Characteristics of Heart Urchins, or Sea Potatoes

Heart urchins (also called spatangoid urchins or sea potatoes) get their name from their heart-shaped test, or skeleton. These are urchins in the order Spatangoida.

Description

Heart urchins are relatively small animals that are usually not more than a few inches in diameter. They look a little like a cross between an urchin and a sand dollar. The oral surface (the bottom) of these animals is flat, while the aboral surface (the top) is convex, rather than dome-shaped like a "normal" urchin. 

Like other urchins, heart urchins have spines covering their tests. These spines may be a variety of colors, including brown, yellowish-brown, green, and red. The spines are used for movement, including helping the urchin burrow into the sand. These urchins are also known as irregular urchins because they have an oval-shaped test, thus they are not round like typical urchins — such as the green sea urchin. 

Heart urchins have tube feet that extend from petal-shaped grooves in their test called ambulacral grooves. The tube feet are used for respiration (breathing). They also have pedicellariae. The mouth (peristome) is located on the bottom of the urchin, toward the front edge. Their anus (periproct) is located on the opposite end of their body. 

Heart Urchin Relatives

Heart urchins are animals in the Class Echinoidea, which means they are related to sea urchins and sand dollars. They are also echinoderms, which means they belong to the same phylum as sea stars (starfish) and sea cucumbers.

Classification

• Kingdom: Animalia
• Phylum: Echinodermata
• Class: Echinoidea
• Order: Spatangoida

Feeding

Heart urchins feed by using their tube feet to gather organic particles in the sediment and in the water around them. The particles are then transported to the mouth.

Habitat and Distribution

Heart urchins may be found in various habitats, from shallow tide pools and sandy bottoms to the deep sea. They are often found in groups.

Heart urchins burrow in the sand, with their front end pointing downward. They may burrow as much as 6-8 inches deep. So that the heart urchin continues to receive oxygen, their tube feed can continuously move the sand above them, creating a shaft of water. Heart urchins live primarily in shallow waters less than 160 feet deep, although they may be found in waters of up to 1,500 feet deep. Since these are burrowing animals, heart urchins are not often seen alive, but their tests may wash ashore. 

Reproduction

There are male and female heart urchins. They reproduce sexually through external fertilization. During this process, males and females release sperm and eggs into the water. After an egg is fertilized, a planktonic larvae forms, which eventually settles to the ocean bottom and develops into the heart urchin shape. 

Conservation and Human Uses

Threats to heart urchins can include pollution and trampling by beach visitors. 

Sources

• Coloumbe, D. A. 1984. The Seaside Naturalist: a Guide to Study at the Seashore. Simon & Schuster. 246pp.
• Marine Species Identification Portal. Red Heart Urchin. Interactive Guide to Caribbean Diving.
• Marshall Cavendish Corporation. 2004. Encyclopedia of the Aquatic World.
• Smithsonian Marine Station at Fort Pierce. Heart Urchins.