August on Norfolk Island is the coolest month of the year. The southerly winds have been bringing in pounding surf and reduced visibility in the bays, meaning very few opportunities to get some decent images. And for those who are interested, the water temperature has been hovering between 17C and 18C. I’ve observed that a lot of the bustle has died down, and less fish have been around generally.

Sea urchins, known locally as wunna

Did you now that sea urchins are valuable members of our reef ecosystems? They like to eat algae, and, indeed, can play a significant role in maintaining the balance between corals and algae. They are especially important on reefs where other algal eating fish, or herbivores (such as parrotfishes) have been depleted.

Harmony in the reef’s food web

I see plenty of species of wrasse on our reef (which like to eat crustaceans, shrimps and sea urchins), but inside our lagoons I can count the species of parrotfish (I've seen five), and the numbers in each species (just a few of each), almost on one hand, which makes maintaining our sea urchin population important for the reef’s overall health.

Some wrasse LOVE to eat sea urchins, so by taking too many you are also depriving these beautiful fish of their dinner. If you are interested, you can check out exactly which are parrotfish and which are wrasse on my reef website on the fish page. You'll be surprised when you see what you thought was a parrotfish, isn't in fact one! Scroll down to each section (alphabetical).

Of course, there are times when the balance can get out of kilter and we have an explosion of too many sea urchins. I don't believe this is the case at the moment, certainly not inside the lagoons. If you consume sea urchins, do so wisely. Take only what you need (no, you don't need bagfuls), and preferably not from inside the lagoons. It's all about maintaining a balance.

This week, I thought I would showcase the different species of sea urchins found in the three bays here on Norfolk Island. We have a few different ones, but they all share a few things in common.

Here are some fast facts:

• Their life span ranges from between 15 years to as long as 200 years!

• Sea urchins have a five-fold symmetry, inasmuch as they contain five symmetrical sections. In comparison, mammals have two symmetrical sections.

• A sea urchin’s outer skeleton, and the shell that is left behind when they die, is called a ‘test’.

• They have an array of spines – some species have longer ones that others, and some are thicker – as well as tube feet, and tiny pincer-like organs.

• The tube feet extend through the tiny holes in their test and have tiny suckers on the ends. These cling to objects and help them move around.

• The spines are attached to the nodules that you can see on their test. They use these to deter hungry predators, and they may also use them to walk.

• Inside the test is their gut and their reproductive organs.

• Sea urchins don’t like the spotlight. Although they have no eyes as such, it is thought that their entire body acts like a compound eye that is sensitive to light.

• Wrasse especially like to eat them.

• They like to eat marine vegetation and sometimes molluscs and tubeworms.

• There are female and male urchins. Most species release their eggs into the water column to be fertilised by the males’ sperm.

• Their mouth is underneath. They have five, self-sharpening, hard plates that come together like a beak, called an Aristotle’s Lantern, which they use these to eat molluscs tubeworms and algae. These plates are replaced every few months. Some even use these grinding plates to make a crevice in the rock to live in.

• And they poop out of their anus on the top.

Four particularly noteworthy species (I think, anyway) are the collector urchin (Tripneustes gratilla), the Indo-Pacific rock-boring urchin (Echinometra mathaei), the red pencil urchin (Heterocentrotus mamillatus) and the flower urchin (Toxopneustes pileolus).

Collector urchins are rather pretty, with a habit of collecting debris in their spines, including rocks and pieces of dead coral. I’ve even seen a plastic comb and a plastic bag used in this way. It has been suggested that the urchin covers itself as protection from the sun, or possibly to deter enemies. Pufferfish and octopuses will prey on these urchins.

The Indo-Pacific rock-boring urchin creates a niche for itself by burrowing into the rock and coral.

A red pencil urchin is also known as a red slate urchin. They eat algae using the five sharp scraping teeth on their undersides. Their name, ‘slate urchin’, derives from the fact that when chalkboards were made of slate, these urchins’ spines were sometimes used instead of chalk.

The flower urchin, which I have only seen once, should definitely not be handled. It was noted as the ‘most dangerous sea urchin’ in the 2014 Guinness Book of World Records. Its generic name Toxopneustes translates as ‘poison breath’! ‘The severe debilitating pain of the flower urchin sting compounded by muscular paralysis, breathing problems, numbness, and disorientation can result in accidental drowning among divers and swimmers’ (Wikipedia). I suggest that this is one urchin that doesn’t want to go on your dinner table. It is undoubtedly very pretty, though!

You can find more images over on my sea urchin page on this website.

In the photographs, below, you can see the urchin’s tube feet. These extend through the tiny holes in their test and have minature suckers on the ends that cling to objects. They also help them to move around.

The white spines are attached to the nodules that you can also see on their test. They use these to deter hungry predators, and they may also use them to walk.