Sea urchins first emerge in the fossil record around 450 million years ago. Because their bodies are rich in calcium, sea urchin fossils are remarkable well-preserved. Another reason why they may have been so well-preserved is because of the type of water they inhabited. Many modern sea urchins live along the coast in rough waters, but prehistoric sea urchins likely lived in calm waters where they wouldn’t be broken up. It is theorized that prehistoric sea urchins may have been to fragile to survive to conditions modern sea urchins do.
Around 300 million years ago, are seen considerably less than at other times. As it’s unlikely that fossils from this period would be preserved in a worse state than in other periods, scientists believe that sea urchins nearly became extinct, but luckily made a comeback around 250 million years ago. When they began growing in numbers again, they seem to be nearly identical to the sea urchins that live in our oceans today.
Most sea urchins only measure a few inches across in size, although the largest can reach over a foot wide. Sea urchins can be divided into two classifications: regular and irregular. Regular sea urchins are more or less spherical and are covered in long spines which may or may not be venomous. Irregular sea urchins, a group which includes sand dollars, are flat and have a clearly defined top and bottom. This flat design allows them to burrow better. Because most sea urchins are spiny, they were called “hedgehogs of the sea”. “Urchin” comes from archaic words for “hedgehog”.
Sea urchins do not have a skeleton. Instead, they have a hard shell that completely surrounds their body called a test. The test is another defense, aside from the spines, for the sea urchin. Sand dollars and sea stars also have tests. Their spines help some with moving, but most of the sea urchin’s movements are under the power of hundreds of tiny feet on the bottom of their bodies. The mouth of the sea urchin is on the bottom, too, so that they can scrape algae off of rocks and sand as they move. Sea urchins do not have eyes, but they are able to sense light through their spines.
Sea urchins are most herbivorous. They consume algae more than anything else. However, some sea urchins have been known to be omnivorous, eating other animals such as sea cucumbers. Some of the most common predators to sea urchins are eels, especially the wolf eel. They are also very susceptible to bacteria.
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