Eyes Under Puget Sound: Critter of the Month — Sweet Potato Sea Cucumber

With its smooth, plump body, this month’s critter bears a resemblance to items you might find in a grocery store. Meet Molpadia intermedia, the Sweet Potato Sea Cucumber.

Gloved hand holds a critter that looks like a shiny orange corndog with a broken stick on one end and a belly button on the other.

Our Critter of the Month Molpadia intermedia, freshly collected from mud under Puget Sound.

Although the sweet potato sea cucumber may look and sound like a vegetable, you definitely don’t want to go slicing up this slimy mud-dweller on your salad!

Brother from another mother

Sea cucumbers like Molpadia intermedia are part of a larger group called Echinodermata that also includes sea stars, brittle stars, and sea urchins.

Kingdom, Animalia; Phylum, Echinodermata; Class, Holothuroidea; Order, Molpadida; Family, Molpadiidae; Genus, Molpadia: M. intermedia

Superficially, sea cukes don’t look much like their echinoderm kin, but they all have three characteristics in common:

  1. A water vascular system made up of tubes and valves which allows movement, digestion, and breathing by pumping water throughout the body.
  2. Pentaradial symmetry — the body is organized in five symmetrical sections.
  3. An internal skeleton containing calcium carbonate.

Get under your skin

In the case of sea cucumbers, the internal skeleton takes the form of tiny particles called ossicles, embedded in the outer skin. Taxonomists can identify sea cucumbers by examining the shape of their skin ossicles under a light microscope.

M. intermedia has two kinds of ossicles:

  1. “Tables” — flat plates with tall, narrow spires
  2. Plates shaped like tennis racquets, filled with tiny holes


The “table” looks like a flower with beans on its petals. The stem is “spire,” the beans are “phosphatic body.” The racquet: spatula-looking

LEFT: Microscopic images of M. intermedia’s skin ossicles. The table images show a dorsal view (top) and lateral view (bottom). The racquet-shaped ossicle shows the small perforations throughout.
RIGHT: Close-up of M. intermedia’s skin with ossicles (tables and clusters of racquets) visible.

In  addition to ossicles, M. intermedia has something else lurking inside its skin: tiny iron- and phosphate-rich blobs called phosphatic bodies. You can see these microscopic orange blobs in the image to the right above. The animals get more of these as they get older. Scientists think these granules serve to strengthen the connective tissue in the skin.

No foot to stand on

Sea cucumbers typically have rows of tube feet running the length of their bodies; however, M. intermedia lacks tube feet, giving it a smooth and shiny appearance. Its soft, cylindrical body has muscles running its length. These muscles can expand to lengthen it (up to 43 cm) or contract to shorten it into a little ball when it is disturbed.

Fleshy lump in a jar with short orange pokey bits at the tip.

A living Molpadia specimen with its feeding tentacles extended. This individual is waiting to be weighed and measured in the field.

At one end of its body is a mouth surrounded by 15 feeding tentacles. These short finger-like (digitate) tentacles help push food into its mouth as it ingests sediment while burrowing. At the rear end of its body is a short, stubby tail.

Couch Potato

M. intermedia seldom moves, making it an easy target for its main predators, including the sand star, Luidia foliolata, and fish. In some areas, M. intermedia lives in densities of up to 15 per square meter, and individuals tend to aggregate in groups of two to six. Talk about a sea cucumber buffet!

Tubular critter, red and orange, with silvery mud. One end is ovoid with a nipple-looking structure; other end has short pinkish tentacles.

M. intermedia in a sediment sample which has been partially rinsed through a sieve.

Critter of the month

Our benthic taxonomists, Dany and Angela, share their discoveries by bringing us a benthic Critter of the Month. Dany and Angela are scientists who work for the Marine Sediment Monitoring Program. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants.

In each issue we will highlight one of the Sound’s many fascinating invertebrates. We’ll share details on identification, habitat, life history, and the role this critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.