Eyes Under Puget Sound: Critter of the Month — The Sea Mouse

A Sea Mouse from above, labelling the head, setae and elytra. The latter two are visible because part of the body covering is removed.

This juvenile Aphrodita specimen has had some of its dorsal (top) covering removed, exposing the elytra beneath.

This month, love is in the air — and in the mud! With a scientific name that originates from Aphrodite, the ancient Greek goddess of love, we think the sea mouse is a perfectly lovable addition to the fauna of Puget Sound.

Worm and fuzzy

The sea mouse may be brown and fuzzy, but that is about all it shares with its mammalian namesake. Believe it or not, the sea mouse is actually a marine segmented worm, or polychaete! This is more obvious if you flip it over on its back, exposing its segmented underside.

Kingdom, Animalia; Phylum, Annelida; Class, Polychaeta; Order, Phyllodocida; Family, Aphroditidae; Genus, Asphrodita
Sea mice are members of the family Aphroditidae — one of several families of scale worms, characterized by elytra (scale-like plates running down their backs). However, the sea mouse’s elytra are completely hidden under a tangled mat of hair-like setae and mucus that cover the worm’s dorsal (top) surface.

Pulling the wool over your eyes

Two views of a sea mouse. One from above shows the body covered in what looks like fur. The other shows the fleshy, segmented underside.

TOP: Aphrodita japonica captured at 70 meter depth, west of Yellow Island, WA. Photo courtesy of Dave Cowles, wallawalla.edu.
BOTTOM: A juvenile Aphrodita sp., showing its fleshy, segmented underside.

As the sea mouse crawls along in the sediment, its setae pick up mud and silt particles and other debris, adding to its woolly appearance. In fact, you can barely make out any features under this dorsal covering, but if you look underneath the hairs at the worm’s front end, you can see a tiny head with a single antenna, a pair of sensory appendages called palps, and two pairs of eyespots.

The Puget Sound species of Aphrodita are typically scavengers, using their palps to search around in the mud for delicious dead things to munch on. However, some species of sea mice found elsewhere in the world are predators, eating other polychaetes and small crabs.

Mouse trap

On the polychaete size scale, the sea mouse is actually pretty hefty, with some growing to about 15 cm (6 inches) long. They live in soft sediments, like mud or sand, generally in shallow depths (to about 120 meters in Puget Sound). Although we sample in these areas, we rarely see sea mice in our benthic grabs. However, when we do, we have to examine them closely under a microscope to make sure we identify them correctly. We look at the number of segments, the length of the antennae and the type of setae to distinguish the three species we see in Puget Sound: Aphrodita japonicaA. negligens, and A. parva.

 

Of mice and men

The head is shown, with palps on either side labeled.

Head of an Aphrodita specimen, dorsal (top) view.

The sea mouse has two kinds of setae:
  1. Long, soft hairs that cover its back.
  2. Tough, hollow bristles made of chitin that stick out of its parapodia, or feet.

In the Shimmering Sea Mouse, Aphrodita aculeata (which does not occur in Puget Sound, but, rather, in the Atlantic), these bristles are bright iridescent colors, which may be a defense mechanism to scare away potential predators.

 

A. aculeata’s amazing bristles may have other uses too — for humans! Researchers in Norway have found that the properties which give the worms’ hairs their iridescence also lend themselves very well to nanotechnology — the science of studying and controlling very small things, like atoms. The sea mouse’s bristles can be used as tiny wires (called nanowires) to conduct charged ions, making them potentially useful for building miniature electronic devices such as in-vitro health sensors and computer processors. Talk about a mighty mouse!

A close-up view of sea mouse bristles.

Close-up of bristles around the bottom of Aphrodita negligens

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.