Sea urchins are found throughout the world's oceans, from the coast to the deepest waters. They are more than 1,000 species unknown to most people, although it is quite common to see them on rocky beaches. There are those who have even pricked their feet with those who hide under the sand. But what exactly are they? What's under all those spikes? How do they eat?
Although they may seem like very simple animals, they are quite complex and interesting organisms. In this article on our site we summarize the characteristics of the sea urchin: its anatomy, feeding, reproduction and much more.
What group does the sea urchin belong to?
Sea urchins are one of the most unknown organisms in the animal kingdom, as well as their entire taxonomic group. Because of its "shell," many people believe that the sea urchin is a mollusk. However, they are echinoderm animals They are part of the Phylum Echinodermata, a group that encompasses more than 7,000 species, including stars, lilies and sea cucumbers, along with brittle stars and, of course, sea urchins.
Despite their apparent simplicity, echinoderms are very complex animals. In fact, it is one of the groups closest to the edge of the chordates, that is, to us. All of them are characterized by having a calcareous skeleton, an aquifer circulation system and pentameric radial symmetry in the adult state. So these are also the main characteristics of the sea urchin.
Within the echinoderms, sea urchins make up the class Echinoid. They are hemispherical animals with a body covered with spikes and a kind of shell. Let's see what it is about.
Sea Urchin Skeleton
As occurs in all echinoderms, the presence of a calcareous skeleton is the main characteristic of the sea urchin. It is a hemispherical structure, that is, convex at the top and flattened at the bottom. It is composed of 10 double rows of plates or ossicles of calcium carbonate. Unlike other echinoderms, these plates are fused together and enclose the body of the hedgehog like a shell.
The skeleton of sea urchins has five-radial symmetry, that is, it is divided into 5 equal parts, each of them formed by 2 rows of plates. These 5 parts are known as ambulacral zones and are homologous to the arms of starfish. The plates that form it have a series of pores through which the tube feet come out. These are structures that connect to their aquifer system and are used to breathe, capture tiny organisms or expel paralyzing toxins.
Among the ambulacral zones of the skeleton are the interambulacral zones, which connect the lower part with the upper part of the body. At the bottom we can find the animal's mouth, which is surrounded by 5 teeth scrapers. At the top is the anal opening, which is surrounded by a set of plates known as the periproct. In them appear a series of openings that correspond to the genital pores and the madreporite, which communicates the aquifer system with the water.
Sea Urchin Spines
Another of the main characteristics of the sea urchin is its spines, which do not appear in the rest of the echinoderms. The skeleton plates have projections or mamelons that are articulated with a series of upright and mobile spines Their function is movement and defense.
In some species, the spines are not sharp and the skeleton is very reduced. However, they have other methods to avoid predation, such as expulsion of toxins In addition, they have very striking colors that warn predators about their toxicity. This is a case of animal aposematism that appears in sea urchins such as Strongylocentrotus purpuratus.
Jagged sea urchins
The characteristics of the sea urchin that we have related are not always fulfilled. Some have irregular shape and bilateral symmetry, that is, their skeleton has an axis that runs from the mouth to the anus. Therefore, his body is divided into two equal parts, like ours. We're talking sand dollars and heart urchins.
In the sand dollars or sand dollars (order Clypeasteroida) the anus is displaced to the side of the body, meeting in the oral area. Thus, we can say that the area where the anus is located is posterior and, therefore, they have lost radial symmetry.
In the heart urchins (order Spatangoida) this anteroposterior axis is even more accentuated. Thus, both the mouth and the anus are located in the lower part of the body. The mouth is displaced to one side, which constitutes the anterior part of the animal, while the area where the anus is located is considered the posterior part.
Sea urchin habitat
Echinoids or sea urchins are marine animals that are distributed throughout all the world's oceans In them, they can occupy very different depths. Some species live in the intertidal zone, that is, the one that is exposed when the tide goes out. Other species, however, can reach very high depths, even inhabiting the abyssal or dark zone, where sunlight does not reach.
Within the oceans, sea urchins live on the seabed, that is, they are benthic animals Regular or hemispherical urchins they prefer hard, rocky bottoms, while spotted urchins live on sandy bottoms. There, they take refuge in cracks in the rocks, among corals, in algae meadows or under the sand.
Discover the World's Rarest Deep Sea Animals.
How do sea urchins move?
Most echinoderms move by filling and emptying their tube feet with fluid. This is the case of starfish. However, sea urchins use their spines to move These spines are articulated with the plates of their skeleton and attached to a series of muscles. In this way, when the muscles contract or relax, the spines move in a similar way to our limbs.
In some sea urchins that have reduced spines, the tube feet can be very useful in movement, just as in other echinoderms.
In this video by Fernando Vblog we can see a slight movement.
How do sea urchins reproduce?
Sea urchins exhibit sexual reproduction and separate sexes, that is, there are male and female urchins. When it is time to reproduce, the females pour their eggs into the sea and the males do the same with their sperm. Subsequently, these gametes unite and fertilization occurs. Thus, the eggs are formed, which are deposited on the seabed.
When the eggs hatch, they hatch into bilateral larvae known as equinopl u te u s. They are small, planktonic swimmers that live suspended in the water along with other tiny organisms. After several months, they undergo a metamorphosis and acquire pentaradial symmetry. Thus, transformed into adults, they return to the bottom of the oceans and reproduce, starting a new cycle.
How do sea urchins feed?
After reviewing the main physical characteristics of the sea urchin, where it lives and how it reproduces, let's now see what the sea urchin eats. Most sea urchins are omnivorous animals, although some species are exclusively herbivorous or carnivorous. When they are larvae they feed on phytoplankton and other floating organisms. Once they become adults, their main food is algae, usually fleshy brown algae. They also often consume sessile invertebrate animals, that is, they live fixed on a substrate, such as bryozoans, tunicates and sponges.
To feed, sea urchins must sit on top of their food, since their mouth is on the bottom of their Body. Thanks to their 5 teeth, regular hedgehogs can scrape up algae and animals that stick to rocks. Irregular sea urchins also have structures around their mouths with which they remove the sand in search of food. They can also collect particles and small organisms in suspension thanks to modified tube feet known as pedicelaria.
Once they eat the food, they break it down thanks to a complex chewing device known as Aristotle's lantern. The food then travels down the esophagus, which connects to the intestine through a siphon. This prevents the passage of water and concentrates the food, which passes to the intestine for digestion. Finally, the waste exits through the anus, which is located in the upper part of the animal, except in irregular hedgehogs, as we have seen before.
Sea Urchin Customs
The behavior of sea urchins depends a lot on each species. In general, they are sedentary animals that live on the seabed and move very little. During the day, they take refuge in cracks and holes in rocks or among corals. At night, when their predators are less active, they come out to feed in areas near the refuge. To do this, they move by following certain chemical substances present in the food, or else, attracted by the sexual hormones of other hedgehogs.
Some sea urchins are gregarious and form large groups with others of the same species. This is the case of the green sea urchin (Strongylocentrotus droebachiensis), whose individuals form aggregations to feed and also to take shelter, since together they run less risk of being preyed upon. Also, staying together makes it much easier for them to reproduce.
Other hedgehogs are territorial with other individuals of the same species. The rock urchin (Echinometra lucunter) lives in coral reefs, where it takes refuge when not feeding. When an intruder approaches its burrow, it does not hesitate to push it and even bite it, although they can coexist when resources are abundant.
As for irregular hedgehogs, they tend to be much more sedentary. Many of them, such as Echinocardium cordatum, can remain half-buried under the sand for a long time. In this way, they can feed on small organisms that float or pass through the sand without having to move.
The image shows the rock urchin.
