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Pencil Urchins For Saltwater Tanks

By June 20, 2021No Comments
pencil urchin

Pencil Urchins (Eucidaris tribuloides) are a popular invert for saltwater aquariums. They are also called slate pencil urchin or red pencil urchin. These inexpensive urchins usually go for around $10-$15, and they are a great addition to your tank’s clean up crew.

They are efficient at eating away different types of algae, sponges, and coralline algae. They have strong blunt spines that help them cling to rocks and your aquarium glass.

Benefits Of Pencil Urchins in Saltwater Aquariums

Pencil Urchins are amazing members of your tanks clean up crew. They are a bulldozer on a quest to consume anything in their path. If you have any algae in your tank, these urchins will devour any of it in their path. If you have an outbreak of hair algae, they will make quick work of cleaning it up.

These urchins will also get rid of red slime algae (cyanobacteria). Granted, if you have a large outbreak in your tank, these inverts are not the solution to get rid of it all. But, if you do have a couple of spots on your sand bed, they will consume it rapidly if it is in their path.

In addition to the obvious benefit of cleaning up algae, these urchins are fun to watch. While they do move slowly, you can still get a closeup of them when they clean your front glass. Anyone that comes over to your house to view your tank will almost always ask about the urchin in your aquarium.

Are There Any Downsides?

While, these urchins are great, there are some downsides to keeping them in your saltwater aquarium.

As we mentioned, they are very efficient at removing algae. While this is great for most kinds of algae, some people like to keep coralline algae in their tank. This is a favorite food for urchins, so if they can get to it, they will eat it. If you intend to keep coralline in your display, then you do not need to add this urchin.

Pencil urchins are very strong. In their natural habitat, they cling to rocks so that the rough waves will not dislodge them while they are feeding. In your tank, if you have anything loose, they will knock it over if it is in their path. A common complaint is that will knock over unglued frag plugs in coral tanks (more on that later.) If you have live rock that is small and not sturdy, it may knock it over.

Are Pencil Urchins Reef Safe

Pencil urchins are not reef safe because they will eat a variety of corals including soft, LPS, and SPS. Even if they are provided a lot of algae to eat, they will still consume most corals that they come in contact with. They are also not safe for a marine planted aquarium as they will consume those as well.

Some hobbyist have been fortunate enough to have a pencil urchin not eat their corals, but I can 100% guarantee that they will eat some corals. I have seen one eat through green star polyps and Duncan corals. There are also a lot of other reef keepers who have had bad experience with these urchins destroying corals and frags.

Unforunately, there are a lot of sites and even online vendors who mark these urchins as reef safe, but that is not accurate.

Care For Pencil Urchins

  • Scientific Name: Eucidaris tribuloides
  • Common Names: Mine Urchin, Club Urchin, Pencil Urchin, Slate Urchin, Red Slate Urchin
  • Family: Cidaridae
  • Origin: Caribbean
  • Care Level: Easy
  • Size: 2-5 inches
  • Temperament: Peaceful
  • Minimum Aquarium Size: 30 gallons
  • Diet: Algae, Sponges, meaty foods (shrimp, etc.)
  • Temperature: 72-80 Fahrenheit
  • Specific Gravity: 1.023 – 1.026
  • pH: 8.1 – 8.4
  • dKH: 8-12

These urchins will need at least a 30 gallon aquarium with plenty of live rock for hiding and grazing. They do not tolerate high levels of copper, and they do not thrive in tanks with high nitrate levels in it. They are peaceful and fun to watch, but you are taking a risk if you add them to a tank with corals in it.

Their spines are thick and blunt, so they are relatively harmless to pick up. You should not pick them up if they have lodged themselves in some rocks. You will likely break off some spines if you try to forcefully remove one from your tank. These marine creatures can grasp onto rocks, and they have enough strength to withstand waves and tides in their natural environment.


What do Pencil Urchins eat? They will eat almost any algae, sponges, cyanobacteria (red slime), some corals, and meaty foods like mysis shrimp.

Are they poisonous? This urchin is not poisonous like some other species, and their spines are blunt so they will not penetrate your skin and pierce it.


My name is James, and I am the founder of Saltwatercoraltank. I love everything about the ocean, and my main hobby is saltwater aquariums. Currently, I have 3 tanks that I maintain. I have a 130 gallon mixed as my main, and I prefer softies.

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