Echinoids, commonly known as sea urchins and sea dollars, are part of the class Echinoidea. The main characteristics of the sea urchin include its rounded and globose shape in some species and, of course, its famous spikes. However, other species of sea urchins can be round and flattened. The sea urchin has a calcareous skeleton, which makes up its body and this, in turn, is made up of plates that protect its interior as a shell and from which come out quills or thorns that have mobility. They live in all the seas of the world, being able to reach seabeds up to almost 3,000 meters deep, and feed on a wide variety of fish, algae and other invertebrates. In addition, they exhibit a wide variety of colors, which makes them even more fascinating.
Of the approximately 950 species that exist, two types of sea urchins can be found: on the one hand, regular urchins, which they are spherical in shape and their body is covered with numerous spines of different lengths; and on the other hand, the irregular hedgehogs, which are flattened and with much fewer shorter spines, these are the so-called sand dollars. Have you ever wondered what the types of sea urchins are? If you want to know that and the characteristics of each one, don't miss this article on our site where we will show you examples of each type.
Types of regular sea urchins
Among the regular sea urchins, that is, those with a spherical body and full of spikes, the most common species are the following:
Common sea urchin (Paracentrotus lividus)
This species, also known as sea chestnut, is one of the most common in the Mediterranean Sea, as well as being present in the Atlantic Ocean, where it inhabits rock bottoms and marine meadows. It is common to see them at depths of up to 30 meters and They are capable of breaking soft rocks with their spines, and then get into the holes they produce. Its spherical body measures about 7 cm in diameter and has a wide range of colors, which can have brown, greenish, blue and violet tones.
Large sea urchin (Echinus esculentus)
Also known as European edible urchin, this species is found all along the coast of Europe. Generally, it can reach more than 1,000 meters deep and frequents areas with hard and rocky substratum bottoms. Its diameter varies between 10 to 17 cm and it has fairly short spines with purple tips The rest of the body has a striking color red, although it can vary from pink to pale purple or with greenish tones.
It is a species categorized “Near Threatened by the IUCN (International Union for Conservation of Nature) due to overfishing, since it is a species consumed by humans.
Green sea urchin (Psammechinus miliaris)
Also known as shore sea urchin, this species is distributed in the Atlantic Ocean, being very common in the North Sea. Generally, this species lives up to 100 meters deep, in rocky areas with an abundance of algae. In fact, it is very common to find it associated with brown algae. It is also very common in areas of seagrass beds and oyster beds. It measures about 6 cm in diameter and the color of its shell is grayish brown, while its spines are green with tips purple
Fire urchin (Astropyga radiata)
This species is distributed throughout the Indian and Pacific oceans, generally in depths that do not exceed 30 meters and preferably with sandy bottoms. It also inhabits areas of coral reefs. It is a large species and its color varies from dark red to light colors such as beige, however there are also black, violet or orange individuals. Their long red or black spines, which are also poisonous and used for defense, are They are grouped in such a way that some regions of the body are uncovered and a V can be seen, which, in addition, have an iridescence in such a way that they seem to shine. Its diameter can exceed 20 cm and, added to its spines of about 5 cm, makes the fire urchin a very striking and imposing species.
Black sea urchin (Diadema antillarum)
Also known as long-spined sea urchin, this species inhabits the Caribbean Sea and the western Atlantic Ocean basin, where it inhabits shallow on coral reefs. It fulfills an important ecological role, since they are responsible for maintaining stable populations of many species of algae, which otherwise could cover the corals. It is a herbivorous species, but sometimes, when its food is scarce, it can become carnivorous, as we explain in this other article on What do sea urchins eat? This type of sea urchin is black in color and its most striking feature is the presence of long spines, which measure about 12 cm, with large individuals that can measure more than 30 cm.
Types of irregular sea urchins
Now we turn to the irregular types of sea urchins, those whose bodies have a more flattened shape and have fewer spines than regular urchins. These are the most common irregular sea urchin species:
Heart-shaped sea urchin (Echinocardium cordatum)
This species is also known as heart urchin and is found in all the seas of the world, with the exception of the polar areas. It lives up to a little more than 200 meters deep and in sandy bottoms, where its presence can be noticed because, when buried, a depression is observed. Its body can measure about 9 cm and is heart-shaped and completely covered by short, light, almost yellow spines, which give it the appearance of having hair. He lives buried in chambers that he himself digs in the sand and that can be up to 15 meters deep.
Sea urchin (Echinocyamus pusillus)
The sea urchin is distributed from Norway to Sierra Leone, including the Mediterranean Sea. It generally inhabits calm waters and can be seen down to 1,000 meters deep, on sand or fine gravel bottoms. It is a very small species that usually does not exceed one centimeter in diameter and is flattened and oval in shape. Its spines are short and densely packed. Its color is greenish, although its skeleton is whitish.
Pacific Sand Dollar (Dendraster excentricus)
This species, also known as Western sand dollar, is American and is found in the Pacific Ocean, from Alaska to Baja California. It inhabits calm and shallow waters, generally at a shallow depth, although it can reach depths of around 90 metres, where it buries itself in sandy bottoms and many individuals can group together. Its shape is flattened, which allows it to bury itself in the sand. In general, they measure about 8 cm, although they can reach more than 10. Their coloration varies from brown to purple, and their body is covered byfine hair-like spines
Five-hole sand dollar (Mellita quinquiesperforata)
This species of sand dollar is found off the coast of the Atlantic Ocean in North America and from North Carolina to southern Brazil. It is common to observe it both on sandy coasts and rocky bottoms, as well as in coral reef areas, at depths of more than 150 meters. It is a medium-sized species, since in general it does not exceed 10 cm. Like all other sea dollars, it is flattened ventrally and has five openings on the top of its carapace, which act as gills It is covered by fine, short spines that give it a greenish-brown coloration.
Six-hole hedgehog (Leodia sexiesperforata)
This species of sea urchin is native to the Atlantic Ocean, in tropical and subtropical areas, from North America to South America, where it reaches Uruguay. It inhabits shallow waters and soft-bottomed seas that it uses to bury itself and in areas with little marine vegetation, and can be found up to 60 meters deep. Like the other species, this sand dollar is flattened dorsoventrally and its shape is almost pentagonal Its size is variable, since there are individuals from almost 5 cm to more than 13. And as its name suggests, has six holes, called lunules, on the top of its shell, as well as numerous short spines covering its body.
Other types of sea urchins
In addition to the aforementioned sea urchin species, there are many others, such as:
- Melon hedgehog (Echinus melo)
- Red Pencil Hedgehog (Heterocentrotus mammillatus)
- White sea urchin (Gracilechinus acutus)
- Snuffbox (Cidaris cidaris)
- Purple heart urchin (Spatangus purpureus)
- Red snuffbox (Stylocidaris affinis)
- Sea potato (Brissus unicolor)
- Purple sea urchin (Strongylocentrotus purpuratus)
- Gathering urchin (Tripneustes gratilla)
- Variegated sea urchin (Lytechinus variegatus)
- Burrow Hedgehog (Echinometra mathaei)
- Kina (Evechinus chloroticus)
- Flower Sand Dollar (Encope emarginata)
- Sea cake (Arachnoides placenta)
- Red sea urchin (Asthenosoma marisrubri)