Urchins Urchins are really old, even older than dinosaurs, but the changes wrought after the Jurassic Period saved them from a long decline towards extinction and gave them a ‘second wind’. The earliest echinoid fossils date to the upper part of the Ordovician period (circa 450 Mya). During the Carboniferous period, urchins declined in diversity. They neared extinction at the end of the Paleozoic era, with just six species known. Only pencil urchins and the spiny urchins survived from the once diverse range of urchins. By the upper part of the Triassic period, their numbers began to increase again. Pencil urchins proved to be a very robust design and have changed very little since the Late Triassic. From here they went on from strength to strength. Sand dollars are a really new model (circa 66 to 1.8 Mya) and form the latest branch on the still active echinoid tree. The Basics – Anatomy The sea urchin is a good example of a modest but robust design that survives when the going gets tough. It is built like a little armoured tank which protects not a lot more that a mouth and gonads. They are effectively simple eating machines. That said, they have a pretty interesting anatomy. The real artwork has been saved for the mouth, so complex its been given a special name, a “lantern”. This strange hard structure contains the jaws, a ‘heart’ that pumps water around the urchin, and a nerve centre (too small to call it a brain). A circulatory fluid fills the general body cavity and they even have blood. They save on a back end by eliminating waste through their gills and tube feet. So they effectively go to the toilet through their toes. Sea urchins also breathe mostly through their tube feet. If they get low on oxygen they can use five pairs of little gills located around the mouth. Heart urchins and sand dollars live under the sand most of the time and have done away with these gills. Although sea urchins don’t seem to have any problems avoiding predators or finding comfortable dark corners to hide in, they don’t have eyes. Genetic analysis of sea urchins has revealed they have lightsensitive molecules, mostly in their tube feet and in tiny stalked appendages found in among their spines. Although they do not have eyes as we know it, their entire body might function as one big compound eye (like a fly). Sea urchins are also sensitive to touch and chemicals and can tell up from down. They have all the functionality of a more superior organism, but without all the complex systems. Todays common temperate urchins Slate pencil urchin – Goniocidaris tubaria This is a very old animal from which many other urchin designs evolved. These kind of urchins are very common around southern Australia from shallow depths to 630 metres. On reef, the Slate Pencil Urchin uses its spines to wedge firmly into holes and crevices during the day. The Slate Pencil Urchin is a herbivore. It comes out to feed at night. The mouth is on the underside of the body and is equipped with five sharp teeth used to scrape algae from the rocks. The spines of this urchin are often covered with encrusting invertebrates. Two species of Goniocidaris are now thought present in southern Australia—Goniocidaris tubaria, which possesses spiky primary spines and has wide red areas extending down the test between the spines, and Goniocidaris impressa , which can possess spiny or smooth primary spines and has small white spinelets distributed across the red test. Purple or native urchin (Heliocidaris erythrogramma) The native or ‘purple’ urchin Heliocidaris erythrogramma is about the most commonly noticed marine invertebrate on sheltered reefs in the southern coasts. They are basically everywhere. Because urchins are so good at eating algae, their numbers make a big difference to the health of local seaweed beds. If their numbers get out of balance, the area can be turned into a barren desert. 15 The first noticeable thing about purple urchins is that they usually aren’t purple. They come in a vast array of colours, white, purple, dark red, brown, green, or occasionally pink. This big colour variation is not very common among urchins. The urchins contain pigments called spinochromes and echinochromes. They extract the ingredients for these pigments from food and some of them are based on iron minerals they gain from the environment. The final colour variation is determined by genetics and where they live. If the area is very exposed to wave action a white colour is most common. During the winter and spring the purple urchin eats and builds up its condition ready to breed. Its gonads become large and swollen. The urchin is ready to spawn when its shell, called a “test”, is 4-5 cm in diameter. H. erythrogramma usually spawn in summer. When the urchins are ready to breed it seems like the females may try to snuggle up to as many males as they can to improve her chances of one of them being a star fertilizer. The released sperm and eggs mix in the water column and develop into little larvae that settle on the bottom. Like a lot of marine animal larvae they are very fussy about where they land. Urchin larvae appear to have strong preferences for particular kinds of seaweed and are even attracted by particular types of bacterial film growing on the rocks. A stop motion video at the University of Tasmania shows urchins spending much of the winter sheltering in crevices. The winter storms keep them well-supplied with broken bits of drift algae which they wrap around their spines and munch at their leisure. They barely move from the safety of their crevices, especially in shallow exposed areas where the bottom is being lashed by macroalgae in the swell. As the summer progresses and drift algae runs low, they will take more risks and roam around the reef, especially at night, to tackle anything they can get their hands on. The University of Tasmania also has excellent night video of urchins being attacked by crayfish who also roam around the reef looking for exposed urchins. As soon as the crayfish comes near, the urchin bristles its spines and points them towards the crayfish. A battle is then on with the cray trying to flip the urchin onto its back. A small cray will struggle to do this with a large urchin. Studies of black urchins suggest that excessive cray fishing by humans is severely reducing the size and numbers of crays. This is helping urchins to increase to excessive numbers until they eat the reef down to bare rock. Native urchins also form barrens, but generally only in sheltered areas. You may have noticed that native urchins will also be found with lots of shells and other objects lodged in their spines. Like many other species of urchin they will pick up anything to use as a shield including human litter. It was thought that they did this just to cover themselves from ‘sunburn’ by UV rays, or even as a shield from predators. More recent international studies suggest UV is a factor, but more often they are trying to shield themselves from physical damage caused by the lashing of seaweed. Older urchins tend to do it less than more vulnerable smaller urchins. The native urchin is an important grazer on our reef. Even small numbers tend to change the amount of weed growth on the rocks and that changes the other types of animals that can live there. Mostly, they cause only small areas of barrens damage to reef compared to the longspined or black urchin that has recently arrived in Victoria and Tasmania from New South Wales. Native urchins can still be a significant problem in unique areas where some animals are already doing it tough, such as the Red Handfish habitat at Primrose Sands in Tasmania, which has been severely damaged by urchins. 16 Long spined or Black urchin (Centrostephanus rogersii) This species has found it’s way down the east coast from NSW and Tasmania and is seen as a pest in Tasmania. When found in large numbers, these urchins can eliminate kelp and other macroalgae and form urchin barrens of ‘white rock’. They differ from Heliocidaris by having hollow spines rather than solid spines. In NSW they have converted 50% of the reef into a barren area. Climate change is allowing them to breed further south than ever and they are beginning to seriously damage the cold temperate zone. Egg Urchin Amblypneustes ovum is found in sheltered and moderately exposed reef; 0 – 70 m depth form SA to Victoria and around Tasmania. This is a short-spined urchin, often found amongst the fronds of seaweeds. It must spend a long time climbing up the seaweed thallus and then get whipped around in the swell. Hard work, but it ensures a permanent supply of algae to munch on. The inflated egg urchin Holopneustes inflatus is a similar sea urchin that belongs to the Temnopleuridae family. This species is found in the waters of south eastern Australia and is known to occur from the Richmond River in northern New South Wales down to the Derwent estuary in Tasmania. This species looks very similar in appearance to the Holopneustes purpurascens sea urchin. The test (body) of Holopneustes inflatus is generally always brightly coloured and most the common colours observed are orange, red, yellow, pink and mauve. The Holopneustes inflatus sea urchin may grow to a maximum width of 5cm. It can be found on in shallow seagrass meadows and is sometimes associated with kelp and is known to feed on seagrasses. Heart urchins – Echinocardium cordatum In Australia, it is observed as odd specimens along the shoreline of calm estuarine, among the flotsam and jetsam, and occasionally seen in disturbed sand by divers. Depending on the temperature, the species digs a few centimetres to about 20 cm deep into the sediment. A respiratory channel (chimney) leads from the hole to the surface. The animal is isolated from the sediment by a mucus veil, which plasters the burrow, basically it lives inside a ball of snot. lt collects food particles from the sand. It will live in the shallows or in water as deep as 230 metres. In some parts of the world it can reach densities of up to 200/m2. Breeding occurs in summer. The pelagic larvae are sometimes found in enormous quantities. Occasional population explosions have been seen in SE Australia where thousand will wash ashore. In mid-October 1994 in Henderson Lagoon at Falmouth, Tasmania, thousands bred up after a long period of dry weather, probably as salinity became higher in the lagoon, favouring the urchins. When food was exhausted a month later they all perished, causing the locals to unnecessarily worry that something was poisoning the lagoon. The species can live for 10 to 20 years. It is found world-wide in temperate seas. It’s the only sea urchin with a world-wide distribution. The species is only found in sediments with a low mud content (< 20%) and medium-sized sand grains (200 to 300 µm)