Animal, vegetable, or mineral?
How to categorise a coral
Over half a billion years ago, our oceans produced a new kingdom of life.
This kingdom – Animalia – includes 1.2 million described species so far.
Sometimes, animals are easy to identify. Other times, they may be surprising, unfamiliar, or even invisible to the naked eye. Within the kingdom Animalia are creatures whose lives are very different to our own, and whose categorisation stumped scholars for centuries.
Corals are one example. Throughout history, corals have been variously described as "stones" (Theophrastus), "trees" (Rashi) and "neither animals nor plants, but possessed of a third nature" (Pliny the Elder).
How do we know that corals are animals?
Despite their huge diversity, animals share some features that set them apart from other living things.
Corals meet all of these criteria.
1. All animals are made of more than one cell
2. Animals must eat other organisms
3. Animals can move freely at some point in their lives
This piece of Bubblegum Coral is actually a vast colony of individual animals.
In their familiar adult form, these animals are known as polyps.
Bubblegum Coral (Paragorgia arborea) with polyps extended for feeding
Bubblegum Coral (Paragorgia arborea) with polyps extended for feeding
This is a coral polyp, a singular animal...
Coral polyps are made of more than one cell.
Within each polyp are several different types of specialised cell that perform different functions, such as feeding and reproduction.
Coral polyps eat other organisms.
One of the main energy sources for corals are the zooplankton that live in the water column. Polyps are usually armed with tentacles that can catch zooplankton and sweep it into their mouths.
Stinging cells on the outside of the polyp's tentacles can be launched at prey, ensnaring it or debilitating it with toxins. A layer of mucus on the outer surface of the polyp aids with the transfer of zooplankton towards the polyp's mouth and into its stomach for digestion.
Polyps are not solitary animals. They can share food with other polyps in their colony through a tissue known as the coenosarc. Many polyps also live in a close relationship with an entirely different group of organisms known as zooxanthellae. These are algae that live inside polyps, producing molecules that the coral can turn into fats, proteins, and carbohydrates for energy and cell growth.
Corals are mobile during their juvenile stage.
Corals can reproduce sexually, using sperm and egg cells, and asexually, by cloning themselves. Sexual reproduction gives rise to a larval stage in the coral life cycle, known as the planula. Planulae are free swimming organisms that move around using cilia -- tiny, external hair-like structures that move in a wave-like motion to propel the planula through the water. Eventually, the planula will settle at the bottom of the ocean, and mature into a polyp.
What did the earliest animals look like?
Dr Frankie Dunn is an expert in the early evolution of animals. In this video, she introduces us to a 565 million-year-old fossil that has a lot to teach us about what it means to be an animal today.
With Dr Frankie Dunn
Visit the Animal diversity display
Opened between 2022 and 2024, our new displays on biodiversity showcase the variety of life on Earth and consider important questions about preserving this diversity for future generations.
The Animal diversity display explores the astounding diversity we see in animals today using specimens from the Museum's collections
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