Coral Reefs Explained: Biology, Types, and Threats

[00:00:00]Corals are a diverse group of invertebrate animals that are found in both shallow and deep water across the world’s oceans.

[00:00:10]Their appearance may be deceiving, but they are not plants.

[00:00:14]They are tiny, soft-bodied organisms related to sea anemones and jellyfish.

[00:00:22]Thousands of species of corals have been discovered.

[00:00:26]Coral reefs cover an estimated less than 1% of the ocean floor, but they support about 25% of all marine creatures.

[00:00:35]They are often called rainforests of the sea.

[00:00:40]Corals provide habitat for fish and many other species of marine life.

[00:00:46]The largest coral reef is the Great Barrier Reef, located off the east coast of Australia.

[00:00:52]There are several different types of shallow coral reefs.

[00:00:57]The majority of coral reefs are called fringe reefs.

[00:01:01]They fringe the coastline of a nearby land mass.

[00:01:05]Barrier reefs form when a coral reef grows around a volcanic island.

[00:01:10]Over millions of years, the volcano slowly sinks, but the corals continue to grow upward and outward.

[00:01:18]Over time, a lagoon forms between the corals and the sinking island.

[00:01:22]A barrier reef forms around the lagoon.

[00:01:25]Eventually, the volcanic island of a barrier reef becomes completely submerged and only the ring of corals remains.

[00:01:34]This is called an atoll (Marshall Islands pictured).

[00:01:37]Corals share the same simple structure as anemones, the polyp.

[00:01:42]The polyp contains digestive and reproductive tissues.

[00:01:46]The polyp has an open end with a mouth surrounded by a ring of tentacles.

[00:01:51]The tentacles have stinging cells, called nematocysts.

[00:01:56]The stinging cells allow the coral polyp to capture prey.

[00:02:00]The mouth also acts to expel waste.

[00:02:04]Most corals feed at night.

[00:02:06]Depending on the size of the coral polyps, a coral’s prey can range in size from nearly microscopic zooplankton to small fish.

[00:02:16]Individual coral polyps are usually less than a half inch (1.5 cm) in diameter.

[00:02:21]Mushroom corals have the largest polyps, which can be more than 5 inches across.

[00:02:27]Regardless of polyp size, colonial corals can be huge.

[00:02:32]Mounds can be the size of a small car.

[00:02:35]A single branching coral colony can cover an entire reef.

[00:02:41]In addition to catching their own prey, shallow water corals that live in warm water can have another source of food, the zooxanthellae.

[00:02:51]Corals can host zooxanthellae in their tissues.

[00:02:55]Zooxanthellae are single celled algae that photosynthesize and pass some of the food they make from the sun’s energy to their hosts.

[00:03:05]In exchange, the coral gives shelter and nutrients to the algae.

[00:03:11]This type of symbiotic relationship where both parties benefit is called mutualism.

[00:03:18]The algae require water temperatures between 70-85°F (22-29°C).

[00:03:24]Zooxanthellae provide much of the green, brown, and reddish colors of corals.

[00:03:31]The corals that build the hard structures of tropical reefs are reef-building corals called hard corals or stony corals.

[00:03:40]In these corals, each polyp sits in a cup made of calcium carbonate.

[00:03:45]This is called a calyx.

[00:03:46]The base of the calyx upon which the polyp sits is called the basal plate.

[00:03:54]The walls surrounding the calyx are called the theca.

[00:03:57]The coenosarc is a thin band of living tissue that connects individual polyps to one another.

[00:04:04]Periodically, a polyp will lift off its base and secrete a new basal plate above the old one.

[00:04:13]Polyps can extend out of the calyx to feed.

[00:04:17]They can contract when under stress so that they are not exposed above their skeleton.

[00:04:22]This protects the polyp from predators and the elements.

[00:04:27]Stony corals have smooth tentacles, often in multiples of six.

[00:04:32]Colonial hard corals, that may consist of hundreds of thousands of individual polyps, are cemented together by the calcium carbonate that they secrete.

[00:04:42]As colonies grow, they join with other colonies to form coral reefs.

[00:04:47]It takes a long time for a massive coral reef to grow.

[00:04:52]Each coral grows slowly, typically less than an inch per year (<2.5 cm).

[00:04:56]As corals die, they break into smaller pieces and become compacted.

[00:05:02]Just as trees do, each coral lays down annual rings so that the age of a colony can be identified.

[00:05:10]Individual coral colonies often live from decades to centuries.

[00:05:15]Some deep-sea colonies have lived for thousands of years.

[00:05:20]Reef-building corals exhibit a wide range of shapes.

[00:05:24]There are branching corals that branch out like trees.

[00:05:27]Digitate corals look like fingers.

[00:05:30]Table corals form flat structures.

[00:05:34]Encrusting corals grow as a thin layer against a substrate.

[00:05:39]Massive corals are ball-shaped or boulder-like.

[00:05:43]Mushroom corals resemble the tops of mushrooms.

[00:05:46]On the other hand, soft corals do not produce a hard calcium carbonate skeleton and do not form hard reefs.

[00:05:55]They may use more flexible materials or tiny stiff rods to build their skeletons.

[00:06:02]Soft corals have eight tentacles, each of which has tiny branches running along the sides.

[00:06:04]Soft corals are also colonial.

[00:06:05]Colonies often look like trees, bushes, and fans.

[00:06:09]Both stony corals and soft corals can be found in the cold, dark, deep sea, but they do not need sunlight or warm water to survive.

[00:06:20]Deep-water corals can be found at depths down to 20,000 ft (6,000 m).

[00:06:25]Coral reefs have to deal with many threats.

[00:06:29]Some fish, worms, snails, and sea stars prey on adult corals.

[00:06:34]Population explosions of predators like the crown-of-thorns sea star can be devastating to a reef.

[00:06:41]Corals also have to worry about competitors.

[00:06:44]They use the same nematocysts that catch their food, to sting other encroaching corals that are getting too close.

[00:06:52]Seaweeds are also dangerous competitors.

[00:06:55]They typically grow much faster than corals and may contain chemicals that injure the coral.

[00:07:02]Some coral colonies have animals like crabs and shrimp that live within their branches and defend their home against coral predators with their pincers.

[00:07:11]They gain shelter while the coral gains protection.

[00:07:15]Corals have may be male or female or both. They have multiple reproductive strategies.

[00:07:22]They can reproduce either asexually or sexually.

[00:07:27]Asexual reproduction helps to increase the size of the colony, and sexual reproduction starts new colonies and increases genetic diversity.

[00:07:37]Asexual reproduction involves the production of clones.

[00:07:41]This can occur through either budding or fragmentation.

[00:07:45]Budding is when a coral polyp divides, producing a genetically identical new polyp.

[00:07:52]Corals reproduce by budding throughout their lifetime.

[00:07:55]If a part of a colony breaks off and forms a new colony, this is called fragmentation.

[00:08:02]Fragmentation can occur as a result of a disturbance like a storm or being hit by fishing equipment.

[00:08:09]In sexual reproduction, eggs are fertilized by sperm, that are usually from another coral colony.

[00:08:17]Depending on the species, corals may use internal or external fertilization for sexual reproduction.

[00:08:25]Fertilized eggs develop into free-swimming larvae.

[00:08:29]Larvae float in the ocean for days or weeks before attaching to the substrate and growing into a new coral colony.

[00:08:37]Most stony corals are broadcast spawners and fertilization is external.

[00:08:44]Colonies release large numbers of eggs and sperm, called gametes, that are often glued into bundles (one bundle per polyp).

[00:08:53]The gametes rise slowly to the ocean’s surface.

[00:08:57]Spawning often only occurs once a year on cues from the lunar cycle and water temperature.

[00:09:03]In some places, spawning is synchronized for all individuals of the same species in an area.

[00:09:11]This type of mass spawning usually occurs at night.

[00:09:14]The synchrony makes it more likely for fertilization to occur.

[00:09:18]On the other hand, some corals brood their eggs in the body of the polyp and release sperm into the water.

[00:09:26]As the sperm sink, polyps containing eggs take them in and fertilization occurs internally.

[00:09:34]Brooders will often reproduce several times a year on a lunar cycle.

[00:09:39]Human pressures place a large toll on coral reefs and threaten their survival.

[00:09:46]Since coral reefs are often thriving with an abundance of fish, overfishing can affect the entire reef ecosystem.

[00:09:55]Fish play important roles on coral reefs, such as eating the seaweeds that would otherwise smother corals, Fish also eat the predators of corals.

[00:10:06]Rising water temperatures can cause corals to lose their symbiotic algae.

[00:10:11]This results in coral bleaching, where coral tissues lose all their color and turn near transparent.

[00:10:18]Coral bleaching can make them more susceptible to other threats and lead to coral death by starvation or disease if conditions don’t improve.

[00:10:28]Ocean acidification affects coral health by making less calcium carbonate available in ocean waters, making it harder for corals to form their skeletons.

[00:10:38]If acidification gets severe enough, it could even break apart the existing skeletons that already provide the structure for reefs.

[00:10:48]Boats and anchors can harm reefs by breaking or scarring them.

[00:10:53]Chemicals in certain sunscreens can also harm reefs.

[00:10:56]Runoff from lawns, sewage, cities, and farms feed algae that can overwhelm reefs.

[00:11:04]Seawater with too many nutrients speeds up the growth of seaweeds and increases the food for coral predators in their larval stage.

[00:11:13]Deforestation increases soil erosion, which clouds water and smothers corals.

[00:11:19]Coral reefs around the world are on the decline.

[00:11:23]Marine protected areas or (MPAs) are an important tool for keeping reefs healthy.

[00:11:30]Large MPAs protect areas like the Great Barrier Reef and the Northwestern Hawaiian Islands.

[00:11:36]Well-protected reefs typically have much healthier coral populations and are more resilient to storms.

[00:11:45]In the long run, the future of coral reefs will depend on efforts of reducing carbon dioxide in the atmosphere that is caused by the burning of fossil fuels.

[00:11:56]Carbon dioxide is both warming the ocean, causing coral bleaching, and changing the chemistry of the ocean through ocean acidification.

[00:12:06]Along with providing for marine organisms, coral reefs provide opportunities for recreation, education, scientific research and public inspiration.

[00:12:18]The fish we catch for food rely on corals to build the reef structure where they can breed and grow.

[00:12:25]Coral reefs support tourism and fishing industries worth billions of dollars.

[00:12:30]They protect coasts from waves and storms.

[00:12:35]Many current medicines that combat things like cancer, pain and inflammation have also been derived from coral reef organisms.

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