Coral Reefs

Coral Reefs

Habitats & Adaptations

The oceanic habitats support a great diversity of life and ecosystems (Ocean Literacy Principle #5)

Corals are found throughout the world’s ocean in shallow and deep water however, reef-building corals are found mainly in tropical waters.   These corals require clear, warm and shallow water for their zooxanthellae (photosynthetic symbiotic algae) to capture the necessary light and nutrients for photosynthesis.  Most reef building corals contain zooxanthellae that help corals make their hard-outer exoskeleton from CaCO3calcium carbonatethey extract from the seawater (corals can produce the skeletons on their own but much slower).  However, there are many different types of corals including feathery like flowing sea fans, solidary corals and even deep-sea cold-water corals.

There are three main types of tropical coral reefs - fringing reef, barrier reefand atoll.  Fringing reefs are the most common and grow in a narrow band (fringe) nearby the shore.  Barrier reefs are also found along the coast but are typically farther out from shore than fringe reefs and are separated from the shore by a deep lagoon.  Atolls are a ring of coral reefs that surround one central lagoon and are usually found far from land.  Coral atolls can be on top of a subsided (submerged) volcano, seamount or formed from corals growing on top of a subsiding island. 

These slow growing animals build reefs that cover less than 1% of the earth’s surface yet are the most biologically diverse and complex structures in the marine environment!  A quarter of all marine species depend on coral reefs for food and shelter.   Brightly colored small fish dart in and out of the reef crevices, molluscs and sponges attach directly to the coral structure, crustaceans like shrimp and crabs can be found hiding among the many nooks while larger fish like groupers and predators such as sharks roam the waters keeping the ecosystem in balance.  Tropical coral reefs are home to a myriad of species and complex interactions that creates one of the most productive ecosystems that is vital to the sustainability of our earth.     

Learning Objectives:

  • Describe how flora and fauna are uniquely adapted to thrive in tropical coral reefs 
  • Discuss the symbiotic relationships between corals and zooxanthellae 
  • Differentiate between how animal species rely on coral reefs  

Analyzing Adaptations Flora and Fauna in Coral Reef Habitats: Species Found in Coral Reefs

A healthy reef is a biologically diverse ecosystem supporting a great number of species that in turn help make the reef more resilient and able to withstand disturbances.  Coral Reefs are home to a variety of species that utilize the unique structure of the reef environment for various needs – some live there year-round, some use the habitat for nurseries, some pop in for a cleaning or to feed on the ample prey.  Sharks, rays, octopus, seastars, anemones, worms, snails, fish of all sizes and shapes and more rely on coral reefs for survival.  When coral reefs are damaged and destroyed the cascading effects on all species is widespread. 

Year-round Residents

There are many marine species that live year-round on the coral reefs – some are sessile attached to the corals while other swim and dart about.  Daily life on the reef consists of searching for food, a mate (or two!), carving out a home (competition for space is fierce!) or resting.   

Coral reefs are home to a variety of hard corals (the reef building types) and soft corals (such as sea whips that are bendable).  The many invertebrates that inhabit the reef year-round include sponges, seastars, shrimp, crabs, sea cucumbers and worms.  In addition, there are a variety of fish speceis that call the reef home. 

The Giant clam is the largest marine bivalve and can live up to 100 years!   Growing as large as 4 feet and weighing over 500 pounds, these sessile bottom dwelling species have a similar symbiotic relationship with zooxanthellae as corals.  The symbiotic algae live inside the clam’s tissues providing it with nutrition and are responsible for the clam’s bright colors. In return the algae are afforded protection and when the clams open up their large shells during the day, the algae have access to the sunlight needed for photosynthesis.  In addition to nutrients supplied by the alae, giant clams are filter feeders meaning they filter small particles of plankton from the water.  If the clam is unhealthy and their algae are dying, the clams will bleach white in the same fashion as stressed corals that have expelled their zooxanthellae.   

Grey Reef sharks are typically gray with a white ventral side.  All their fins are black tipped except for their dorsal fin which is white tipped. The gray reef shark prefers the warm, shallow waters of coral reefs and is one of the most common sharks found in the Indo-Pacific region.  It can grow up to 8 feet and, in the wildlive for 25 years.  Sociable animals, gray reef sharks gather in schools of over 100 individuals!  They feed on squid, bony fish, crabs and octopus.  These sharks are active both day and night!  Most notable about the grey reef shark is its highly sensitive sense of smell which aids in hunting their prey as they cruise along the open waters over the coral reef.  Like many sharks, the gray reef shark is an apex predator helping to keep the coral reef ecosystem healthy and in balance by keeping populations from growing too large. 

Visitor Species

Coral reefs are a host to a variety of species - all that have a different need or niche the reef play a role in fulfilling even if they are just visiting the reef.  For the Green Sea Turtle that need is a good cleaning!   Green sea turtles live throughout the topics and subtropical ocean basins but visit coral reefs when they become dragged down by too many barnacles, algae or other organisms that attach to their shell.  Cleaner fish and  cleaner shrimp and their “cleaner stations”   are well known to other species. Moray eels and  green turtles will visit these cleaner stations on coral reef engaging in a symbiotic relationship with the cleaner fish or shrimp.  The eels and turtles get a good cleaning, ridding their skin and shells of parasites, algae, bacteria, or other nuisances and the fish/shrimp get a free meal! 

Nursery Grounds

Many reef fish spawn in waters over coral reefs.  Chemical cues within the water column and the moon phases indicate to these fish (and many other reef species) that it is time to swim up into the water column and release their eggs.  The eggs are carried away on the ebbing tide helping to ensure enough eggs escape the waiting mouths of predators for the fish populations to survive.  Once hatched, the same young ride the incoming tides back to the coral reef to settle continuing the cycle. The multiple nooks and crannies of the reef provide ample hiding spots from hungry predators and the abundance of food is just what a young fish needs to grow strong! 

Flora and Fauna Adaptations

Coral reefs are often referred to as the rainforests of the sea!  These biodiverse ecosystems are home to many different species of flora and fauna that are adapted to live in shallow, low nutrient waters.   Corals, invertebrates, and reef fish have adaptations necessary for survival in an ecosystem where waters ebb and flow with the tide as well as the challenges of competition for substrate and limited hiding places from predators. 

Flora Adaptations

Zooxanthellae, microscopic algae have adapted to live inside coral polyps.   These single celled plants provide the corals with up to 60% of their nutrition from photosynthesis and aid in the production of the coral's outer calcium carbonate skeleton.  In turn the corals provide protection,CO2 and other nutrients the zooxanthellae require for photosynthesis.   

Fauna Adaptations

Corals have more than one way to reproduce.  A single polyp can break off or divide into new polyps and they can also reproduce sexually during which corals release eggs and sperm into the water column.  This massive broadcast spawning occurs on during certain nights of the year, usually just after a full moon between October and December.   Both male and female corals release their gametes into the open waters at the same time.  This simultaneous release increases the chances of fertilization, escape from predators and greater genetic diversity as gametes from different corals combine. 

Reef fish have also evolved to survive in the coral reef environment by adapting their body shape and structure.   Coral reefs have many nooks and crannies of which some are tight spaces.  These fish have a flatter body that allows them to maneuver better and seek shelter among the crevices in order to hide from a predator.   The brightly colored patterns of reef fish are also an adaptation.  Some fish, such as the Butterfly fish, have a false eye – a dot on its dorsal fin that confuse predators into thinking its rear is actually its head.  Additionally, the bright coloration of reef fish helps them blend into the surround corals, camouflaging them from predators.  

Some corals have adaptations to survive coral bleaching. That is, they have their own natural protection. They produce a kind of sunblock, called a fluorescent pigment. These pigments control the amount of light entering the coral tissue by reflecting visible and harmful UV rays. During times of high temperatures, the fluorescent pigments help the corals resist bleaching.   

Key Vocabulary 

Barrier Reef
Bony Fish
Broadcast Spawning
Calcium Carbonate
Fringe Reef

Environmental Importance

The ocean ecosystems made the Earth inhabitable (Ocean Literacy Principle #4)

Coral reefs are highly productive and diverse ecosystems that provide a multitude of ecosystem services and are vital to the health of the planet and its inhabitants.   Some of these services include protecting coasts from storms and waves, providing habitat and shelter for various species of fish, invertebrates and other marine life.   Coral reefs also provide economic benefits in the form of fishing, tourisms and recreation.   

More than 25% of all marine life rely on coral reefs for all or portions of their lives. Humans also depend on coral reef ecosystems for survival.    Approximately 1/8 of the world population live near the coast and rely upon coral reefs and their surrounding waters for subsistence.    As storms become more frequent and intense, coral reefs play an even more important part in protecting our coastlines.  These immense structures act as a breaker absorbing storm and wave energy thereby mitigating the amount of surge hitting the coastline and reducing erosion.   

Many corals utilize a type of chemical warfare in defense from predators including other corals trying to overtake their space.  Modern medicine is looking at the chemical compounds of corals for potential treatment of diseases. 

Coral reefs have so many benefits and functions on a global scale that any imbalance or damage to these ecosystems will have cascading effects, some of which are still unknown. 

Learning Objectives:

  • Identify 3 ways that coral reefs are environmentally important 
  • Explain the significance of reducing storm and wave energy 
  • Comprehend the significance of indicator species and importance of biodiversity for healthy resilient ecosystems. 

Analyzing the significance of coral reef functions in the global environment  

Tropical coral reefs are one of the most diverse ecosystems providing vital and valuable ecosystem services.  Coral reefs protect shoreline from storms reducing the wave energy, provide habitat for over 25% of all marine species, provide economic and recreational opportunities to millions of people and are a source of food and medicine.  


Coastal Protection

Coral polyps build their calcium carbonate exoskeleton upon previous generations of corals.  In doing so, large, vertical structures are created.   The coral’s symbiotic  zooxanthellae require ample amounts of sunlight for photosynthesis therefore most of the coral reefs are built in shallow offshore waters.    When storms such as hurricanes approach land the coral reef acts as a natural barrier causing the waves to break or slow down farther away from the coastline. In doing so, the wave energy impacting the shoreline is abated which in turn reduces erosion and coastal flooding.  Did you know coral reefs can absorb a lot of energy from waves...over 90%!

Indicator Species

An indicator species is a species whose presence, population size and health are a direct connection to the health of their habitat.  Coral reefs are sensitive to changes in temperature, water quality and pH levels of the ocean.   As sea surface temperatures rise corals become stressed and respond by expelling their zooxanthellae.  The colorful corals become pale and eventually turn white or bleached.  Corals also will expel their zooxanthellae when the normally clear water become cloudy from too much sediment (erosion), when an influx of nutrients result in algal blooms or when the pH of the waters become more acidic as a result of the oceans absorbing excess atmospheric CO2

By monitoring the condition of corals we are alerted to changes in the health of the ocean waters surrounding the coral reefs and can take steps to mitigate potentially deadly results.

Biologically Diverse Ecosystem

Biodiversity refers to the variety of and population size of different species inhabiting an ecosystem. The more diverse an ecosystem is, the more resilient it is and better able to withstand disturbances including effects from the changing climate. 

Coral reefs are found in 109 countries around the world and play a very important role in protecting the earth’s biodiversity.  Approximately 25% of all marine species rely on coral reefs either directly or indirectly for survival.      

Carbon Sink

The ocean ecosystems are efficient in recycling nutrients, especially carbon.  The ocean absorbs atmospheric carbon dioxide which is readily take up by phytoplankton and used in the process of photosynthesis.  These organisms are prey for different types of herbivores, which in turn are food for larger predators.  As the large predators die and decompose on the ocean bottoms, bacteria and other decomposers continue the recycling the nutrients and carbon dioxide – essentially locking away the atmospheric carbon dioxide through the efficient biological pump cycling.  Corals and other organisms that build their exoskeletons  and shells out of calcium carbonate also play a role in locking away carbon dioxide.  Clams, oysters and corals extract carbonate and calcium from the seawater to form their shells.   


Want to learn more about Biodiversity?  Watch our short videos lesson on the importance of Biodiversity

Key Vocabulary  

Atmospheric CO2
Indicator Species

The Changing Climate

The ocean is a major influence on climate (Ocean Literacy Principle #3)

Corals have experienced many extinction events over the past 500 million years due in large part to substantial changes to the environment.    The changing climate and human activities can lead to the destruction of coral reefs and reduce or eliminate the ecosystem services provided by coral reefs.  The Great Barrier Reef formed about 20 million years ago and is currently in a state of decline due to similar drastic climatic changes that ultimately led to the disappearance of coral reef throughout the millennia.  Twenty-two coral species are listed on the Endangered Species Act as threatened and three as endangered. 

Learning Objectives:

  • Develop an understanding of the interdependence humans have with coral reefs including how our actions directly or indirectly change coral reef dynamics 
  • Identify and understand sources of climate change, describing and explaining the causes and discuss the adverse effects as well as long-term solutions 

Anthropogenic Actions and Climate Change

Human activity has altered the climate through increased input of carbon dioxide (and other greenhouse gases) into the atmosphere, decreased biodiversity through overfishing and habitat loss, disrupted the intricate ocean food webs, decreased populations of apex predators and loss of critical habitat  These actions increase the earth’s global average temperature, add excess nutrients and runoff into the sea as well as speed up the melting of land and sea ice of which all have an adverse effect on coral reef ecosystems. 

Sea Level Rise

Humans have been burning fossil fuels since the industrial revolution.  This process of using coal, oil and gas for energy  produces gases including CO2,  CH4 and N2O collectively known as greenhouse gases, as a by-product.   In the atmosphere, these gases in appropriate levels allow a certain amount of the sun’s energy in while allowing for excess heat to be radiated back out to space warming the earth from the outer atmosphere inward.  Just the right amount of radiated heat allowed in enables the earth to be at the right temperature for life to exist.  However, with  the increase of more greenhouse gases trapped within the atmosphere the earth is heating up faster.  

As the global surface temperature rise glaciers are melting and the meltwaters are flowing into the ocean.   Since 1993 the ocean has risen by 95 mm.  This may not seem like a big increase but approximately 1/3 of  people worldwide live in low lying areas near the coast.  Many in major cities like New York, Miami, Bangkok and Shanghai plus many small island nations such as the Maldives, Grand Bahama, Solomon Islands and Micronesia are already under threat of rising seas. The warming of the earth is also causing the ocean sea surface temperatures to increase  - 90% of the increased heat is trapped by the ocean - which results in thermal expansion which indirectly results in increased sea levels.  

Ocean Acidification

The ocean on average absorbs approximately 1/3 of anthropogenic atmospheric CO2.   As increased amounts of CO2 is emitted into the atmosphere the ocean is absorbing more and more CO2 ultimately changing the chemistry and pH.  Prior to industrial times, the pH of the ocean was on average 8.2, however the pH has since dropped to 8.07.  This may not seem like a big change but remember the pH scale is logarithmic meaning a one unit change is equivalent to a tenfold change - the current decrease represents about a 30% change towards acidity!  Even seemingly small changes in ocean pH can be detrimental to species that require carbonate ions to build their outer calcium carbonate shells/ exoskeleton like oysters, corals.  As the oceans absorb CO2chemical reactions take place that cause more hydrogen ions to be available.  These hydrogen ions (H+) will bond with carbonate ions (CO3 -2) forming bicarbonate ions (HCO3-) locking away carbonate ions corals and shell building organisms  need.  A 0.1 unit drop in pH has resulted in a 20% decrease in available carbonate ions. Increasing amounts of CO2 and acidity make it difficult for organism to form their exoskeleton and shells or weakens their existing exoskeleton/shell leaving them vulnerable to predation and disease.  Research conducted on the Great Barrier Reef has found the coral skeletons have a lower density than pre 1950s. Not only will corals struggle to build their calcium carbonate exoskeleton, but studies have also  shown that larval corals are being impacted as well.  Some species of planktonic larval corals are unable to find suitable substrate to settle on thereby impacting the success of future generations of coral reefs and all the organisms that rely upon the reef ecosystem for survival.   

Invasive Species

An invasive species is a plant or animal that causes harm, either ecologically or economically, in an ecosystem where it is not native.  These aliens have  multiple  advantages  over the indigenous species and can cause serious damage in a short time to the invaded ecosystem altering the community structure, decreasing biodiversity and function of the ecosystem.  Most invasive species are opportunistic easily adapting to the new environment finding plentiful sources of food, reproduce rapidly and do not have to worry about predators (most invasive species are not palatable to local predators).   

The lionfish is native to coral reefs in the Indo-Pacific waters and are known to eat over 100 different species of fish!  It was introduced into the waters of the Caribbean and along the coasts of Florida in the mid 1980s where its populations have exploded threatening the health of the coral reefs.   Lionfish eat herbivorous fish such as the parrotfish that eat algae.  These fish help keep the algae under control and protect the corals from being taken over by algae and ultimately killing the corals.     A single lionfish can reduce the recruitment of   native fish by 79% meaning fewer young fish settling in the coral reef waters resulting in a less diverse ecosystem. 

Marine Debris

Most marine debris originates from land-based sources, some from areas far away from the ocean, however the effects on the marine ecosystem are far reaching. Wind, ocean currents and gyres move trash throughout the ocean surface and water column.  Plastic water bottles, plastic bags, discarded fishing line, plastic bottle caps, packaging material are just some of the marine debris that is wreaking havoc on the marine environment including coral reefs.  As societies increase their use of single use and throwaway products, the more trash and damage is done to the marine environment. 

Plastic bags can smoother coral colonies, floating debris such as fishing nets, lines or traps get caught on coral, especially branching corals, damaging and/or breaking the corals.   As corals become damaged or destroyed, algae and other opportunistic species take over drastically  altering the reef ecosystem and food web.  Many species that are part of the coral reef ecosystem are directly affected by marine debris – sea turtles mistake floating plastic bags as jellyfish – their favorite food, small invertebrates and zooplankton ingest  various types of plastic debris.   These animals die from starvation with their stomachs filled with plastic or choke on pieces of trash. 


Key Vocabulary 

Apex predators
Bicarbonate ion
Carbonate ion
Invasive species       
Marine debris  
Opportunistic species

Shipping and Navigation

The ocean and humans are inextricably interconnected (Ocean Literacy Principle #6)

Humans have been taking to the sea since the 1200s when the Phoenicians began using celestial navigation to voyage across the ocean. Since then, we have made tremendous strides in advancing technology for sea-going voyages, navigational tools, harvest of ocean resources and in our understanding of our actions impact on the ocean ecosystems

Learning Objectives:

  • Identify efforts being made to reduce the impact of vessels on coral reefs 
  • Understand how changing ocean water color indicate the presence and depths of coral reefs 

Balancing Commerce and Conservation

As more and more people flock to tropical islands for vacation and recreational activities, the greater the chance is for damage to be incurred on coral reefs – a popular tourist and fishing spot.  Anchors from boats of all sizes can cause serious damage to coral reefs destroying and/or breaking corals.  The damage extends beyond the corals themselves to the species that rely on the coral reef ecosystem for survival.  Diminished coral reef coverage reduces nursery habitat for many species, fewer hiding spaces for small reef fish and decreases the ecosystem services such as storm protection for coastal areas. However, with the installation of permanent mooring buoys and an increased awareness of the location and management of coral ecosystems, coral reefs and boats can coexist

Permanent Anchor Mooring Sites

In the Florida Keys National Marine Sanctuary, efforts to conserve and prevent anchor damage to coral reefs has been underway since 1997.    Permanent mooring buoys are installed and used in areas where boats typically anchor.  These collaborative projects bring together governments, divers, fishermen, sailors and the public to select, install and monitor the stations.    Boats tie up to the moored buoys instead of dropping anchors which damage or destroy fragile coral reefs The initial installation of the moored buoys  requires research into the best area to drill into the seafloor, takes into consideration the number and size of boats in order to balance the need for conservation along with commerce.   Scientific research has shown a  decrease in amount of damage to coral reefs at  permanent moored buoy sites compared to non-moored sites throughout the tropics.  

Tips for safe navigation near coral reefs

Many people enjoy sailing, snorkeling,  fishing or scuba diving near coral reefs.  The clear blue waters and abundant marine life is a constant draw for people all around the world.  However, the influx of  visitors to coral reefs can cause much damage to these fragile and vital ecosystems.  Many people rely on navigational chartsGlobal Positioning System (GPS) and maps to boat through and near coral reefs.  Unfortunately, not all maps are current (as we read earlier a new coral reef was recently discovered!) nor contain  complete details and information.   A few tips for safe charting of waters near coral reefs are: 

  • Keep the sun at your back!   This reduces glare on the water which can make spotting corals more difficult 
  • Pay attention to changes in water color (wearing polarized sunglasses helps!) which is an indication of depth. As the waters become light green and pale yellow you are entering an area  that may be 12 feet or less in depth and the possibility of shoaling (grounding) is danger. 
  • The color of the sea bottom is also an indicator to the presence of corals or sea grasses.  Dark brown or black patches are the location of corals and sea grasses 
  • When in doubt – stay out!

Key Vocabulary 

Celestial navigation
Global Positioning System (GPS)
Mooring buoy
Navigational Chart


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Next Generation Science Standards

MS-ESS-3 Earth and Human Activity. Application of scientific principles for monitoring and minimizing human activity on the environment  

 LS1A-Structure and function of animals as it applies to their habitat  

LS1B- Growth and development of organisms  

LS2A- Interdependent relationships in organisms  

LS2C- Ecosystem Dynamics, Functioning and Resilience  

LS4A- Evidence of common ancestry and diversity  

LS4C- Adaptation  

HS-LS2-2 Ecosystems: Interactions and Dynamics 

HS-LS4-6 Biological Evolution: Unity and Diversity 

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