Nature’s underwater ‘pencil pusher’

By Conservancy volunteer Bill Rhodes

Visitors to the touch tank at the Conservancy of Southwest Florida’s Dalton Discovery Center easily spot the animals that are moving; this includes the wary pinfish, hiding in plain sight in the back corner of the tank, the sea stars adhering to the tank’s walls, the whelks and conchs smoothly gliding along on the sand, and the horseshoe crabs busily burying themselves into the bottom.

With a little bit of effort, sharp-eyed observers will also pick out an odd, round animal, which looks like an inflated, thickened pin cushion — covered in round projecting ‘arms’, it has squeezed itself tightly in the crevices of a coral outcropping in the middle of the tank.

Algae that has collected on it spines, or ‘pencils’ has turned it a reddish-tan color, which blends well with its surroundings. These pencils, which are actually short stiff projections of its shell (which is called a ‘test’), help wedge it tightly into the rock, protecting it from predators. This is the Pencil Sea Urchin, a member of the genus Eucidaris. Sea urchins are found in all major oceans of the world and in some places they are harvested for their eggs, or roe, which are eaten raw — sushi lovers may recognize them as the ‘uni’, found on many sushi restaurants’ menus.

They are members of the Phylum Echinodermata, a term that was taken from the Greek words for ‘spiny’ and ‘skin’, and are relatives of the sand dollars and sea stars.

Visitors to the touch tank may touch the Pencil Sea Urchin, but removing it from its hiding place would be difficult, and breaking the pencils is to be avoided. While it appears to be an animal that never moves, that is definitely not the case. It is a nocturnal feeder, and at night will move about the tank, opportunistically feeding on whatever it can find. Sea urchins are omnivores — they will eat algae and other plant life, as well as decaying pieces of animal matter.

Urchins are often prized in home aquaria, as they are excellent scavengers of algae and bits of decaying animals, doing their work through the night to help clean the tank. In the wild, they feed on living coral reefs and rocks, eating the algae and detritus they find there.

They have a hard round shell (the test) within which their soft, tender and vulnerable body parts are protected. In the center, underneath, is a mouth with a horny beak, which is used to scrape food as it moves about the ocean floor. Equipped with hundreds of small fleshy tube feet, similar to its relative the sea star, the sea urchin is an efficiently mobile animal, but only at night — so look carefully in the rock and you will finding it hiding there, waiting for visitors to leave and night to fall, before making its rounds.

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