Sea Urchins

Picture credit : Tim Long

The Diadema Urchin, commonly referred as Long spined black urchin has exceptionally long black spines which can grow to 10-12cm. Most other urchins have spines that only grow to 1-3cm.

Mating season of sea urchins takes place in the spring. Just like many other sea creatures, sea urchins reproduce by releasing eggs and sperm cells in the water. This type of reproduction is called external fertilization. In the fertilization process, male and female urchins excrete fluid to alert other urchins to respond by releasing their eggs and sperm in mass reproduction. With more gametes available, there is a higher chance of fertilization. However, in areas of low-population, a few sea urchins may not be enough to initiate fertilization.

A fertilized egg undergoes larval stage before it becomes adult sea urchin. During the larval stage, sea urchin swims with other tiny animals as a part of zooplankton. Most species of sea urchins live up to 30 years but the Red sea urchin can live for up to 200 years.

Their long pointy spines help them to deter predators. Underneath the spines of a sea urchin is a hard outer body called a ‘Test’ It resembles the hard outer skeleton of other sea creatures that also belong to the phylum echinodermata, such as sea stars, sand dollars, sea lilies and sea cucumbers. The outer shell is constructed of 10 fused plates that surround the entire body in the same structure as the slices of an orange.
Every other section of the outer body contains holes that allow sea urchins to extend their tubed feet using a water vascular system when they need to walk or hang onto objects when they feed. Sea urchins can also walk using their spines or their teeth. Urchins have a special type of mouth, called “Aristotle’s lantern”. The mouth is equipped with five sharp teeth that are able to drill a hole in the rock.

The long spined black urchins are ecologically important because they consume algae that can otherwise grow to such an extent that they can smother coral reefs thus are a keystone species. This species usually lives at 1–10 metres in depth on coral reefs. However on Koh Tao you will find many of them on our deeper sites on the sand at around 30m. They will often lodge themselves in a crevice, so that only their spines can be seen, but individual urchins who can’t find a suitable crevice will live in more exposed situations. Individuals that have been able to find a crevice usually will roam about one metre from their crevice at night during feeding.
They are very sensitive to light, and will often pick its crevice or resting place based on how much shade there is. Urchins are omnivores. This means they eat both plants and animals. Sea weed, algae, plankton and decaying organic matter.

If you want to find out more about coral reef monitoring, how keystone species affect the workings of the reef, then take a look at eco courses

More fun facts can be found on our Ocean Facts page… check it out and learn about the ocean!