Coral is a well-known example of symbiosis, or a relationship in which multiple organisms live together. A coral consists of the coral organism itself, which forms the part of the animal that is visible to humans, and a microscopic alga living inside the animal’s tissues. Scientists have recently discovered a third organism in the relationship. They’ve named this organism a corallicolid. It’s an intriguing organism with some unusual features. Its function in coral hasn’t yet been discovered.

Part of a coral reef in Australia (Photo by Toby Hudson, CC BY-SA 3.0 license)

What Is a Coral?

Corals are animals belonging to the phylum Cnidaria and the class Anthozoa. Their body is a polyp. In zoology, a polyp is a tube that is closed at one end and open at the other. The open end is the mouth and is surrounded by tentacles. The tentacles contain stinging cells known as nematocysts. These capture small organisms and send them into the mouth. The prey is then sent into the gastrovascular cavity in the centre of the polyp.

The gastrovascular cavity got its name because it’s the site of food digestion and also of the transport of materials. In the illustration below, it’s called the stomach. Nutrients are absorbed through the lining of the cavity. Waste products are expelled through the polyp’s mouth.

The polyp of a coral organism is joined to other polyps of the species, forming a colony.  The coenosarc shown in the illustration below is a layer of living tissue extending between the polyps. A polyp is attached to the substrate and stays in one place during its life, so it’s classified as a sessile animal.

A stony coral polyp (Illustration from NOAA, public domain license)

Corals are divided into two major groups: stony corals and soft corals. A polyp of a stony coral is surrounded by a cup made of calcium carbonate. This chemical is secreted by the polyp, which can retract into the cup. Stony corals often form reefs. In some cases, these become huge structures over time.

Soft corals are sometimes known as gorgonians. Their polyps lack a calcium carbonate cup, but they often have spicules made of the chemical in their walls. Spicules are tiny, needle-like structures. Soft corals don’t form reefs.

Corallium rubrum, a soft coral (Photo by Parent Géry, CC BY-SA 3.0 license)

Zooxanthellae and the Symbiodinium Genus

Corals often have an additional way to obtain food besides catching it with their tentacles. Most reef-building corals contain microscopic organisms called zooxanthellae, which are often referred to as algae. These organisms absorb light and use it to produce food during photosynthesis. The coral absorbs some of this food for its own use. The zooxanthellae benefit from the protection that a coral’s tissues provide and the presence of chemicals that they need. The relationship is an example of mutualism (a relationship in which both organisms benefit) as well as symbiosis.

Light and confocal view of Symbiodinium in a jellyfish (Photos by Alisonmlewis, CC BY-SA 4.0 license)

Zooxanthellae are a type of dinoflagellate. Dinoflagellates are unicellular and photosynthetic organisms that belong to the phylum Dinoflagellata. The organisms have flagella in at least one stage of their life cycle. As these whip-like appendages move through the medium in which the dinoflagellate is living, they enable the organism to move.

Most zooxanthellae in corals belong to the genus Symbiodinium, which often has a golden-brown colour. The motile stage of this organism’s life cycle has flagella, as shown in the photo below. The other stage is known as a coccoid. This has a round shape and lacks flagella. It’s a metabolically active form that lives in the host’s cells.

Life stages of Symbiodinium (Photo provided by Todd LaJeunesse, CC BY 3.0 license)

Corallicolids

The discovery of a third type of organism living in corals is exciting. A team of botanists at the University of British Columbia (UBC) has been studying them for some time and has recently discovered more about the creatures. The researchers have found that about 70% of corals around the world contain corallicolids. The organisms have been found in corals of different types, including stony and soft corals.

This organism poses completely new biochemical questions. It looks like a parasite, and it’s definitely not photosynthetic. But it still makes chlorophyll. — Patrick Keeling, UBC (with reference to a corallicolid)

Chlorophyll is a green pigment that absorbs light. The light energy normally allows carbohydrate to be produced in the process of photosynthesis. The carbohydrate acts as food for the organism possessing the chlorophyll. Corallicolids make chlorophyll. It’s strange that they don’t make some other essential molecules needed during photosynthesis.

The researchers say that since the chlorophyll in corallicolids absorbs light energy without passing it to molecules involved in photosynthesis, the situation could be dangerous for the cell. More research is needed in order to discover what is going on.

The Link Between Corallicolids and Apicomplexans

Corallicolids live in corals, so it might be thought that they have nothing to do with humans. They have been linked to certain parasites in humans with respect to their evolutionary history, however. The link is summarized below.

• The phylum Apicomplexa contains obligate parasites that can only survive inside the cells of another organism. Examples of apicomplexans affecting humans include the organisms that cause malaria (genus Plasmodium) and the organism that causes toxoplasmosis (genus Toxoplasma).
• Apicomplexans evolved from non-parasitic organisms that could carry out photosynthesis.
• The cells of corallicolids have some apicomplexan features, including the presence of an organelle called a plastid. (Plant cells have a plastid, too. In plants it’s known as a chloroplast and carries out photosynthesis.)
• Researchers have found that corallicolids lack the genes needed for making the proteins involved in photosynthesis but contain the genes needed to make chlorophyll. They may therefore be an intermediate stage between the photosynthetic ancestors of apicomplexans and the apicomplexans that affect our health today.

We don’t know why these organisms are holding on to these photosynthesis genes. There’s some novel biology going on here, something we haven’t seen before. — Waldan Kwong, UBC

Corallicolids appear to be parasites of corals, although their exact role in the life of the animals is unknown. As the researchers say, there are some very puzzling aspects about the presence and biology of corallicolids. Future investigations will hopefully reveal much more information about these very interesting organisms.

References

• Information about corals from the Smithsonian Institution
• The role of zooxanthellae in coral from NOAA (National Oceanic and Atmospheric Administration)
• Symbiodinium information from the Tree of Life Web project
• Coral’s curious symbiont from the University of British Columbia
• An apicomplexan with chlorophyll biosynthesis genes from Nature (Abstract)