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OCH BLOG

Sea Urchins

Updated: May 9, 2023

By Jessica Brogna


The first time I ever saw a sea urchin, I thought it was a sedentary animal that did not have anything all that interesting to it. However, urchins are a lot more complex and have a bigger impact on our ocean ecosystems than most think. Urchins are in the phylum echinodermata (referred to as echinoderms), along with sand dollars, sea stars, sea cucumbers, and a few other species. Urchins can be found all over the globe, in fact, there are over 900 species of urchins in the world. California is home to two different types of sea urchins, the red (S. franciscanus) and the purple (S. purpuratus).



Urchins are easily distinguishable by their spines. They use their spines for defense and can actually move them towards a potential threat. The spines contain venom as well. Arguably one of the coolest things about sea urchins is their tube feet. These are used as more than just “feet,” although they do use them to move. Urchins use their tube feet to breathe and to see! National Geographic reports that urchins’ bodies are covered in light receptors that function as a sort of “eye,” although urchins do not have actual eyes.


Urchins reproduce externally by releasing their eggs and sperm, also known as gametes, into the water column. The gametes meet in the water column and an embryo forms. These larvae are planktonic and have two different planktonic stages before they grow into a benthic (seafloor dwelling) organism. The young urchin grows inside, and when it is ready to settle on the seafloor, it flips itself inside out by reaching its tube feet out. It then settles on the seafloor and grows into what is now visually identifiable as an urchin. According to the National Park Service, the average lifespan of an urchin is 20 years, however, some red urchins have been found to be 200 years old!


Photo Credits: Cynthia Bradham

Sea urchins are omnivores, meaning they eat both plants and animals. Their diet mostly consists of algae (AKA kelp or seaweed), but they also eat animal matter, such as plankton, that floats by. The urchins catch them with their tube feet and move it to their mouth. Because of their algae diet, they can pose a huge threat to kelp forests if the food web is not functioning optimally. In a properly functioning ecosystem with a balanced food web, sea urchins are helpful at maintaining a healthy balance of algae. In addition, their predators help ensure that the urchin population remains at a healthy size.


Photo Credits: Morgan Rector

Sea urchins have a few main predators, CA sheephead (​​Semicossyphus pulcher), sea otters (Enhydra lutris), sunflower stars (​​Pycnopodia helianthoides), and humans. Otters are the primary predator, in fact, they are what’s known as a keystone species. According to Oxford Languages, a keystone species is, “a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically.” Unfortunately, sea otters were hunted to near extinction in the late 1800s, and their populations were completely wiped out in Southern California, Oregon, and Washington. The US Department of Fish and Wildlife stated in an otter reintroduction feasibility study that they completed, that they are not planning on reintroducing otters in Southern California. Their reasons for this were 1) there is “a greater redundancy” of sea urchin predators in the OC area, 2) the gene flow between Northern and Southern sea otter populations would not be impacted. Sunflower stars are another urchin predator, but were greatly affected by sea star wasting syndrome in 2013-14. This caused a massive die off and allowed urchin populations to boom. As for us humans, we gather sea urchins and eat them for food! It is most common in Asian dishes, and is known as uni.


Photo Credits: David Hagerman

Some of the threats that urchins face range from illness to the effects of climate change. Sea urchins fell victim to sea star wasting syndrome (SSWS) and black spot disease. SSWS causes urchins’ spines and sea stars’ legs to be degraded. Black spot disease causes lesions on the test where a portion of the spines fall off completely, leaving the test exposed. Urchins can have both at the same time, however they are not related diseases. Ocean acidification is also harmful to urchins i. The lowering pH of the ocean can cause the spines and test of the urchin to break down leaving the urchin vulnerable to predation and starvation. Sea urchins are a native species and even though they can be harmful to our kelp forests, we still want them around because they play an important role in our marine ecosystem here in Orange County.


Urchins have important impacts on their environments, either positive or negative depending on the stability of the ecosystem. OC Habitats is starting a sea urchin monitoring program in collaboration with Get Inspired to watch the population sizes of sea urchins in the intertidal zones of beaches in Orange County. The aim of this project is to do surveys in tide pools along the Orange County coast in order to get an idea of how many urchins are living in the tide pools. High numbers in the tide pools can indicate high numbers in the subtidal zone, which is when our kelp forests could suffer. Once the population sizes are known, then further steps can be taken to help maintain a healthy population. If anyone is hoping to get involved, visit our website and check out the get involved tab to sign up for future monitoring opportunities!



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