, cellular organs that enable photosynthesis. Yep, these slugs can glean energy right from the sun. They can also use molecules from the algae for defense, and some of them could help defend human health, too. "Through my work as a marine microbiologist, I worked with a team of scientists to ultimately help discover an anti-cancer compound that is produced by a marine bacterium associated with alga that is hijacked by a sacoglossan and used as a defense molecule," Davis said.
Scientists studying Kingman Reef, a mostly submerged reef near Palmyra atoll and Kiribati, about 1,600 kilometers south of Hawaii, were surprised by what they found. In reef that was about as untouched and far from human influence as was possible, they found lost of colorful coral and sponges but instead of large schools of colorful fish they found large numbers of predators — including whitetip and grey reef sharks and masses of aggressive snappers — forcing smaller fish to seek hiding places in the reef. Large predators accounted for 85 percent of the fish biomass, the scientists said, a larger percentage than has been found at any other reef worldwide. [Source: Kennedy Warne, National Geographic, April 2008]
The finding raised questions about what is a healthy reef. Was one teeming with schools of colorful fish actually one in distress? Enric Sala, a marine biologist studying Kingman reef, told National Geographic, “Worldwide, there are maybe 50 reefs in this sort of condition.”
Kennedy Warne wrote in National Geographic, “Here the biomass pyramid is turned in its head. At first glance, an upside-down pyramid is counterintuitive. On land, we are familiar with the notion that an apex predator, such as a lion, must eat many wildebeest to survive. But imagine a world with one pound of wildebeest for every five pounds of lion, the only way an inverted pyramid can function is if there is rapid turnover of biomass at the lower levels. Prey must be fast growing and quick to replenish; predators must grow slowly and live long.”
“In the warm tropical waters many prey species spawn several times a year, replenishing their stocks as rapidly as predators deplete them. Even so the prey barely manages to sustain the predators.” Researchers found the stomach of red snappers at nearby Palmyra Atoll, another protected reef, mostly empty. The picture that emerges of life on a healthy reef is one of abundant predators living in perpetual hunger and scarce prey living in perpetual fear.” The red snappers were so hungry they sometimes attacked diver, “They wanted to taste us” Sala said.
What happens when the large predators are removed through fishing? As has been the case at other islands which have had the predators fished out, initially there is a vibrant boom of smaller fish but as time passes the system begins to break down. As the number of harmful microbes increases, large numbers of algae flourish. As large grazing fish are taken the activity of the algae increases carbon dioxide production, boosting bacteria growth — ultimately turning a healthy coral reef into a sediment-caked wasteland. “Eliminating the top predators speeds the turnover rate of the entire reef community,” Sala said. “It’s like removing vital parts from a machine and expecting it to keep functioning.”
Day and Night Creatures at the Reef
About half to two thirds of all reef species are most active in the day. Daytime fish species such as butterflyfish, angelfish, damselfish, parrotfish and wrasses, generally have good color vision and eyes that function well under water in bright light. Most daytime fish feed on plankton or graze on algae or aquatic plants. Some seek crabs, worms, snails and other creatures that hide in the crevasses of the reef. To avoid trouble day time fish stay near a crevasse — large enough for them but too small for predators that feed on them — where they can make a quick escape. [Source: Joseph Levine, Smithsonian magazine and the book “ The Coral Reef at Night” ]
Daytime fish that wander far from the safety of the reef do so to feed on concentrations of plankton. They avoid threats from predators by traveling in schools. During the day huge schools of grunts and snappers — which are often most active at night — hide inside crevasses and two or three moray eels may huddle together inside a single cave.
The character of the reef changes dramatically at night. After sunset the colorful polyps from hard coral emerge from their exoskeletons and soft corals expand their bodies four or five times and sway gently in the current. Crabs and invertebrates such as sea slugs and sea urchins — all of which are vulnerable to triggerfish attacks in the day — emerge to graze on algae and other things. Starfish began roaming around, giant clams open their bulbous lips, and sea lilies crawl slowly to a perch and unfurl their arms.
Between a quarter to a third of all reef species sleep or rest during the day and roam around at night. A few are active day and night. Some are most active during the twilight periods of dawn and dusk. At night spiny lobsters, octopuses and moray eels emerge from their burrows to feed. Spiny lobsters forage for small invertebrates. Moray eels seek bigger prey. Snapper, porgies and jacks graze in nearby grass beds. Day time fish such as parrotfish and angelfish change from bright to dull colors and seek the safety of crevasses and caves.
Some night-time fish have big eyes. Squirrelfish, soldierfish and cardinalfish have sensitive eyes that are blinded by strong light. They spend their day beneath coral ledges and in the mouths of caves. Sweepers that usually hang out in caves and young grunts that spend their days around coral heads meet at landmarks and then disperse to the floor of the open sea where they feed on worms, small shrimp and other sanddwellers.
Reef Predators at Twilight
reef squirrel fish Twilight is often a chillingly quiet time on the reef. Most daytime fish have sought shelter for the night and the nighttime fish haven't emerged yet. Many chase-and-gulp predators, such as reef sharks, barracuda, schooling jacks and grunts, semi-solitary snappers and reclusive groupers, prefer to do their hunting at dawn and dusk, taking advantage of their acute twilight vision and the confusion that takes place as sea creatures adjust to the change in light and the changes in rest and wakefulness. Larger predators like reef shark patrol reef lagoons and channels looking for fish that are in some kind of trouble. [Source: Joseph Levine, Smithsonian magazine and the book “ The Coral Reef at Night” ]
These predators have little success during the day because the sharp eyesight and quick reactions of their prey during the day makes them difficult to catch. In the twilight their prey lose their edge and stragglers and isolated and confused individuals are sitting ducks.
Schooling predators such as jacks hunt like a pack of wolves trying to bring down a caribou. Working in teams and individuals, the fish try to break up schools of prey into small groups and separate victims a few at time and corral them against the surface, where they are easy targets. Sometimes individual jacks dash into the center of a school of prey in an effort to scare individuals and distract them enough to be separated from the school so they can be picked off. The hunt continues until darkness.
Surgeonfish often mate in the twilight. Around dusk at certain times of the year, surgeonfish suddenly stop eating and organize themselves into long chains, with individuals about half a meter from one another, and head to the open ocean side of the reef. Many chains of surgeonfish join together to form dome-shaped school with around 2,000 fish that begins an unusual reproduction ritual. [Source: Joseph Levine, Smithsonian magazine and the book “ The Coral Reef at Night” ]
Describing the mating ritual in the Red Sea, Joseph Levine wrote in “The Coral Reef at Night”, "As the light dims noticeably, a dozen individuals gather more closely together within the pulsing mass. They separate slightly from the group as the dome reaches it peak. Finally, as though released from a tautly strung bow, first one group, then two groups, then five, sharply fling themselves toward the surface, pivot sharply and dash back down into the milling swarm. At the apex of each group's ascent, its members leave something behind — small clouds of transparent eggs and white sperm that float slowly away...After the mating dance, the surgeonfish return single file to their reef haunts and dart into their favorite hiding place.
Why do surgeon fish and other reef species spawn at dusk and loose themselves in what they are doing when they are most vulnerable to predators? They do it for their offspring — eggs and larvae — which are fed on by sharp-eyed daytime reef fish that see less well and seek cover in the evening.
Sounds and Coral Reefs
Jennifer S. Holland wrote in National Geographic: A reef is full of music. And motion. Clicks and taps, squeaks and gurgles accompany the shimmying of soft corals, the quivering of shrimp, the nibbling of fish, the skittering of crabs. Moray eels poke from hollows like open-mouthed puppets, reef sharks battle for a bite, curious cuttlefish loiter, then bolt. With thickets of elkhorn corals, and boulder corals like massive cakes iced in pinks and greens, their tiers decorated with lacy sea fans, tube worms, and feather dusters, reefs are fantastical — a Seussian stage set for a recurring daily drama. From every nook, cartoonish critters wave fins or claws or tentacles, each defending its little place in the world. [Source: Jennifer S. Holland, National Geographic, April 7, 2021]
Angie Teo of Reuters wrote: When a team of scientists listened to an audio clip recorded underwater off islands in central Indonesia, they heard what sounded like a campfire. Instead, it was a coral reef, teeming with life, according to a study scientists from British and Indonesian universities published in May 2022, in which they used hundreds of such audio clips to train a computer programme to monitor the health of a coral reef by listening to it.[Source: Angie Teo, Reuters, June 6, 2022]
A healthy reef has a complex "crackling, campfire-like" sound because of all the creatures living on and in it, while a degraded reef sounds more desolate, life sciences specialist and the team's lead researcher Ben Williams said. The artificial intelligence (AI) system parses data points such as the frequency and loudness of the sound from the audio clips, and can determine with at least 92 percent accuracy whether the reef is healthy or degraded, according to the team's study published in Ecological Indicators journal.
The scientists hope this new AI system will help conservation groups around the world to track reef health more efficiently. Indonesian conservationist and lecturer at the marine sciences faculty of Hasanuddin University Syafyudin Yusuf said the research would help in monitoring reef health in Indonesia. The researchers also hope to collect underwater recordings from reefs in Australia, Mexico and the Virgin Islands to help assess the progress of coral restoration projects.
Sounds also can boost the health of ailing coral reefs. National Geographic reported: Healthy coral reefs are pretty noisy places. “The crackle of snapping shrimp and the whoops and grunts of fish combine to form a dazzling biological soundscape” that draws juvenile fish looking for a place to settle, says marine biologist Steve Simpson. When a coral reef gets degraded, inhabitants disappear and the reef becomes “ghostly quiet,” he says. “But by using loudspeakers to restore this lost soundscape, we can attract young fish back again.” [Source: Annie Roth,National Geographic, October 2020]
In 2017 Simpson and an international team of scientists placed loudspeakers along Australia’s Great Barrier Reef right after a mass bleaching event to see if playing the sounds of a healthy reef could entice fish to repopulate a damaged one. After six weeks, twice as many fish settled on bleached patches of reef where sound was played as on patches where no sound was played, according to the team’s study, reported in Nature Communications. “Fish are crucial for coral reefs to function as healthy ecosystems,” says marine biologist Tim Gordon, the study’s lead author. “Boosting fish populations in this way could help to kick-start natural recovery processes.”
Poisonous Reef Creatures
Reefs are also very poisonous environments. According to one study at the Great Barrier reef, 73 percent of the 429 species of common exposed invertebrates were toxic to fish. There are hundreds, maybe thousands, of poisonous marine organisms.Among those found in reefs are lion fish, cone snails, blue ring octopus, box jellyfish, sea urchins, scorpionfish and sea snakes
If you get stung by a sea creature you should: 1) put hot water on the wound. Sometimes hot water will break down the poison and relieve the pain. 2) remove the spines if any and do so carefully so they don’t break and remain inside the skin; 3) wash the wound with soap and bath it in hot water for 90 minutes or more; 4) rest with the limb elevated; 5) seek medical help. Try to get an antivenin if it is available.
Peter Marren wrote: It is some of the smaller beasties that really make your flesh creep. The nasty little fish so adept at slinking into human orifices and getting stuck is, unfortunately, far from an urban legend. Those fishing for cone shells risk being stabbed with a toxic mini-harpoon that can be fatal. The larvae of the thimble jellyfish are almost invisible but make themselves known painfully when they get inside swimming trunks.Nor did I like the sound of the vampire moth that "occasionally drills into a human victim, causing a few hours of itchy irritation". [Source: The Independent, December 9, 2011]
Katherine Harmon Courage wrote in Smithsonian magazine, Striped eel catfish are an ornamented catfish species have fins that hide spines that deliver venom and can be fatal to the touch; fishermen cleaning nets in the Persian Gulf and Red Sea have reportedly been killed by exposure. Curiously, specimens that migrated to the Mediterranean Sea are thought to be less toxic. [Source: Katherine Harmon Courage, Smithsonian magazine, December 2020]
Book: The Book of Deadly Animals, By Gordon Grice
See Lion Fish, Cone Snails, Blue Ring Octopus, Box Jellyfish, Sea Urchins, Scorpionfish and Sea Snakes
Ciguatera Fish Poisoning
Ciguatera fish poisoning (CFP), also known simply as ciguatera, is a foodborne illness caused by eating reef fish whose flesh is contaminated with certain toxins. Cigatera poisoning is a danger but rarely fatal. Barracuda is the most toxic fish and always should be avoided. Red snapper, grouper, amberjack, moray eel, sea bass, sturgeon fish and a wide range of tropical fish contain the toxin at unpredictable times.
Reef fish sometimes ingest toxin-containing algae of the dinoflagellate genus Gambierdiscus, often found in subtropical and tropical coral reefs. By ingesting contaminated fish, humans may contract ciguatera poisoning. Symptoms include gastrointestinal issues (diarrhea, vomiting, and nausea), cardiovascular distress (hypotension, bradycardia), and neurological problems (fatigue, joint and muscle pain, and numbness or tingling of extremities).
Symptoms: Diarrhea, vomiting, numbness, itchiness, sensitivity to hot and cold, dizziness, weakness. Heart difficulties such as slow heart rate and low blood pressure may also occur.
Usual onset: 30 minutes to 2 days after eating the poisoned fish
Duration: Few weeks to months. Diarrhea may last for up to four days. Some symptoms typically remain for a few weeks to months.
Diagnostic method: Based on symptoms and recently eating fish
Differential diagnosis: Paralytic shellfish poisoning, neurotoxic shellfish poisoning, scombroid food poisoning, pufferfish poisoning
Treatment: Mannitol, gabapentin, amitriptyline
Prognosis Risk of death less that 0.1 percent. [Source: Wikipedia]
The toxins that cause they illness are ciguatoxin and maitotoxin. They are originally produced by a small marine organism, Gambierdiscus toxicus (microalgae), that grows on and around coral reefs in tropical and subtropical waters. It attaches to seaweed. Fish absorb the toxin when they eat seaweed. Fish eat fish that eat the the seaweed and the the toxin accumulates and becomes concentrated. Food poisoning is caused by eating toxin-carrying fish,
Ways to avoid it include not eating reef fish, not eating high-risk fish such as barracuda, and not eating fish liver, roe, or fish heads. Ciguatoxin has no taste or smell, and cannot be destroyed by conventional cooking. There is no specific treatment for ciguatera fish poisoning once it occurs. Mannitol may be considered, but the evidence supporting its use is not very strong. Gabapentin or amitriptyline may be used to treat some of the symptoms. In 2017, the United States Centers for Disease Control (CDC) estimated that around 50,000 cases occur globally each year. Other estimates suggest up to 500,000 cases per year.
Ciguatera fish poisoning (CFP) is the most frequent seafood poisoning. It occurs most commonly in the Pacific Ocean, Indian Ocean, and the Caribbean Sea between the latitudes of 35°N and 35°S. The risk of the condition appears to be increasing due to coral reef deterioration and increasing trade in seafood. Descriptions of the condition date back to at least 1511. The current name, introduced in 1787, is of Cuban Spanish origin and originally referred to the univalve mollusc Cittarium pica.
Mesophotic Coral Ecosystems
Mesophotic coral ecosystems exist in low light — "meso" means middle and "photic" refers to light. They are found in tropical and subtropical regions at depths ranging from almost 30 to over 150 meters (100 to over 490 feet) below the ocean’s surface. The dominant communities providing structural habitat in the mesophotic zone are corals, sponges, and algae. [Source: NOAA]
Little is known or understood about these ecosystems because until recently, studies were hampered by lack of technology. The upper limit of mesophotic coral ecosystems coincides with the diving limit for conventional scuba diving (130 feet), but is too shallow and costly for most deep-diving technologies, such as remotely operated vehicles and submersibles, to operate in. However, advances in undersea technologies in the past decade now make it possible to investigate these ecosystems.
In an era of significant changes occurring on shallow coral reefs, it is important for scientists to understand the role of mesophotic coral ecosystems in tropical and subtropical regions. These ecosystems are regarded as extensions of shallow coral ecosystems and share common species. As a result, scientists hypothesize that mesophotic corals may serve as potential sources to reseed or replenish degraded shallow-water reef species.
Mesophotic coral ecosystems also serve as essential fish habitat for some economically and ecologically important fish species, which use these areas for spawning, breeding, feeding, and growth to maturity. Similar to shallower coral ecosystems, mesophotic coral ecosystems contain organisms with specialized defenses to ward off predators and microbial infections. These specialized defenses often yield compounds that can be used to develop natural products that benefit human health.
Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated March 2023