Eyes Under Puget Sound: Critter of the Month —The Heart Urchin

Be still, my heart (urchin)! During the time of year when candies and valentines are everywhere you look, it is only fitting that we present to you the most romantic creature in Puget Sound as February’s critter: the Heart Urchin.

Spiny oblong creature with bear-paw-shaped spine design in the center. A heart is sketched around it.

Happy Valentine's Day from this adult specimen of Brisaster latifrons collected from Puget Sound.

The shape of my heart

Heart urchins are a type of sea urchin, which are in the phylum Echinodermata (meaning “spiny skin”), along with sea stars and sea cucumbers. Unlike most sea urchins, which are round, heart urchins appear heart-shaped because their bodies are elongate, with a small depression for the mouth at one end.

A living heart urchin is brownish in color, with its many spines giving it a furry appearance. Once the animal dies, however, the spines fall off, leaving the smooth white external shell, or test, exposed. The heart urchin test is made up of hundreds of hard interlocking plates, but it is actually quite brittle in dead specimens, which is why you will hardly ever find one washed up on the beach.
 
White stone- or skull-looking object seen from top, bottom and side. The shape on the top looks like a sand dollar.

Different views of Brisaster latifrons missing a majority of its spines and revealing its white test. Photo courtesy of Linda Schroeder (PNW Shell Club).

How deep is your love?

Damp tan spiny critter in a stainless steel basket. Arrow indicates the mouth.

Brisaster latifrons, freshly collected by our team from Puget Sound sediments.

Only one species of heart urchin, Brisaster latifrons, occurs in Puget Sound, and it is also the most common sea urchin we encounter during our sediment monitoring. B. latifrons can be found from Alaska to southern California, inhabiting soft muddy bottoms in deeper water depths of 51 – 1,166 meters (although Puget Sound depths only reach about 300 meters).

Growing up to 73 mm, the heart urchin is a fun and welcome sight in our sediment grabs. We just have to handle their fragile outer shells carefully — we don’t want to break any hearts!

Flower power

Pink preserved specimen shows bear-paw or flower-shaped section.

This juvenile Brisaster latifrons displays the five petals of the ambulacra on its dorsal (top) surface.

Heart urchins have little in common with flowers, but they do have some surprisingly flower-like features. For example, underneath their spines are five flower petal-shaped outlines called the ambulacra.

The pedicillariae are another type of flower-like appendage that stick out of the test. They have serrated claw-like pincers that open and close at the end of their long stalks. They use these pincers for a variety of functions like defense, capturing food, parasite removal and more! When the pincers are open, they resemble flower blossoms, and when closed, they look like tulips. Now that’s a bouquet I’d like to give to my ex-boyfriend.

Diptych: microscopic closed pincers look like a tulip. The open pincer looks like a three-fingered hand.

Close-up of the pedicillariae showing the claw-like pincers closed (left) and open (right).

Eat your heart out

The tube feet are tinier than the spines. The close-up shows that the feet look like tiny tube anemones or mushrooms with pompons.

Top: Tube feet come out of the pores that make up the ambulacra petal outline. Bottom: Close-up of the tube feet.

Like other sea urchins, heart urchins have tube feet— slender tubes with suction cups on the ends that extend from pores in the ambulacra petals. These amazing feet serve multiple functions:

  • Feeding: As the urchin burrows slowly through the mud, it ingests sediment and delicious organic matter along the way (this is called “deposit feeding”). The tube feet help by delivering food particles to the mouth.
  • Locomotion: Thousands of tube feet work together at the same time to move the heart urchin wherever it needs to go. As the urchin moves along, the tube feet act like feelers or antennae to sense its surroundings.
  • Respiration: Heart urchins primarily live in the mud. Modified tube feet that stick out above the mud’s surface act as a kind of snorkel to allow the urchins to obtain oxygen.


We hope after reading this blog, you too have fallen in love with the heart urchins.

Kingdom, Animalia; Phylum, Echinodermata; Class, Echinoidea; Order, Spatangoida; Family, Schizasteridae; Genus, Bristaster: B. latifrons