Urchins - Hedgehogs of the Sea
They resemble hedgehogs, but aren't quite as cuddly. Here's a closer look at the sea urchin, aka sea hedgehog.
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Their Skeleton is a Test
The domed, fragile endoskeletons are called tests. On each test, you can see five symmetrical pieces that represent the five-fold symmetry that most echinoderms have.
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Brainless
Like other echinoderms, sea urchins don't have a brain. Their nervous system is centralized around their mouth, which is on the bottom of their bodies and known as Aristotle's Lantern. Their gills are also near their mouths.
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Ball and Joint Spines
Their spines are mobile within a ball and joint type socket. They can angle their spines in any direction, and when touched they will often move all spines toward the threatened area.
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They're Not Lazy
If you thought they just sat around, you'd be wrong. They can actually walk using their spines combined with tube 'feet' that are mobilized by a water vascular system. They can use these feet to pick up things as well.
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They Have Claws
These tiny claws, known as pedicellaria, are found on all echinoderms. On sea urchins, they're found adjacent to their tube feet. Some species of sea urchins have longer claws than others, and some are venomous.
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They Use Rocks for Cover
Some species of urchins will pick up rocks, shells, or dead coral and hold them close their bodies. This behavior, called 'covering', is thought to protect them from predators and strong currents.
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They Aren't Cuddly
There are a couple of species that have venom in their spines, but one in particular - the flower sea urchin - has the potential to be deadly. They're found primarily in the Indo-West Pacific.
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Into the Abyss
Urchins can be found in very shallow waters all the way down to 20,000 feet deep. They've been seen in the Java Trench.
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Like it Hot - or Cold
While they're most often found in warmer waters, some do live in the polar regions. The ones pictured here are Antarctic sea urchins living under the ice.
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They Eat It All
They're omnivores, so they have a wider range of food sources. They feed on algae, sea cucumbers, mussels, sponges, and brittle stars.
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They Get Eaten Too
Despite their unappetizing appearance, urchins are preyed upon by otters, crabs, wolf eels, triggerfish, and more. This is a good thing, since urchins can devastate an ecosystem if left unchecked.
