What are Corals? Animal Types, Polyps and Coral Bleaching

Table of Contents

What are Corals?

Corals are animals or marine invertebrates of the phylum Cnidaria within the class Anthozoa. They are the important reef builders that secrete limestone (calcium carbonate) to form a hard skeleton and inhabit tropical oceans. Corals form compact colonies of genetically identical sac-like animals called polyps. Almost all corals are colonial organisms and compose of hundreds to hundreds of thousands of polyps.

Also, the term ‘coral‘ is applied to the skeletons of these animals, especially those of the stonelike corals.  Corals are characterized by external or internal skeletons of a hornlike, stonelike, or leathery consistency.

These hard outer skeletons of limestone (calcium carbonate) attach either to the dead skeletons of other polyps or rocks. However, the shape and appearance of a coral are dependent upon its species location, type, depth, water movement, and many other factors.

Most corals get the majority of their nutrient and energy from unicellular dinoflagellates (algae) of the genus Symbiodinium that are photosynthetic.

These algae are commonly known as zooxanthellae and live within the tissues of the corals, giving the coral color. There is mutualism between these corals and algae. These microscopic algae are well protected and use the metabolic waste products of corals for photosynthesis. However such corals that rely on zooxanthellae grow in clear shallow waters and require sunlight. Usually, they are found in waters with depths less than 60 meters (200 feet).

These corals benefit from zooxanthellae because they remove wastes,  produce oxygen, and supply the organic products of photosynthesis that the corals need to thrive, grow and build up the coral reef. However, this mutual exchange is the reason why coral reefs are the largest structures of biological origin, globally.

However, some corals catch small fishes and plankton using stinging cells on their tentacles. Such corals live in much deeper water and do not rely on zooxanthellae. For instance, some can survive as deep as 3,300 meters (10,800 feet) in water habitats. Some have been found as far north as the Darwin Mounds, Scotland, northwest of Cape Wrath,  and off the coast of Washington state and the Aleutian Islands.

Coral Polyps

Since corals are colonial organisms, they are composed of hundreds to thousands of individual animals. These individual animals are called polyps. This means each individual coral is a polyp and most structures people call coral are made up of hundreds to thousands of tiny coral polyps.

Corals are madeup of hundreds to thousands of polyps
A: Colony of Antipathes (Black coral)
B: Enlargement of a single polyp
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson

A polyp is a sac-like animal that is usually a few centimeters in height and has a diameter of few millimeters. These coral polyps live on the limestone exoskeletons of their ancestors and add their own exoskeleton to the already existing coral structure.

Therefore, each coral polyp excretes an exoskeleton near the base and the coral reef gradually grows as the centuries pass. With one tiny exoskeleton at a time, the reef becomes a massive feature of the marine environment.

Each soft-bodied polyp secretes hard outer skeletons of limestone (calcium carbonate) that attach either to the dead skeletons of other polyps or rocks. Stony or hard coral polyp colonies grow, die, and endlessly repeat the cycle of laying the limestone foundation for coral reefs and gives shape to the familiar corals that reside there.

Many coral colonies can live for a very long time, due to the cycle of growth, regeneration, and death among the individual polyps.

A polyp has a stomach that opens at only one end. This opening is the mouth and a set of tentacles surrounds it. These tentacles are for defense and the polyps use them to capture small animals for food. Also, they use them to clear away debris.

However, food enters the stomach of the polyp through the mouth. Additionally, waste products are expelled through the same openings after the food is consumed.

What do Corals eat?

What corals eat varies from organic particles to zooplankton or small fishes. Most corals feed at night and in order to capture their food, they use stinging cells. These stinging cells are called nematocysts. The nematocysts are located in the outer tissues and tentacles of the coral polyps. These nematocysts are often powerful and lethal toxins that are essential for the corals to capture their prey.

Depending on the size of the coral polyp, its prey ranges from nearly microscopic animals (zooplankton) to small fishes. Hence they feed on zooplankton and small fishes. In addition to using their tentacles to catch their prey, many corals collect and draw into their mouth, organic particles in mucous strands and film.

Are Corals animals?

Corals are sessile animals. They take root on the ocean floor and that is why most people think they are plants or sometimes rocks. However, they do not manufacture their own food as plants usually do. Rather they have tiny tentacle-like arms which they use to capture food from the water and sweep it into their mouths.

Also, corals are alive, unlike rocks. The coral animal body consists of a polyp which is a hollow cylindrical form attached at its lower end to some surface. It has a mouth at the free end, surrounded by tentacles. The tentacles gather food and are armed with stinging structures (nematocysts) that paralyze prey.

Polyp of a hexacorallian coral( order Scleractinia)
Polyp of a Hexacorallian coral
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson Pg 279
Polyps of an Octocorallian coral
Polyps of an Octocorallian coral
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson

A coral polyp is an invertebrate and has a saclike body. Each polyp builds a hard cup-shaped skeleton by using calcium carbonate from the seawater. The skeleton helps protect the body of the polyp which is delicate and soft.

Currently, corals are classified as animals in the sub-classes Hexacorallia and Octocorallia of the class Anthozoa. They belong to the phylum Cnidaria.

The sub-class Hexacorallia consists of stony corals. These groups, generally have polyps that have a 6-fold symmetry.

The sub-class Octocorallia consists of soft coral and blue coral. These species of Octocorallia have polyps that have 8-fold symmetry, with each polyp having eight tentacles and eight mesenteries.

How do corals reproduce?

Individual colonies of polyps grow by asexual reproduction of the coral polyps. Also, corals breed sexually by spawning. Usually, around a full moon, polyps of the same species release their gametes simultaneously overnight. The fertilized eggs then form ciliated larvae called planulae. The planula is a mobile early form of the coral polyp that settles to form a new colony when mature.

Furthermore, the eggs and sperm produced by separate individuals, develop as an outgrowth in the gastrovascular cavity. These eggs and sperm are then expelled into the open water through the mouth. However, fertilization usually occurs in the water but occur sometimes in the gastrovascular cavity. The larva (planula) swims around for several days or weeks and then settles onto a solid surface. The planula then develops into a polyp.

Also, reproduction occurs by budding. A bud remains attached to the original polyp. Then a colony eventually develops by the continual addition and growth of new buds. As these new polyps develop, the old polyps beneath die, but the skeletons remain.

How do coral reefs form?

A coral reef begins when the coral polyp attaches itself to a rock on the seafloor. This one coral polyp begins to bud or divides into thousands and thousands of clones of itself. These polyps and their calcareous skeletons connect to one another. This creates a colony that acts as one. Hence, as this colony grows over the years, it will join with other colonies and form a reef.

Additionally, each coral polyp excretes an exoskeleton near the base and the coral reef gradually grows as the centuries pass. With one tiny exoskeleton at a time, the reef becomes a massive feature of the marine environment.

Coral Types

  • Soft corals
  • Sea whip
  • Sea pen
  • Sea fan
  • Black corals (Antipatharians)
  • Dendronephthya
  • Alcyonium digitatum
  • Hard corals
  • Blue coral
  • Pillar coral
  • Leptopsammia
  • Acropora
  • Great star coral (genus Montastraea)
  • Tubastraea
  • Boulder Star Coral (Orbicella annularis)

Generally, corals are classified into soft corals and hard corals.

  • Soft corals

Soft corals don’t have a rock-like calcareous skeleton as hard or stony corals. Instead, they are soft and bendable and often resemble trees and plants. They usually resemble brightly colored plants and are easy to differentiate from stony corals. Soft corals grow fleshy rinds for protection and wood-like cores for support. These woodlike cores comprise structural proteins like gorgonin and those similar to those proteins of animal horns and nails. Their polyps have tentacles that give a feathery appearance and occur in numbers of eight.

Soft corals are found generally in caves or ledges in oceans and are referred to as Ahermatypes. They are called Ahermatypes because they are non-reef building corals. These corals do not always have a symbiotic association with zooxanthellae. However many of them still utilize the presence of zooxanthellae.

Red whip corals are soft corals
Red Whip Coral
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson
Dendronephthya sp. is a soft coral on a Pacific coral reef
Dendronephthya sp. is a soft coral on a Pacific coral reef
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson

Examples of soft corals are Dendronephthya sp, seas fans, and sea whips. They do not produce calcium carbonate, instead, they contain spiny skeletal sclerites. These spiny skeletal sclerites give them some protection and support. They have internal skeletons consisting of separate calcareous needle-like structures (spicules). Soft corals prefer to live in waters with less intense light and are rich in nutrients. Fleshy true soft corals have no rigid internal skeleton at all in other parts of the world.

Some species of soft corals are platelike in form and others have fingerlike projections.

Sea fan

The horny corals like sea fans are actually the most numerous in shallow tropical waters. These corals are ribbonlike or branching in form. Soft corals also include the precious coral which is also called rose coral or red coral used in jewelry.

Soft corals Examples

  1. Sea whip
  2. Sea pen
  3. Sea fan
  4. Black coral (Antipatharians)
  5. Dendronephthya
  6. Alcyonium digitatum
  • Sea whip

Sea whips (Leptogorgia virgulata) or colorful sea whips are species of soft coral that belong to the family Gorgoniidae. These corals are about 20 cm in height and most times between 15-60 cm as an adult. They do not have a rigid calcium carbonate skeleton as true corals but their stalks have an internal axial skeleton that is stiffened by sclerites and covered by coenenchyme.

These sea whips can be found on shallow-water reefs, in estuaries, and bays. This is because they can tolerate low levels of salinity. Even though they prefer high salinity environments. The sea whips are seen growing on rocks at depths down to 20 meters (66 ft) and on the edges of rocks and limestone. They are found along the western fringes of the Atlantic Ocean and commonly found on the coastal shelves of Southern states. Their range extends from the Chesapeake Bay south to the Gulf of Mexico. Also, the sea fan species occur in Brazil.

  • Sea pen

The sea pens are soft corals belonging to the order Pennatulacea of marine cnidarians. However, they are grouped with the octocorals as with sea whips (gorgonians). They have a cosmopolitan distribution and are found in tropical and temperate waters. Inhabiting from the intertidal to depths of more than 6100 m.

  • Sea fan

Species of sea fans are sessile colonial soft coral. Instead of attaching themselves to hard substrates, they anchor themselves in mud or sand. Most of them are nocturnal and only extend their polyps during nighttime hours. However, they consume plankton and need strong currents to carry their food.

The sea fan (Gorgonia flabellum) is also known as the common sea fan, venus fan, or west Indian sea fan, purple gorgonian sea fan, or venus sea fan. This sea fan has an untidy shape and short stubby side growths that come out of the main plane.

In Gorgonia flabellum their branches are flattened at right angles to the fan plane. For Gorgonia ventalina, their branches are either flattened or round, parallel to the fan plane. Wide-mesh sea fan (Gorgonia mariae) is smaller and many of their branchlets do not interconnect.

  • Black corals (Antipatharians)

Antipatharians are also known as black corals or thorn corals. They are an order of soft deep-water corals. Black corals can be distinguished by their jet-black or dark brown chitin skeletons. These corals were initially classified in the subclass Ceriantipatharia but were later reclassified under Hexacorallia.

Black corals are whiplike, featherlike, or treelike in form and occur in the Mediterranean Sea, West Indies, and off the coast of Panama. Presently it is used for making jewelry. Historically, it was used in rituals and for medical treatment by pacific islanders. However, the black coral’s population has been declining due to climate change, poaching, and ocean acidification.

Antipatharians exist at nearly every depth and location except brackish waters. However, these corals are mostly found frequently on continental slopes under 164 feet deep.

  • Dendronephthya

Dendronephthya is a genus of soft corals belonging to the family Nephtheidae. There are over 250 species of soft corals in this genus.

These corals are sometimes kept in aquariums. However, they are notoriously difficult to keep because they require an almost constant supply of small foods (phytoplankton).

  • Dead man’s finger (Alcyonium digitatum)

Alcyonium digitatum or Dead man’s finger is a species of soft corals found around the coasts of the northern Atlantic Ocean. It belongs to the family Alcyoniidae. These colonial coral forms clumps of white,  yellow, or cream-colored fleshy masses of finger-like lobes. Their individual polyps are translucent and white, projecting from the leathery surface when feeding. This gives the colony a furry appearance. The polyps attach to bedrock, stones, and sometimes the shells of gastropods and crabs. One can see them plenty in areas with strong water movement and insufficient sunlight for algae to predominate.

They are usually seen in the sublittoral zone down to about 50 meters. These corals are found from Portugal to Norway along the Atlantic coasts of northwest Europe. Also, they are seen in parts of Canada and the northeastern coast of the United States. Alcyonium digitatumis is common around the coasts of Britain and Ireland.

  • Hard corals

Hard corals or stony corals are the most familiar and widely distributed forms. They are both solitary and colonial in their habit. These corals have more than 8 septa and simple tentacles. Most living stony corals, depending on the color of algae living on them are olive, brownish, or yellowish. However, their skeletons are always white. Hard or stony corals are referred to as hermatypic corals. This is because they are known as reef-building corals. These corals are actually a fundamental part of building the coral reef.

Staghorn coral is a stony coral
Staghorn coral
Photo credit: https://ocean.si.edu
Brain corals are hard corals
Brain coral

Their polyps of colonial forms are 0.04-1.2 inches in diameter. These polyps secrete skeletons of limestone which eventually become rock. The hard coral skeleton is almost pure calcium carbonate and is deposited in a cup-shaped form with the polyp inside. Hard corals require zooxanthellae that live within their tissues for their survival. These organisms are what give corals their colors, which is dependent upon where they live. These coral species can be found in all of the world’s oceans, though their populations are expected to decrease as a result of ocean acidification and global changes.

pillar coral is a hard coral
Pillar coral  Photo credit: https://sta.uwi.edu
Cup coral Tubastrea sp.
Cup coral Tubastrea sp.
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson Pg 279

There are about 800 known species of hard coral that occur in all oceans. These corals occur from the tidal zone to depths of nearly 20,000 feet. Hard coral examples are mushroom coral, brain coral, star coral, staghorn coral, and elkhorn coral.

Boulder star corals are hard corals
Boulder star coral
Photo Credit: Textbook (Integrated Principles of Zoology Fifteenth edition) By Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson Pg 279

Hard corals Examples

  1. Blue coral
  2. Pillar coral
  3. Leptopsammia
  4. Acropora
  5. Great star coral (genus Montastraea)
  6. Tubastraea
  7. Boulder Star Coral (Orbicella annularis)
  • Blue coral

Blue corals are hard corals found in reefs exposed to waves, intertidal regions, flats, and sometimes marginal habitats. They are seen in reefs with depths below 2 meters. The blue coral is endemic to the eastern central, western central, northwestern, and southwestern Pacific oceans. They are also seen in the eastern and western Indian oceans. Their range includes the Ryukyu Islands, Great Barrier Reef, Japan, and Australia. However, the blue corals’ largest colony is believed to be located in southwestern Japan, off Ishigaki Island in the Yaeyama Islands.

They have been declared a vulnerable species in the list of endangered species by the IUCN because their population tends to be decreasing in line with the destruction of reefs, globally. They are threatened by bleaching, acidification of the ocean, habitat destruction, climate change, and aquarium harvesting.

  • Pillar coral

Pillar corals (Dendrogyra cylindrus) are hard corals in the order Scleractinia and the only species in the monotypic genus Dendrogyra. They resemble fingers that are growing up from the seafloor without any secondary branching. Pillar corals are one of the few types of hard coral that the polyps can be seen feeding during the day. Unlike most corals, their skeleton is not usually visible. This is because the polyps are typically extended during the daytime.

They are large and can grow on both sloping and flat surfaces at depths between 3 feet to 65 feet. These corals are found in the warmer parts of the Caribbean sea and the western Atlantic Ocean.  The north coast of Jamaica and some islands in the Bahamas have plentiful numbers of pillar coral colonies. These corals seem to be absent from the coasts of Panama, Colombia, and Bermuda. Initially, they were common on the reefs off the coast of Florida but not anymore because, there, they have suffered from over-collection.

  • Leptopsammia

Leptopsammia is a genus that consists of stony cup corals. They belong in the family Dendrophylliidae. Hard corals of this genus do not contain zooxanthellae and are found at depths down to about 3,000 feet. This genus includes these coral species:

  1. Leptopsammia crassa
  2. Leptopsammia formosa
  3. Leptopsammia stokesiana
  4. Leptopsammia poculum
  5. Leptopsammia britannica
  6. Leptopsammia pruvoti
  7. Leptopsammia queenslandiae
  8. Leptopsammia chevalieri
  9. Leptopsammia columna
  10. Leptopsammia trinitatis
  • Acropora

Acropora is a genus in the phylum Cnidaria of small polyp stony coral. Some of the corals in this genus are known as elkhorn coral, table coral, and staghorn coral. The Acropora species are some of the main reef corals. They are responsible for building the immense calcium carbonate substructure. This structure supports the thin living skin of a reef.

Acropora corals can grow as plates, slender or broad branches, depending on their species and location. Their polyps can withdraw back into the coral in response to disturbance or movement by potential predators. However, they protrude slightly when undisturbed. These polyps extend further at night to help capture food from the water. Furthermore, Acropora corals are distributed in the Caribbean and Indo-Pacific.

  • Great star coral (genus Montastraea)

Great star coral is a colonial stony coral of the genus Montastraea. Montastraea is the only genus in the monotypic family, Montastraeidae. The genus contains a single species, which is the great star coral (Montastraea cavernosa).

Occasionally, the great star coral has a fluorescent red or orange color during daytime and its polyps are the size of a human thumb that extend fully at night. The coral however harbors endocellular symbiotic cyanobacteria in addition to the symbiotic zooxanthellae. This is probably to help it fix nitrogen.

Great star coral colonies domes in waters of shallow and moderate depths. They are found in the Caribbean seas and form into massive boulders and at times develop into plates. The coral has been observed to grow as a plate formation in deeper waters. This coral happens to be the predominant coral at depths of 40–100 feet.

  • Tubastraea

Tubastraea is a genus of large-polyp stony corals in the phylum Cnidaria. They are also known as sun polyps or sun coral. They have polyps in a variety of colors like orange, yellow, and black. Tubastraea is a cup coral in the family Dendrophylliidae and does not build reefs while they produce a hard skeleton. Tubastraea is seen in deep waters. This is because they do not need sunlight for nourishment. Instead, you see them colonize on artificial surfaces such as shipwrecks.

  • Boulder Star Coral (Orbicella annularis)

The boulder star coral lives in the western Atlantic Ocean and is the most abundant species of reef-building coral in the Caribbean. Originally, this coral was known as Montastraea annularis. This coral is the most thoroughly studied and has a comprehensive fossil record within the Caribbean.

The boulder star coral is known to exist at depths between 0 and 80 meters. They grew into varying colony shapes of heads, plates, or columns in response to different light conditions. Recently,  Orbicella annularis has been shown to be a complex of at least 3 separate species.These species are divided into Orbicella annularis, Orbicella franksi, and Orbicella faveolata.

Do Corals have brains?

Brain corals have the appearance of the brain, hence the name “Brain Corals“. Corals do not have brains but some corals have grooves and sulci similar to that of the brain and are therefore called brain corals.

Coral bleaching

Corals expel the zooxanthellae (symbiotic algae) that live in their tissues when they are stressed by changes in light conditions, temperature, or nutrients. This eventually causes them to turn completely white. This is what is known as coral bleaching.

A comparison of healthy and bleached polyps within a colony
A comparison of healthy and bleached polyps within a colony
Photo Credit: Dr. Ernesto Weil, Department of Marine Sciences, University of Puerto Rico.

However, a coral is not dead when it bleaches. Even though they can survive a bleaching event, they are under more stress and can be subject to death. Moreso, coral bleaching is of particular concern nowadays especially as our climate changes and atmospheric temperature rises.

A bleached elkhorn coral stands out among the healthy coral and turbinaria seaweed
A bleached elkhorn coral standing out among healthy coral and seaweed
Photo credit: https://www.news.ucsb.edu