Sea urchins are spiny sea creatures that are a part of one of the best-preserved groups of animals in the fossil record. They’ve been around for some 200 million years, so they must be doing something right. Their preservation in the fossil record is thanks to their mega-hard shells. And sea urchins are still considered animals despite their lack of brains or blood.
Sea urchins are echinoderms, that is, marine invertebrate animals. The famous starfish (or sea star) and sea cucumbers also belong to this group. There are around 950 species of sea urchins.
You can find sea urchins in a wide range of colors, from red to purple and green, making them one of the most visible and striking marine animals on the seafloor.
Sea urchins live in groups of five to ten individuals among rocks and on the seabed. The life expectancy of these animals often exceeds 30 years, but scientists have found specimens over 200 years old. Amazing!
The sea urchin looks like a ball, so you might wonder where its mouth is. You can’t see it because it’s on the bottom of its body. So what’s that opening at the top? That’s its anus.
Sea urchins don’t have brains. Instead, they have simple nerves that reach throughout the body and radiate from a neural ring around the mouth. These nerves allow them to control all parts of their body, in addition to giving them the sense of touch.
An average sea urchin is small, measuring about 1.2 to 4 inches (3 to 10 cm) in diameter. However, the largest species can reach 6.7 inches (17 cm).
Sea urchins’ internal organs are protected by a skeleton made of calcareous plates (made of calcium carbonate). The plates fuse together to form a hard, compact shell from which sharp spines protrude.
This structure made of small segments of plates almost completely covers the animal (except for the mouth and anus). This is their main form of protection. However, this skeleton is quite heavy, which makes them slow animals.
A curious fact about these structures is that you can divide them into two exactly equal halves. And when they become adults, they develop five-fold symmetry, meaning you can divide their bodies into five exact sections.
Slowly but surely, the sea urchin does indeed move. However, it does so little by little due to the great weight of its shell.
These creatures move using so-called tube feet. These are a type of tentacle with thick and muscular walls found between the animal’s spines. These tentacles end in suction cups that allow them to move, adhere to surfaces and capture prey.
Even the sea urchins’ spines and mouths themselves serve to grip and move. Additionally, the spines move in all directions.
Another fascinating fact about sea urchins is they have the ability to regenerate their spines and tentacles.
The tips of sea urchins’ spines have pincers that allow them to remain in the skin. If you’re ever stung by a sea urchin, stay calm, try to remove the spines as soon as possible, keep the sting area clean, and seek medical help if the pain persists for several days.
It is important to remove the spines because if they break, they can migrate to deeper tissues and touch bone or nerves. In this case, the spines would have to be removed surgically.
While some species are venomous, their stings are not fatal. If the spines are removed promptly so that the venom does not spread further and the wound is thoroughly cleaned, this should be sufficient treatment.
Despite their beauty, you have to be especially careful with flower sea urchins, as they are one of the most dangerous types. These are found in the Indian Ocean. Their body appears to be covered in white, pink and beige flowers. But the poisonous spines are short and well camouflaged, so their appearance is similar to that of small flowers.
There are very few documented deaths from this type of sea urchin, but what is certain is that death is not caused by the venom itself, but by drowning due to the shock and paralyzing effects from the venom.
Sea urchins live in all oceans, although they are found to a lesser extent in the polar regions. Thus, it’s more common to see them in temperate and tropical areas among rocks and reefs or on the seafloor up to 1.5 miles (2,500 m) deep.
Although resistant, they still become meals for seabirds and fish, otters, lobsters, crabs, starfish and even foxes. And of course, they can’t escape human cooking pots …
The sea urchin is very important when it comes to maintaining a certain balance in the sea. That’s thanks to its eating habits, because it is an opportunistic and omnivorous animal. Basically, it feeds on what it finds in greatest abundance.
In general, sea urchins feed on algae, small animals, mussels, sponges and barnacles. However, on some occasions they even eat dead animals deposited at the bottom of the sea.
Sea urchins reproduce sexually. The female releases millions of tiny eggs into the water that will later be fertilized by the male’s sperm.
The tiny eggs of the sea urchins float for a few months among the plankton. The young urchins will remain in the plankton for between two and five years, depending on the species, and then descend to the seabed. They reach sexual maturation at five years of age.
Bottom line: Sea urchins possess mega-hard shells. They’ve been around for some 200 million years. And they’re considered an animal even with no brains or blood.
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