Tiny animals with remarkable relatives
Corals are made up of animals that look like tiny sea anemones. Together these animals form colonies, and colonies join together to create reefs. Coral is often found as fossils, showing that these creatures date back over 500 million years.
Coral is often mistaken for a plant, but it’s actually made up of tiny, fragile animals called polyps. A polyp has a sac-like body and an opening, or mouth, encircled by stinging tentacles. The polyp uses calcium carbonate (limestone) from seawater to build a skeleton to protect its soft body. It’s this skeleton which is usually fossilised.
Most corals live in colonies, formed when new polyps bud off the initial animal. But others remain as individuals. Solitary corals found off the Dorset and East Devon coast include the Devonshire Cup Coral and the very rare Sunset Coral (right) on the East Tennants Reef, off West Bay. They look like anemones but have a hard internal skeleton.
Corals have tentacles and stinging cells which are used to catch and paralyse prey (tiny floating animals). This is why they are classified in the same group as sea anemones and jellyfish. But sea anemones and jellyfish have no hard parts, so they are very rarely fossilised.
Swim or stay put?
Corals and their two cousins are very different in their life cycle:
Corals are permanently fixed to the seabed. They reproduce by budding, simply splitting to form new individuals, or by releasing eggs into the sea which form free floating larvae. These later settle on the seabed and grow into corals.
Jellyfish begin as polyps attached to the sea floor. They then mature into free-swimming adults. Sea anemones do the opposite. They begin as free-swimming larvae, then they fix themselves to a suitable habitat and grow into adults. They reproduce through eggs that grow into new jellyfish-like larvae before in turn settling down as anemones attached to the sea floor. Sea anemones and jellyfish have no hard parts and therefore are very unlikely to become fossilised.