The sea urchins of Sydney

Sydney sea urchins are diverse and live in a variety of habitats – you may find them in rockpools and kelp forests. They are also an important part of what makes our local biodiversity so special. Learn more about these Sydneysiders!

Sea urchins are not just spiky kelp-eating machines – they are an important part of our biodiversity. In Sydney, we are fortunate to have an amazing diversity of sea urchins, with over 40 species recorded to date. Some of these species are long-time local residents, others have migrated far from home and often come and go. However, all of these sea urchins are important herbivores that help to keep our ecosystems balanced.

What species are common in Sydney?

Recently, the barren-forming sea urchin Centrostephanus rodgersii, commonly known as the long-spined sea urchin, has been getting a lot of attention because of its range extension into Tasmania. This is a native of NSW and is the only barren-forming species in Sydney.

There are two other common species of sea urchin in Sydney that are in the same genus; the short-spined sea urchin (Heliocidaris erythrogramma) and the red sea urchin (H. tuberculata). They are often seen on intertidal and shallow subtidal reefs of Sydney. Heliocidaris erythrogramma and H. tuberculata are sister species, meaning they shared a common ancestor in the past, which split to become the two species that live in Sydney today. They split only about 6 million years ago – a blink of an eye in evolutionary time. H. tuberculata is long-lived and a big one might be over 80 years old. Heliocidaris tuberculata is bright red and Heliocidaris erythrogramma is purple/green while sometimes pink depending on where you are on the coast.

There are many less common species of sea urchins in Sydney that will pop up when you start looking for them. These include the very cute Holopneustes purpurascens, which is usually found wrapped up in some kelp or sea grass. It has a very distinct purple/lavender colour and is about the size of a golf ball when fully grown. You may also see Phyllacanthus parvispinus (pencil sea urchins) tucked away between rocks or boulders. This species, as the name suggests, has very thick pencil-like spines, and often has other marine invertebrates such as barnacles, amphipods, sponges or bryozoans living on their spines. If you look closely, you can see all the different animals living on their spines. Another common sea urchin in Sydney Harbour is Pseudoboletia indiana. This sea urchin is all white, with purple tips at the end of its spines, and it often covers itself in shells or other items from the seafloor to protect itself from predators and the sun. This species is far from home, as it is typically found in tropical areas, but it is a tough species that travels down the East Australian Current in its larval form and can survive winter in Sydney. Finally, you may see Tripneustes kermadecensis (the lamington sea urchin). This species has black skin and short white spines. Despite being described from urchins living in the Kermadec Islands in New Zealand, this species is very common on the east coast of Australia.

What is the largest sea urchin in Sydney?

Centrostephanus rodgersii, the long-spined urchin mentioned earlier, is the largest urchin in Sydney and is important in contributing to the patchwork mosaic that is present in many near shore areas. They have long purple/black spines and form barrens as a natural part of Sydney’s marine ecosystem. The term barren, previously called coralline flats, refers to areas of rocky reef covered in pink encrusting algae where kelp does not grow. Barrens are not deserts; they are full of life and some species prefer to live in barrens! There has been a concern that sea urchin barrens are expanding in NSW, but recent research by the Department of Primary Industries (Fisheries) suggests this is not the case. Sea urchin barrens are a dominant and stable part of the Sydney seascape.

Should I be concerned if I find a sea urchin?

Despite concern about the long-spined sea urchin and barrens in NSW, we mustn't forget the rest of the diverse and amazing sea urchin species we see in Sydney and that our native sea urchins are beneficial to our local ecosystems. If you are out rock pooling, snorkelling or diving, keep an eye out for them! If you are out fishing for sea urchins, bear in mind that we have many different species in Sydney, so be sure to adhere to bag and size limits as stipulated by NSW Fisheries.

Emily McLaren, PhD Candidate, The University of Sydney in association with the Marine Invertebrates team, Australian Museum.