What really is coral?

It's well publicised that coral reefs are dying. News stories focus on climate change and its impact on coral health, but they rarely focus on the incredible properties of coral or how it is formed. By understanding this, we can learn how best to protect coral populations around the world.


Coral Polyp


The actual organism that creates the reefs of the world is an animal. Specifically, a tiny creature called a polyp, closely resembling an upside-down jellyfish. After finding a suitable location, polyps anchor to rocks and begin to asexually reproduce (clone themselves) to form huge identical colonies, with each colony forming a coral. So, one coral is actually made up of thousands of individual animals living and working together, and a group of corals growing in an area becomes a reef.


However, this is only the start. When you look at a coral reef and see the striking colours, you're not looking at polyps at all (which are colourless), you’re actually looking at the pigments inside a plant - a type of phytoplankton called Zooxanthellae to be exact. These phytoplankton are a type of microscopic algae that live inside the tissue of polyps that form the coral.


Acropora Coral Polyp


There is a symbiotic relationship between the polyps and the phytoplankton, meaning both parties benefit from the relationship. Coral reefs are nutrient deserts. The only resource in abundance at a coral reef is sunlight, but polyps don’t have the ability to harness this. In return for the polyp providing a safe haven, the Zooxanthellae shares some of the food it creates from photosynthesis, and this is the basic relationship fundamental to the survival of all coral.


LPS Coral


So far, we've seen how one coral forms from many identical polyps and a symbiotic relationship, but how does one coral transform into vast reefs, some of which can even be seen from space? This is where the second form of reproduction comes into play; sexual reproduction (sperm and egg).

The biggest problem with reproduction in coral is that males and females are static - they can't move. To get around this, corals literally fire their sperm and eggs (gametes) at one other. However, whilst this solves the challenge of separation, the gametes are now at the mercy of the ocean, with its vast scale and powerful currents, bringing the chance of the gametes mixing close to zero.


Coral Bleaching


To overcome this, every coral on a given reef will spawn over 2 nights, once a year. Picture a section of reef, made up of thousands of coral, each made up of millions of polyps, potentially spread over several kilometres. The ability to organise a mass spawning event of this scale just once a year baffled scientific minds for years.

Whilst we're still not certain how it works, we're closer. Using the moon to signal changes in the tide, every coral on the reef releases prepackaged bundles of sperm and eggs into the water column, creating a pink slick at the surface that mixes together. If two bundles from the same species mix, surface based enzymes react and the gametes combine and fertilise. There is now a free swimming baby polyp called a planula.


Great Barrier Reef Heart Reef


This is where the cycle starts again. The free swimming planula (polyp) finds a suitable section of rock and begins to reproduce asexually (clone itself) once again to form a coral.


Boulder Coral with Christmas Tree Worms


This is the basic life cycle of a coral. It's a great starting point to understand how we can protect the reefs. We'll be releasing a blog soon on how human activity impacts reefs and what we can do to preserve them.

At FourState, we've got a range of products that can help preserve the reef; browse through them in the Water state!



Leave a comment

All comments are moderated before being published