What the shell? The tusk shells are in a class all their own

Eyes Under Puget Sound - Critter of the Month

Welcome to the jungle! Take a walk on the wild side with this month’s group of critters, which look like something you might find on an elephant’s face.
Creature with long, thin shell; the aperture/ opening has some fuzzy tissue exposed.

Rhabdus rectius, a species of tusk shell found in Puget Sound. Photo courtesy Dave Cowles, wallawalla.edu.

Come shell or high water

Tusk shells belong to the Class Scaphopoda, meaning “boat foot,” which refers to lobes near the animal’s foot resembling a boat hull. Scaphopods are strictly marine organisms that can live at an incredible depth of up to 4,570 meters — or more than 13,000 feet deep! 

Kingdom, Animalia; Phylum, Mollusca; Class, Scaphopoda

While there are more than 900 species worldwide, we only encounter two species during our sediment sampling in Puget Sound. Most are collected from our northern locations in Bellingham Bay, San Juan Islands, and the Strait of Georgia.

The elephant in the room

In contrast to a real elephant’s ivory tusk, a scaphopod’s conical shell is open on both ends. This design serves a purpose. They live buried in the sand or mud with their head – the wider end of their shell – pointed downward while the tapered end sticks above the sediment to allows the animal to expel waste and exchange water. 
Two images: top looks like a ragged elephant tusk, and the bottom looks like a bloodless severed limb

Top: Pulsellum salishorum in its shell. Bottom:  A P. salishorum head up close up with foot and captacula withdrawn. Bright pink color comes from stain used to pick animals out of sediment samples.

The shell’s inner surface is lined with the mantle, a soft inner body wall all mollusks share. Unlike other mollusks, however, scaphopods lack gills for taking in oxygen. Instead, they have tiny hairs or cilia that move water around the mantle cavity. The beating cilia suck water in through the shell’s smaller opening and push it back out the same way after the oxygen has been used up.

Head to toe

Scaphopods lack gills and a heart and blood vessels for circulation. Instead, their blood is pumped by the movement of the scaphopod’s muscular foot, located at the end of its head. The foot pulls double duty: It is also responsible for locomotion. To move, the animal stretches out its foot to anchor it into the sediment. Then, it pulls the entire body after it when it retracts.

All you can eat

Scaphopods are selective deposit feeders, sifting through the sediment and picking out particular things to eat.

Collection of tiny creatures of various colors with classic spiral-whorled shells

Foraminiferans, favorite food of scaphopods, are about poppy-seed size. Photo courtesy Burke Museum.

Their favorite foods are crunchy, microscopic one-celled organisms called foraminiferans or “forams” for short. While scaphopods lack eyes, they do have sensory organs called statocysts which help them detect food.

Sticky fingers

Once scaphopods find a nice place to eat, they probe around with sticky, finger-like tentacles called captacula.

Top shows tusk shell and bottom shows the captacula that looks like long-stemmed mushrooms with a softly pointed fleshy foot.

Top: 6cm Rhabdus rectius in its shell. Bottom: Head end of R. rectius 

The captacula have ciliated (hairy) ends covered with an adhesive goo, making it easy for them to grab food and bring it to the mouth. Before crunchy foods like forams and small clams can be digested, they need to be ground down. This is where the radula comes in. This set of hardened teeth resembles a zipper, with two hinged parts that come together to crush the unfortunate prey item.

Shell to pay

Occasionally you may find an empty tusk shell washed up on the beach, or see them in shell shops or sold as jewelry. These uses originated long ago, when prehistoric tribes collected scaphopod shells for decorations. Some American Indian tribes used the shells as jewelry and monetary currency. The Sioux and Kiowa tribes used scaphopod shells to decorate their armor. 

Woman in profile wears shell-covered gown, a headdress like a scarf, and shell earrings. Necklaces of strung shells are to the right.

Left: Wishram woman wears bridal headdress, earrings, and shawl adorned with tusk shells

Critter of the Month

Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see in order to understand the health of Puget Sound and detect changes over time.

Dany and Angela share their discoveries by bringing us a Benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.