Eyes Under Puget Sound: Critter of the month – the skeleton shrimp

There are more than a half dozen species of skeleton shrimp in Puget Sound. The Washington State Department of Ecology profiles this unusual crustacean in its Eyes Under Puget Sound series. 

Skeleton shrimp. Image courtesy of Dave Cowles, wallawalla.edu.
Skeleton shrimp. Image courtesy of Dave Cowles, wallawalla.edu.

Skeleton crew


If you can put aside their alien appearance, skeleton shrimp are fascinating creatures. Unlike most of their cousins in the order Amphipoda (which actually do resemble tiny shrimp), the skeleton shrimps (or “caprellids”) are specialized for clinging rather than swimming in marine habitats. If they want to move, they have to crawl along like inchworms. They also have adaptations for camouflage, changing colors to blend in with their surroundings. You can find them hiding among other invertebrate critters on docks and pilings, using their hook-like back legs to hang on to the plant-like fronds of colonial hydroids and bryozoans. They can also be found in beds of seaweed and eelgrass, their lanky bodies waving about in the water current.

Skeleton at the feast

Skeleton shrimp have varied feeding methods – some species are filter feeders that wait for food to come to them, while others are predators, hunting for worms and crustacean larvae. Some eat detritus (dead particulate material), and some species like to munch on tiny microalgae called diatoms. In fact, skeleton shrimp are so good at grazing on diatoms that their presence is thought to keep eelgrass beds healthier by removing diatoms that encrust the blades. Fish and larger invertebrates pass these nutrients up to the higher trophic levels when they eat the skeleton shrimp.

The male gnathopod of Caprella cf californica.
The male gnathopod of Caprella cf californica.

Poison punch

A skeleton shrimp’s unusually large claws, called gnathopods, play an important role in their mating behavior. In some species (including species of Caprellafound in Puget Sound), the male’s gnathopods are armed with a poison-producing gland. The toxins they make aren’t harmful to humans, but when two males fight over a mate, the outcome can be deadly.

Naked truth

Mating can be tricky for these critters because their window of opportunity is short. Like all arthropods, they shed their exoskeletons to grow in a process called molting. The female is only able to mate right after molting, when her new exoskeleton is still soft. She carries her eggs in special flaps called brood pouches, and the young hatch out looking just like miniature adults, immediately inching away to find their own little branches to attach to.

Skeleton key

For taxonomists, the scariest thing about skeleton shrimp is identifying them! We encounter over half a dozen species in Puget Sound, and telling them apart can be quite the daunting task. We look for key features such as the length of the antennae, whether or not the animal has a spine on its head, and the shape of its gnathopods. Despite the taxonomic challenges, we do enjoy seeing these weird little beasts in our samples, although the swaying and claw-waving of the live ones can be a bit unnerving. So if you really want to get spooked on Halloween, head down to a dock, peer into the murky water, and hunt for tiny skeletons!

Read more species profiles on the Washingtom State Department of Ecology's "Critter of the Month" page. 

View a list of benthic invertebrates in Puget Sound.

About the Author: 
Benthic taxonomists Dany Burgess and and Angela Eagleston identify and count sediment-dwelling organisms as part of the Marine Sediment Monitoring Program at the Washington State Department of Ecology.