“Why know me? Well I’ve always been known for my quills, but I can do more than just sting!”
Who is the sea urchin?
Sea urchins include approximately 950 species that are very easily recognisable, their specific characteristics give us little margin for error:
The shape can be regular and therefore spherical, or irregular and therefore oval or heart-shaped;
Its skeleton is calcareous and consists of several plates welded together;
“When we die, it is washed ashore by the waves and currents, you have surely seen it before!”
The color is very varied: black, purple, red, white, pink...
“We are beautifully colorful”
Their body is covered with spines that can be all the same length, some longer and some shorter, thick or thin, pointing backwards or perpendicular to the body;
“In any case, you won't have any trouble recognizing them”
They have an oral opening facing downwards and an aboral opening (anus) facing upwards;
They have small structures between one spine and another called pedicellariae: these are pincer-like structures that are responsible for cleaning the surface of the body from debris but also for defending the animal;
“We’ll explain to you more about what I can do soon!”
With a little attention we can see them “walk” thanks to the coordinated movement of the spines to grip the seabed and the pedicels to drag them along.
“What are pedicels? You’ll find out in a moment!”
Strange relatives
Sea urchins, also called echinoids, are part of the echinoderms like starfish, brittle stars, sea lilies and sea cucumbers. Apparently it seems that all these organisms have nothing in common with each other but instead they all have:
A pentamerous symmetry, probably developed as an adaptation to a sedentary lifestyle
A very ingenious aquifer vascular system for movement: it is made up of a series of water-filled channels that extend outward through projections called ambulacral pedicels passing through openings in the calcareous skeleton.
“the ambulacral pedicels are made of a small bulb filled with water and a kind of suction cup that comes out of our body. These, by inflating, allow us to move on the seabed”
An internal skeleton of calcium carbonate made up of bony plates called ossicles.
The skeleton of the urchins is easily distinguishable from that of its relatives because it has a fairly large opening in the "lower" part, i.e. oral, where the masticatory apparatus was located, and the "upper" part, i.e. aboral, with strange "decorations".... Let's see:
At the centre of the aboral part we find the madreporite, which is a set of small plates fused together with many holes, its function is to put the aquifer system in communication with the outside;
Most sea urchins have perforated plates with alternating colours: from some of them the spines emerge, while from others the pedicels and pedicellariae emerge;
In other sea urchins, like the one in the photo below, 5 perforated "petals" called genital plates are clearly visible, representing the openings of the gonads towards the outside to expel the gametes.
Where the rich man of the sea lives
Sea urchins have colonized seas all over the world, both temperate and tropical seas, both sandy and rocky bottoms. It is very easy to see them in the crevices of rocks even at a few meters depth. Regular sea urchins prefer rocky bottoms while irregular sea urchins prefer sand.
For example, 26 species of sea urchins live in the Mediterranean Sea: 11 regular and 15 irregular.
What and how does the sea urchin eat?
Sea urchins are omnivorous, which is a big plus for them to be able to adapt everywhere. They eat mostly bryozoans, algae, coral polyps, small invertebrates and remains of dead animals.
Some sea urchins have specialized in eating algae and are also very voracious! An excessive abundance of herbivorous sea urchins in a certain area, not balanced by the presence of predators, could create a real desertification.
Hedgehogs have a particular mouth structure called Aristotle's Lantern, made up of 35 ossicles with associated muscles. It owes its name to the great Greek philosopher who first described it, giving this structure the resemblance of a lantern. The Aristotle's lantern is a very robust and efficient chewing apparatus that allows the hedgehog to break up the food to ingest.
Pedicellariae also contribute to food capture, trapping planktonic larvae that end up between the urchin's spines and directing them to the mouth.
Sea Urchin Weapons
Although it may seem like a static spiny ball, the hedgehog is not so calm, in fact it knows how to both defend itself and attack!
First of all, the spines are among the most effective weapons: they can be sharp or not, hollow and contain poisonous substances.
The second weapon is the pedicellariae which, in addition to the functions already seen, can contain poison that they inject into their main enemies: starfish.
Furthermore, all the structures of the body, including the calcareous skeleton, can be quickly regenerated in case of breakage.
In addition to starfish, the sea urchin must guard its spines from fish such as breams, some large gastropods and crustaceans, sea birds and sea otters.
“We don't have an easy life either”
The life of the sea urchin
Sea urchins have separate sexes so there will be males and females that are difficult to distinguish from the outside. Some species congregate towards the coast to mate, fertilization is external with the release of gametes into the water.
During the reproductive season, the 5 orange gonads (sacs that contain eggs or spermatozoa) swell a lot and occupy a large part of the inside of the animal. These are famous from a culinary point of view and are mostly eaten raw.
“The problem is that they cannot be extracted without killing us and there is not always the certainty of finding them inside us.”
The gametes are then expelled through the madreporite or the genital plates.
From fertilization, small planktonic larvae will be born that, after numerous transformations, will settle on the seabed and give life to a new small sea urchin.
Friendly Fins and Sea Urchins
We can easily encounter the sea urchin while snorkeling or diving. They are often seen more easily at night or at dusk, when they come out of their hiding place and start hunting.
Obviously it goes without saying that it is better not to touch or step on them!
The sting is very painful and its spines break once they enter our skin and it is as complicated as it is painful to be able to remove all the fragments. In addition to them we have seen that there are pedicellariae that contain poison ready to be used for defense.
"It's not a lethal poison for humans, but it's not a walk in the park either!"
Remedies in case of sting
As soon as we are stung we feel a great pain that we can alleviate with ice and some anesthetic substance.
It is also advisable to put hot water on the sting, as the venom is thermolabile and therefore inactivates with heat.
Finally, we arm ourselves with saintly patience and manually remove the spines and any fragments with the utmost care, because there is an easy risk of an infection that will have to be treated with antibiotics.
“Translation = holiday over! So I recommend Friendly Fins always check where you put your feet and hands, especially in the first few meters of water and then avoid touching anything underwater!”
But once we have recognized the hedgehog… Why don’t we try to distinguish one hedgehog from another?
Follow us to discover their differences!