The main predators of sea urchins are sea otters, triggerfish, wolf eels, California sheephead, lobsters, and crabs.
Sea urchins are spiny invertebrates that live in rocky seabeds, coral reefs and seagrass forests. They move slowly along the sea bottom with rows of clingy tube-shaped feet, along with their spines.
Sea urchins have a good sense of smell, and when they sense that no predators are around, they move in diverse ways and patterns. But when the urchins smell a predator, the range of movement patterns vanish, and there is only one motion – straight, fast and directional – typical of escape.
What Animals Eat Sea Urchins?
Any sushi lover will tell you that humans are one of the animals that eat sea urchins. The gonads of male and female sea urchins, called sea urchin roe, are delicacies in many parts of the world, particularly Japan.
Another animal that prizes sea urchins as a delicacy is the sea otter. Sea otters are marine mammals that hunt in short dives, often going to the sea bottom.
They are pretty handy with their paws and are the only marine mammal that catches fish with their forepaws rather than with their teeth. When it finds something good to eat, a sea otter will bring it back to the surface, where the otter eats it while floating on its back.
To eat spiny sea urchins, a sea otter will bite through the underside where the spines are shortest and lick the soft center out of the urchin’s shell. Whether it is painful for the sea urchin is not clear, but the otters clearly enjoy it.
A fine-tuned balance exists between sea urchins, kelp, and sea otters. Sea otters maintain the balance of kelp forest ecosystems by controlling populations of sea urchins, which graze ravenously on kelp.
What Fish Eat Sea Urchins
Sheephead fish (Semicossyphus pulcher) eat sea urchins, as do triggerfish in tropical and subtropical seas. Both fish have giant mouths and powerful jaws that can handle sea urchins’ spines and outer shells.
Other fish that consume sea urchins are certain pufferfish types, like the narrow-lined pufferfish (Arothron manilensis) and wrasses like the parrotfish. Hogfish, spot-fin porcupinefish, balloonfish, burrfish, beakfish, moray eels and wolf eels will also feed on sea urchins.
In kelp forests off the coast of southern California, where sea otter populations have declined, sheephead fish and spiny lobsters may be helping control sea urchin populations, a 2019 study found.
Other Species That Consume Sea Urchins In The Ocean
Lobsters and crabs are armed with strong claws that let them take on the spiny defences of sea urchins. Starfish also feed on them.
The sunflower star is a significant predator of sea urchins, as was learned in 2014 when a disease of epidemic proportions hit starfish on the US west coast. The sunflower star’s disappearance flowed through the entire kelp forest food chain in what scientists call a trophic cascade.
Before the outbreak of sea star wasting disease, the abundance of sunflower stars had the most substantial effect on urchin populations. But after the episode, the best predictors of urchin numbers were the abundance and size of the remaining predators – spiny lobsters and California sheephead fish.
Researchers say that the ocean north of San Francisco is too cold for sheephead and lobsters. The otters that are well established on the Central Coast haven’t been able to get established north of the bay.
That led to the urchin population growing relatively unchecked once the sunflower star disappeared. The spiny swarm has since decimated the kelp forests of Northern California and the Pacific Northwest.
- Eisaguirre, J. H., Eisaguirre, J. M., Davis, K., Carlson, P. M., Gaines, S. D., and Caselle, J. E.. 2020. Trophic redundancy and predator size class structure drive differences in kelp forest ecosystem dynamics. Ecology 101(5):e02993. 10.1002/ecy.2993
- Jordi F. Pagès, Frederic Bartumeus, Javier Romero, Teresa Alcoverro. The scent of fear makes sea urchins go ballistic. Movement Ecology, 2021; 9 (1) DOI: 10.1186/s40462-021-00287-1
- Robert P. Dunn, Kevin A. Hovel. Experiments reveal limited top-down control of key herbivores in southern California kelp forests. Ecology, 2019; 100 (3): e02625 DOI: 10.1002/ecy.2625
- Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning. pp. 896–906. ISBN 978-81-315-0104-7.
- Witman JD, Smith F, Novak M (2017) Experimental demonstration of a trophic cascade in the Galápagos rocky subtidal: Effects of consumer identity and behavior. PLoS ONE 12(4): e0175705.
Skin Can See For Fish That Change Color
Scientists speculate that fish that change color with active camouflage don’t depend on their eyes to alter their appearance — they also sense light with their skin. But exactly how “skin vision” works remains an enigma.
Pacific herring have a bluish-green back and silver-white sides and bellies; they are otherwise unmarked. The silvery color derives from guanine crystals embedded in their sides