🐾 Sea Urchin

Sea Urchin

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Who it is

• Scientific name: Class Echinoidea
• Family & relatives: Sea urchins are echinoderms, related to sea stars, sand dollars, and sea cucumbers.
• How to spot it: A round, spiny ball with a hard shell covered in movable spines.

Its body & abilities

• Size: Most are about 3 to 10 centimeters across, not counting the spines.
• Weight: They are light animals, usually weighing well under half a kilogram.
• Body & adaptations: Their round shell, called a test, is made of fused plates, and a mouth with five teeth sits underneath.
• Top speed: They move very slowly using their spines and tiny tube feet.
• Strength: Their sharp spines are a strong defense, and some species carry a mild venom in them.
• Senses: They have no eyes but can sense light and touch across their whole spiny body.
• Growing up: They start as tiny swimming larvae that drift with the plankton before settling to the bottom.

Where it lives

• Habitat: They live on rocky reefs, kelp forests, and the sea floor in shallow and deep water.
• Where in the world: Sea urchins are found in oceans all over the world.
• Territory: They stay in small home areas and sometimes carve hollows in rock to shelter in.

What it eats

• Diet: Most are grazers that mainly eat plants.
• Favorite foods: They feed mostly on algae and kelp, plus tiny bits of animals and debris.
• How it hunts: They graze by scraping algae off rocks with a special five-toothed mouth called Aristotle's lantern.

How it behaves

• How it moves: They creep along on rows of tube feet, using their spines to push and grip.
• When it is active: Many hide in crevices by day and come out to graze at night.
• Family life: They are not truly social but often live packed close together in feeding grounds.
• How it talks: They rely on chemical and touch cues rather than sight or sound.
• Babies: They release clouds of eggs and sperm into the water, where the eggs are fertilized.

Its place in nature

• Role in nature: As grazers they shape kelp forests, and too many can strip a reef into a bare 'urchin barren.'
• Conservation: Many species are common, but overgrazing and warming waters can throw their numbers out of balance.
• Dangers: Sea otters, fish, crabs, and sea stars are among their main predators.

Life & legacy

• How long it lives: Many live around 10 to 30 years, and a few species can live much longer.
• In our culture: Their eggs, known as uni, are prized as food in many coastal cuisines.
• Where to see it: You can see them in tide pools, on rocky reefs while snorkeling, and in aquarium touch tanks.
• Fun fact: A sea urchin walks on hundreds of tiny see-through feet that reach out between its spines!