Black Sea Urchins

Black Sea Urchins
Black Sea Urchins
The main sea urchin we see around Sydney is the Black Sea Urchin (Centrostephanus rodgersii). Although there are other species that live in our waters I will mainly focus on the Black Sea Urchin.

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Sea urchins are relatives of starfishes and belong to a group of animals called echinoderms. They are mostly encountered on rocky shores, beaches and coral reefs. Sea urchins are characterised by moveable spines that occur in a variety of shapes and colours. There are approximately 800 described sea urchin species worldwide, over 200 of which are recorded from Australia. Sea urchins generally feed by grazing on plant material growing on surfaces, or by burrowing in sediments to find decaying material. They can reach high population densities and, through their feeding activities, can be key organisms in determining the community composition and appearance of marine ecosystems.

Most of you should know what a sea urchin looks like! Although its common name is the Black Sea Urchin, sometimes they can have a red or a green sheen to the spines depending on the angle you are looking at it and the way the light reflects. The sea urchin's body is made up of what they call the " test " that is the round ball like part, and the spines. The spine operates on a ball and socket type joint which allows the urchin to move its spines in all directions. It has a simple nervous system and no brain! There are three main parts that make up the inside of the sea urchin. The stomach, the gonads (this is actually the part the Japanese eat) and this thing with an amazing name called "Aristotle.s lantern".

According to Under Southern Sea's there are about 20 species of Sea Urchin in southern Australia and 18 of these are unique to the region. The Black Sea Urchin can be found in Tasmania, Victoria and up through most of NSW.

Eating, food & hunting
The Sea Urchin has 5 teeth, they are controlled by this structure called Aristotles lantern. The lantern also helps form the food into pellet type packages, which are covered in mucus; these pellets are then moved into the stomach for digestion. Hmm sounds delightful doesn't it.

The Black Sea Urchins main food source is algae (underwater plant life) it prefers large brown algae (in common terms, seaweed). But apparently it will eat almost anything. Sea urchins help create what is known as the "barren Habitat" around NSW, barren rocks devoid of any kelp or large algae.

Most Black sea urchins hunt at night. During a night dive you can always see the urchins out and about, moving around the sea floor. They leave their crevice and travel up to 10m in search of brown algae. Most will return to the same crevice at dawn however most of the urchins jockey for positions within the crevice. Its basically first one back gets the bets position.

In areas where there are no crevices to hide black sea urchins group together to help protect them selves from their predators.

The black sea urchin's main predators are Port Jackson sharks, the Blue Groper and of course man. Divers also kill a great number is sea urchins every year to feed other creatures. These days more and more divers are being educated not to kill urchins as they too play an important role in the underwater ecosystem. Sea urchins are also fished commercially due to demand for their gonads in countries overseas. Currently the fisheries for sea urchin gonads is in its infancy here in NSW but it is set to expand as fisheries in other parts of the world collapse.

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