Stony corals facts for kids
Quick facts for kids Stony corals |
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| Scleractinian corals, illustration by Ernst Haeckel, 1904 |
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Stony corals, also known as hard corals, are amazing marine animals. They belong to a group called Cnidaria, which also includes sea anemones and jellyfish. What makes stony corals special is that they build hard skeletons.
Each individual coral animal is called a polyp. A polyp looks like a tiny cylinder with a mouth at the top, surrounded by tentacles. While some corals live alone, most live together in large groups called colonies.
When a new polyp settles down, it starts to make a hard shell from calcium carbonate. This shell protects its soft body. Solitary corals can grow quite large, up to 25 cm (10 in) wide. But in colonies, polyps are usually only a few millimeters across. These tiny polyps make copies of themselves through a process called budding. They stay connected, forming a big colony of clones that share one large skeleton. Some of these colonies can grow to be several meters wide or tall!
The shape of a coral colony changes based on its species and where it lives. Things like depth, water movement, and light all play a role. Many corals that live in shallow water have tiny, plant-like helpers inside their bodies. These helpers are called zooxanthellae. They give the coral its color and help it get food.
Like sea anemones, corals have special stinging cells called cnidocytes. Corals can reproduce in two ways: sexually and asexually. Most species release tiny eggs and sperm into the water, where they mix. The baby corals, called planula larvae, then float around before settling down. Some corals keep their eggs safe inside their bodies. Asexual reproduction often happens when a piece of a coral colony breaks off and starts growing in a new spot.
Stony corals live in all the world's oceans. They are super important because they build most of the structure of modern coral reefs. Corals that build reefs usually live in shallow, sunny waters. Other corals don't build reefs and can live in very deep, dark parts of the ocean.
Stony corals first appeared about 240 million years ago. Today, their numbers are expected to go down. This is mainly because of global warming and ocean acidification, which make the ocean warmer and more acidic.
Contents
Coral Body Parts
Stony corals can be solitary (living alone) or colonial (living in groups). Colonies can become very large, made up of many individual polyps.
Soft Parts of a Coral
Coral polyps are like tiny cylinders. They have a mouth in the middle, surrounded by tentacles. The bottom part of the polyp makes the stony material for the coral skeleton. The polyp's body wall has two layers with a jelly-like layer in between.
The mouth leads to a tube that goes into a central stomach area. This area is divided by thin sheets of tissue called mesenteries. The parts that make eggs or sperm are also in this area. The polyp can pull itself back into its stony cup, called a corallite, using special muscles.
In colonial corals, the polyps are connected by thin sheets of tissue called coenosarc. These sheets cover the outside of the skeleton. They also have pathways that connect the stomach areas of all the polyps. This means food and water can move around the whole colony.
When polyps make copies of themselves, they stay connected, forming colonies. Sometimes, two colonies of the same species can even join together to form one big colony! Most colonial polyps are tiny, only 1 to 3 mm (0.04 to 0.12 in) wide. But solitary polyps can be much larger, up to 25 cm (10 in) wide.
Coral Skeleton
The skeleton of a single coral polyp is called a corallite. It's made by the outer layer of the polyp's body. It starts as a cup around the lower part of the polyp. Inside this cup, there are plates that stick up from the bottom. These plates are called septa. Each plate is next to a pair of the polyp's internal tissue sheets.
All modern stony coral skeletons are made of calcium carbonate in a crystal form called aragonite. The skeletons of both single and colonial corals are light and have many tiny holes. This is different from some older fossil corals that had solid skeletons.
How Corals Grow
In colonial corals, new polyps grow by budding. There are two main ways this happens. In one way, a new polyp grows on the mouth disc, inside the ring of tentacles. This can form separate polyps or a line of polyps that share a long mouth area.
In the other way, new polyps grow outside the tentacles. This always creates separate polyps, each with its own stony cup. For corals that look like bushes, like Acropora, new polyps grow sideways from the main polyps to form branches.
How fast a coral colony builds its skeleton depends on the species. Some branching corals can grow about 10 cm (4 in) taller or longer each year. Other corals, like dome-shaped ones, grow slower, about 0.3 to 2 cm (0.12 to 0.79 in) per year. Corals grow differently during the day and night, and in different seasons. If you look at a cross-section of a coral, you can see growth rings, just like in trees. These rings can tell us how old the coral is.
Solitary corals don't bud. They just get bigger as they add more calcium carbonate and create new rings of septa. A large Ctenactis echinata coral, for example, can be about 25 cm (10 in) long and have over a thousand septa.
Coral Appearance
The size of individual polyps in a coral varies a lot depending on the species. In some corals, each polyp has its own wall. In others, polyps share a common wall with their neighbors.
Coral colonies come in many different shapes. For example, a colony might grow as a flat crust, or look like a branching tree, or be a big, solid mound. The same coral species might even have different shapes depending on where it lives. For instance, it might be crust-like in deeper water but have a more distinct shape in other parts of the reef.
Within a single species, corals often have different colors. The color of a coral usually comes from two things: the light it gives off itself, and the brown colors made by the zooxanthellae living inside it.
Where Corals Live
Stony corals are found in all the world's oceans. There are two main types. Reef-building corals mostly live in clear, shallow, tropical waters. They are the main builders of coral reefs. Other corals don't build reefs. They can be found in tropical waters, temperate seas, polar waters, or even in very deep parts of the ocean, up to 6000 m (20,000 ft) down.
Coral Ecology
Stony corals fit into two main groups:
- Reef-forming corals: These usually have zooxanthellae (tiny plant-like helpers) inside them.
- Non-reef-forming corals: These usually do not have zooxanthellae.
In reef-forming corals, the zooxanthellae live inside the coral's cells. There can be millions of these tiny helpers in just one square centimeter of coral tissue! The zooxanthellae are good for the corals because up to half of the food they make is used by the polyps. The oxygen they produce and the extra energy from sugars help these corals grow up to three times faster than similar corals without these helpers. These corals like to grow in shallow, sunny, warm water with good water movement and lots of oxygen. They prefer to settle on firm, clean surfaces.
Most stony corals stretch out their tentacles to catch tiny animals called zooplankton for food. Corals with bigger polyps can catch larger prey, like small invertebrates or even tiny fish. Besides catching prey, many stony corals also make a sticky slime. They use tiny hairs to move this slime over their bodies, trapping small food particles. Then, they pull the slime and food into their mouths. For a few corals, like Acropora acuminata, this is their main way of eating, and their tentacles are very small or missing. In the Caribbean, stony corals usually feed at night, pulling their polyps into their skeletons during the day to let their zooxanthellae get sunlight. But in the Indo-Pacific region, many species feed both day and night.
Corals without zooxanthellae usually don't form reefs. They are found most often in waters deeper than 500 m (1,600 ft). They can live in much colder temperatures and in total darkness. They get their energy by catching plankton and other tiny bits of food floating in the water. These corals grow much slower than corals with zooxanthellae. Their skeletons are also less strong and can be damaged more easily.
Coral Life Cycle
Stony corals have many different ways to reproduce. They can reproduce both sexually and asexually. Many species have separate sexes, meaning a whole colony is either male or female. But others are hermaphroditic, meaning individual polyps have both male and female parts.
Some species keep their eggs safe inside their bodies until they hatch. But in most species, sexual reproduction creates a free-swimming larva called a planula. This larva eventually settles on the seabed and changes into a polyp. In colonial species, this first polyp then divides many times to create the entire colony.
Coral History
We don't have much information about how stony corals first appeared. We know a lot about modern corals, but very little about their fossils. The first stony coral fossils show up about 240 million years ago. It took another 25 million years for them to become important reef builders. Their success might be because they started working with symbiotic algae.
By the end of the Triassic period, nine groups of stony corals existed. Three more appeared by the Jurassic period (200 million years ago), and another group appeared in the Middle Cretaceous (100 million years ago).
Scientists are still trying to figure out if stony corals came from a common ancestor. This ancestor might have been an anemone-like coral without a skeleton, or a type of coral called a rugose coral. However, rugose corals had different skeletons and septa arrangements, so they might not be direct ancestors. It's also possible that stony corals developed from an ancestor similar to a Corallimorpharia (a type of sea anemone). It seems that building skeletons and forming partnerships with algae might have happened more than once in coral history. DNA studies suggest that all stony corals came from a common ancestor.
The earliest stony corals were not reef builders. They were small, single corals or small groups. Stony corals were probably most diverse during the Jurassic period. But then, most of them disappeared in a mass extinction event at the end of the Cretaceous period. Only about 18 out of 67 types survived. Recently, scientists found ancient corals from the Paleozoic era that had similar skeletons to modern stony corals. This supports the idea that stony corals might have developed on their own.
It's not clear if the early stony corals had zooxanthellae. This partnership seems to have developed many times later on. Some modern coral groups even have both corals with and without zooxanthellae.
How Corals Are Classified
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Classifying stony corals is very difficult. Many species were described a long time ago, before people could use scuba diving to see corals in different places. Scientists didn't realize that corals could look very different depending on their habitat. Collectors mostly saw corals in shallow reef areas and couldn't observe how they changed in deeper, murkier waters. More than 2,000 names were given to corals during this time.
Even the idea of a "species" is tricky for corals. They live in huge areas with many smaller groups. Their physical features can blend with other species, making it hard to tell them apart.
In the past, scientists tried to classify corals based on their skeletons. In the 19th and early 20th centuries, they looked at how the skeleton was built and the patterns of its internal plates. Later, in the 1940s and 1950s, scientists like Thomas Wayland Vaughan and John West Wells used the patterns of these plates to divide corals into five main groups. They also looked at polyp features, like how tentacles grew.
In the late 20th century, new methods using DNA changed how scientists looked at coral relationships. Studies of mitochondrial RNA showed that while species grouped into existing families made sense, the older, larger groups (suborders) did not. For example, some types of corals that were in different suborders were actually closely related. This means there aren't clear physical features that separate these groups, only genetic differences.
Australian zoologist John Veron and his team also studied ribosomal RNA and found similar results. They confirmed that all stony corals came from a single common ancestor. But they also found that stony corals are divided into two main groups: "robust" and "complex" corals. Veron suggested that future classification should use both physical features and genetic information.
Images for kids
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Settlement and early life stages of scleractinian corals. This figure highlights on the first steps of a coral larvae (searching, attachment, and metamorphosis) toward an adult coral.
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Montastraea annularis can be fragmented to form new colonies.
See also
In Spanish: Escleractinios para niños
