The pencil urchin (Eucidaris tribuloides) is a species within the genus Eucidaris, and can also be known as the mine or club urchin. It has a brown body that is often mottled, with stubby, thick spines that radiate out in all directions. Each spine is surrounded by a basal plate that has a strong grip. Therefore, do not try to pull it out of a hole, otherwise you will end up breaking off its spines if it hasn’t let go. As the pencil urchin ages, the spines begin to become covered in coralline or other types of algae. 

During the day it will seek refuge in and around the rocks; at night it will come out to feed, searching in particular for algae lawns and smaller invertebrates such as sea squirts or sponges. It should also be noted that this urchin can be destructive to corals. It needs an aquarium with several hiding places and a living rock on which to graze. The pencil urchin has been known to dislodge rocks as it explores the tank, so ensure that any rock formations are sturdy. It is important to monitor the tanks, as the pencil urchin is very sensitive to high levels of copper-based medications and will not tolerate high levels of nitrates. If the water conditions are not suitable, the pencil urchin will shed its spines.

Species Information

On initial inspection, the pencil urchins look like a large ball of spines, but there’s more to them than that. They typically have a diameter of between 5 and 6 inches. Their thick spines, which provide some protection from predators, are attached to their globular core. They range from brown to red or tan, with dull white spines. Their underside is where they become interesting, as they have hundreds of transparent tube feet that move them around and a mouth with five wedges radiating towards each other that appear to look like five sharp teeth. Whilst these relatively small, spiny creatures appear to have no eyes, legs or method of moving themselves, due to their tube feet they are surprisingly mobile when they choose to be.

The pencil sea urchin belongs to the taxonomic phylum Echinodermata, which comprises other species including starfish, sand dollars and sea cucumbers. A key trait they share with other members of their phylum, is that they have five-fold radial symmetry, but it’s not clear when you glance at their dried shells. These urchins are very abundant in the wild and are currently not known to be threatened or endangered.

Native Habitat

Pencil urchins are known for their strikingly thick, blunt spines. The spines allow urchins to wedge themselves into gaps in rocks throughout the day, even in places with a strong current. Mostly nocturnal, pencil urchins can be found in shallow, rock-bottomed areas in reefs and coral crevices. Their most common habitat though is in turtle grass beds. They can be found at a variety of depths, even though they are very common in shallower waters, about 20 feet deep, they have been collected at depths of up to 2,300 feet. Their native habitat extends across the  western Atlantic, from South Carolina to the Caribbean, as well as on the eastern coast of Mexico to Brazil. 

Although their spines provide some protection from predators, they do not completely protect them. Sea otters will happily feed on the peaceful sea urchin, a trophic relationship that has been seen to sustain the kelp forest habitat occupied by these and many other species. Other known predators are eels, starfish, triggerfish, and, of course, humans.


Despite the fact that males and females appear almost the same, slate pencil urchins have very distinct and separate sexes. Pencil urchins reproduce by external fertilization. Females release their eggs and males release sperm into the water at the same time as they enter and become fertilized. Thousands, or even millions, of eggs can be produced by the female in just one go. Once the eggs are hatched, these tiny urchins start out their lives as larvae and take roughly two years to reach their full adult size.

Aquarium Care 

Pencil urchins are nocturnal, so will shelter during the day and will come out to feed at night. Therefore, it needs live rock to find a hiding place and also so that it can graze on any algae mats. Do be aware that they can be destructive to some coral and rock formations. You will need to ensure their food levels are popped up and rock formations are sturdy to prevent damage. Therefore, we do not recommend this species for all reef aquariums. The pencil urchin is very sensitive to high levels of copper-based medications and will not tolerate high levels of nitrates, so monitoring your water parameters is important. If water conditions are not suitable for the urchin, it will begin to shed its spines. 

Feeding and Nutrition

Although mostly herbivorous, slate pencil urchins are opportunistic feeders so will scavenge for food too. They normally go out at night to drink, walk about, and use their rough, horned teeth to brush algae and other plant matter off the rocks and corals. They can also eat sponges, barnacles, mussels, dead fish and other marine animals. Your urchins should be fed with dried seaweed and some animal-based supplemental foods, which will also prevent opportunistic scavenging. 

Urchin’s Aiding Biological Research

Sea urchins are of interest to biomedical researchers for a whole host of reasons. Primarily, researchers are interested in the embryonic development from a bilaterally symmetrical embryo to a five-fold symmetrical adult for decades. This may have significant implications for the general understanding of human and animal developmental biology. More recently, however, the phylum has also attracted considerable interest from evolutionary biologists. 

In addition, genes that are linked with human diseases have been discovered. This has led researchers to try to potentially develop gene therapies through understanding the genome of urchins. Certain genes in some species, for example, have been found to have the power to return cells to a ‘stem-cell like’ state. This type of research has implications for a wide range of treatments for diseases including Alzheimer’s disease and many forms of cancer. Urchins also live a relatively long time considering their small size, as they have the ability to damage aging tissue. This too has obvious implications for human biomedical research and longevity therapies to help slow or stop aging. 


COMMON NAME: Pencil Urchin

SCIENTIFIC NAME: Eucidaris tribuloides

MINIMUM TANK SIZE:  30 gallons




DIET:  Omnivore

CARE LEVEL:  Moderate