An animal beneath the tentacles
On a branch, a crown of tentacles surrounds each mouth. It belongs to a polyp, one of the animals making up the colony. The mouth leads to a digestive pouch where food is broken down.
Red coral has white polyps with eight pinnate tentacles: each has small side branches, giving it a feathery shape. Stony reef corals generally have unbranched tentacles.
Red coral has white polyps with eight pinnate tentacles: each has small side branches, giving it a feathery shape. Stony reef corals generally have unbranched tentacles.
What enters through a mouth
The current brings plankton: organisms that live suspended in water and drift with currents. Tentacles hold small prey and move it towards the mouth. Stinging cells, which can release a thread that helps catch prey, may take part.
Digestion breaks down the meal. Cells absorb nutrients, the usable substances obtained from food. Undigested remains can leave through the mouth: a polyp has no anus.
Digestion breaks down the meal. Cells absorb nutrients, the usable substances obtained from food. Undigested remains can leave through the mouth: a polyp has no anus.
A meal’s route through the colony
On a sea fan branch, living tissues join the polyps. Gastrovascular canals, small passages connected to their digestive cavities, run through these tissues.
Substances obtained from digestion can move to other parts of the colony. There is no heart pumping blood. Exchanges depend on the colony’s organisation; they do not guarantee that every polyp receives the same amount.
Substances obtained from digestion can move to other parts of the colony. There is no heart pumping blood. Exchanges depend on the colony’s organisation; they do not guarantee that every polyp receives the same amount.
Growing by budding
A new polyp can develop from the colony’s tissue. This is budding, a form of multiplication without reproductive cells joining. The young polyp develops a mouth and tentacles while staying connected to the others.
This process enlarges an existing colony. Depending on the coral, growth may produce branches, plates or rounded masses.
This process enlarges an existing colony. Depending on the coral, growth may produce branches, plates or rounded masses.
How does a colony begin?
In red coral, colonies are generally male or female. Males release sperm, the male reproductive cells. Fertilisation, the joining of male and female reproductive cells, takes place inside female polyps. Larvae begin developing there.
A released larva may settle on a surface and change into the first polyp. Budding may then enlarge the colony. Other corals have a different cycle, with fertilisation in the water.
A released larva may settle on a surface and change into the first polyp. Budding may then enlarge the colony. Other corals have a different cycle, with fertilisation in the water.
When polyps close
Muscles allow a polyp to contract. In red coral, retraction, the withdrawal of a polyp into the branch’s tissue, can be complete.
Touch may cause it to close. Currents and other conditions may also affect its activity. Seeing a closed colony does not tell us why it closed.
Touch may cause it to close. Currents and other conditions may also affect its activity. Seeing a closed colony does not tell us why it closed.
Breathing without lungs
Oxygen dissolved in the water passes through thin tissues to reach cells. Carbon dioxide can pass in the opposite direction. These exchanges occur by diffusion: molecules move from areas where they are more concentrated to areas where they are less concentrated.
Water movement helps renew the water close to the tissue. Polyps have neither lungs nor gills.
Water movement helps renew the water close to the tissue. Polyps have neither lungs nor gills.
Different kinds of support
Red coral has a hard axis, the central support inside its branches. Living tissue surrounds it. This tissue also contains sclerites: small mineral pieces.
The purple sea fan has a more flexible central support rich in organic material, substances made by living organisms. Stony corals form a mineral skeleton with cups in which the polyps sit. These supports are built differently.
The purple sea fan has a more flexible central support rich in organic material, substances made by living organisms. Stony corals form a mineral skeleton with cups in which the polyps sit. These supports are built differently.
Light, for some corals
Some corals shelter symbionts: organisms living in a lasting association with them. These microscopic partners use photosynthesis: they make organic material using light, water and carbon dioxide. Some of their products feed the coral.
Red coral and the purple sea fan do not host these photosynthetic partners. What happens in one reef coral does not apply to all corals.
Red coral and the purple sea fan do not host these photosynthetic partners. What happens in one reef coral does not apply to all corals.
Connected polyps, one colony
A polyp can catch prey or close while remaining connected to the colony. Shared tissues allow exchanges between its different parts.
In some corals, a limited wound can close. This does not mean a colony survives every injury. The species, the extent of the damage and living conditions all matter.
In some corals, a limited wound can close. This does not mean a colony survives every injury. The species, the extent of the damage and living conditions all matter.
Interactive quiz
Try it yourself
Five questions about what you have just read. Choose one answer for each question.
Interactive quiz
Try it yourself
Question 1 out of 5Score: 0
1
How many tentacles does a red coral polyp have?
Its eight tentacles have small side branches that give them a feathery shape.
2
What do gastrovascular canals, passages connected to digestive pouches, allow?
Substances obtained from digestion may reach other regions of the colony.
3
How does budding differ from a colony starting from a larva?
Budding forms a polyp in existing tissue; a settled larva may begin another colony.
4
Why might a colony be closed?
Touch and other conditions may affect activity. One observation does not establish the cause.
5
What does photosynthesis, making material using light, produce in some corals’ partners?
Some products made by the microscopic partners can feed the coral.
Final result
Quiz completed
correct answers
Understanding what we observe
A colony combines polyps that can feed with living tissues connecting them. Budding enlarges it, while a larva may start another colony. Connections allow exchanges, but corals do not all have the same skeleton or the same microscopic partners.
Sources
Natural history accounts and publications used to check the mechanisms described.
NOAA — What is Zooxanthellae?oceanservice.noaa.gov↗Bouderlique et al. (2022). Surface flow for colonial integration in reef-building corals. Current Biology.doi.org↗Palmer et al. (2011). Corals Use Similar Immune Cells and Wound-Healing Processes as Those of Higher Organisms. PLoS ONE.doi.org↗Newcastle University — Surface flow for colonial integration in reef-building coralseprints.ncl.ac.uk↗
