The pencil urchin is a species of sea urchin from the Atlantic ocean.
There are very many different kinds of sea cucumbers, some are colorful and flamboyant, others have more mundane colors. This was my first sea cucumber when I get involved in the marine aquarium hobby.
Sea urchin are voracious algae eaters. Unfortunately they also tend to try to rearrange the rockwork in your reef tank, and they grow big, and strong enough to cause some real damage.