The dumbbell worm

The genus Sternaspis is comprised of sedentary invertebrates with short and thick anterior setae. The dumbbell worm (Sternaspis affinis) can be found on the West Coast of North America, from Alaska to the Gulf of California.

Sternaspis affinis
Ventral (bottom) view of Sternaspis affinis with anterior end retracted

Benthic invertebrates are the tiny water animals that live in the soil at the bottom of Puget Sound. "Benthic" comes from the term benthos which means the community of organisms which live on, in, or near the seabed, also known as the benthic zone.

These mysterious benthic invertebrates often resemble something straight out of a science fiction novel that comes from a planet far away in space, but in reality they live in our own backyard.

Call me dumbbell worm, mud owl, or gooseberry worm

Known to our scientists as Sternaspis affinis Stimpson, 1864, the dumbbell worm has many names. It has been nicknamed the "mud owl" because of two colorful protective shields, called the "caudal shield", that cover its gills when its head is buried in the soil searching for tasty edible specks of organic matter. The shields look so much like eyes that scientists felt that the worms looked like tiny owls in the soil.

Classification of Sternaspis affinis

The caudal shield and "branchiae", or gills, (see diagram below) were originally believed to be located at the head end, leading to the genus name Sternaspis, which is Greek for “breast-shield”. The actual head end includes a mouth and rows of thick hooks used for burrowing into the mud, and is completely retractable into the worm’s body. On the rear end, the long gills are exposed to the water column to help them pull oxygen from the water.

What does it look like?

Sternaspis affinis

Sternaspis affinis, whole worm

The dumbbell worm is tiny, ranging from 15 to 20 millimeters long and 5 millimeters wide. It belongs to the class Polychaeta, a group of segmented worms within the phylum Annelida. Worms in this class all have tiny bristles along their bodies called "chaetae" or "setae". These bristles help the worms move about the soil and anchor to hold on to one place.

Other Descriptive Characteristics

  • Body grayish to cream-colored, with about 29 segments
  • Some of the body segments have rows of little bumps called "papillae"
  • Caudal shield with radiating ribs and concentric lines, and 5 bundles of bristles on each side of the rear edge

Where and how does it live?

S. affinis

Close-up of mouth and hooks

The dumbbell worm can be found living anywhere along the west coast of North America from Alaska to the Gulf of California. Although there are 32 different species living across the world in the same family as the dumbbell worm, Sternapsis affinis is the only species that occurs in the Puget Sound.

This little critter gets its nutrition by eating tiny particles of organic matter in the soft soils of the sea floor. In order to do this, the worm spends most of its day upside-down with its head buried in the mud. You’ve heard the term, “like an ostrich with its head stuck in the sand” but I bet you’ve never heard of a worm that eats with its head stuck in the sand!

Caudal shield

Close-up of caudal shield, which resembles owls eyes

As you can imagine, eating all day with your face buried in the soil can leave the other end of your body exposed to predators. This can be a dangerous feeding strategy, since part of the worm is left vulnerable above the sediment surface.

To protect it from becoming another creatures snack, the dumbbell worm has developed a unique feature to protect its exposed rear end – a pair of plates made from a tough flexible material called "chitin". These plates are called the “caudal shield”, and they are used to cover the opening of the worm’s burrow. This defense mechanism is called “phragmosis.”

About the Author: 
Dany Burgess and Angela Eagleston are scientists who work for the Marine Sediment Monitoring Program at the Washington State Department of Ecology.