5 Fun Facts About Sea Urchins
Echinoids, commonly known as sea urchins, are a class of the echinoderm phylum. These animals are very different from other groups, mainly due to their radially segmented body rather than the typical bilateral arrangement. We invite you to dive into the many curiosities about sea urchins.
These small animals live only in the oceans, like the rest of the animals in the same phylum. They never colonized fresh waters, so to see them you have to go down to the sea. If it’s a professional dive or using a snorkel, so much better! Here are five fun facts about these beings.
1. The spines of sea urchins are part of their skeleton
One of the biggest curiosities about sea urchins is that they have an endoskeleton. Despite this, they are not vertebrate animals, but invertebrates, since they do not have a spine or embryonic development like that of mammals or birds, for example.
Under the epidermis, sea urchins have a skeleton made of fusing plates, which impedes movement. These plates are also called calcareous ossicles and in other types of echinoderms – such as the starfish – they are articulated and allow the animal to perform its movements.
On many occasions, although not always, the calcareous ossicles have spines or tubercles, which emerge through the epidermis. For this reason, many sea urchins have a characteristic prickly appearance.
2. Another curiosity about hedgehogs: they have feet!
Although the endoskeleton of sea urchins – being fused – prevents the animal’s movement, this does not mean that they cannot move. In fact, the most important and exclusive feature of this group of animals is the presence of an aquifer or ambulatory system.
From the inside of your body, called coelom, derives a system of tubes or channels that have associated appendages that protrude outward. These channels are filled with salt water thanks to a special endoskeleton plate called madreporito, which is located in the upper part of the hedgehog, close to the anus.
Seawater enters the sea urchin through the mother-of-mouth plate. This gives turgidity to the tubular feet, which, thanks to a rudimentary nervous system, allows these animals to move, even if slowly.
3. Some hedgehogs have deadly poisons
The echinoids have structures called pedicelaria, which are found around the spines throughout the upper region of the animal. These formations can have various functions, including defense, feeding or cleaning.
Each pedicel consists of three parts: head, neck and pendulum. The head is usually made up of three calcareous bones, and inside there are sensitive cells that tell you when to open and close.
The normal thing is that pedicelers are responsible exclusively for cleaning the hedgehog. Thanks to them, the animal goes palpating the surface of the body, looking for any parasites that may have installed itself.
On the other hand, pedicellars sometimes contain poisonous glands that are activated defensively. When a person tries to catch a poisonous sea urchin or accidentally steps on it, the invertebrate injects the poison, which is very painful and even deadly.
4. Hedgehogs breathe through their gills
Like fish, sea urchins breathe through their gills, but unlike them, these invertebrates can also excrete waste through their gills.
Around the mouth, at the bottom – which we’ll talk about later – these animals have five gills that are coelom evaginations. Through them, they carry out gas exchange with salt water and, in addition, they also serve to dispose of certain substances that do not come out through the anus.
5. The strange mouth of sea urchins
Inside the body and close to the opening that is the mouth of the sea urchins, there is a structure called Aristotle’s lantern. It consists of five teeth that, together with the endoskeleton and surrounding muscles, support them.
Sea urchins use Aristotle’s lantern to scrape algae from the substrate or break food into smaller pieces. Likewise, they also use it to cling to the ground or climb certain surfaces.
There is no doubt that sea urchins are spectacularly strange animals. If we go deeper into the different species that exist, the curiosities of sea urchins would be almost endless.
