Stuck at home? Get cozy like the tunicate amphipod

Eyes Under Puget Sound - Critter of the Month

White amphipod, side view, on black background. Text reads, Polycheria osborni.
When we find ourselves in times of unprecedented uncertainly and isolation, it’s important to focus on the positive, seek out small wonders, and put things in perspective. While most of us are temporarily sequestered in our homes and feeling a bit stir-crazy, there are those who stay inside their entire lives! Meet the tunicate amphipod, a critter that embraces the comforts of home like no other.


Bringing home the bacon

Text reads, Kingdom Animalia, Phylum Arthropoda, Class Malacostraca, Order Amphipoda, Family Dexaminidae, Genus/Species Polycheria osborni
The tunicate amphipod, Polycheria osborni, makes its home in a very unusual way — by living inside another animal. In Puget Sound, the host of choice is a squishy whitish blob called Aplidium, a colonial tunicate  commonly known as sea pork. Other members of the genus Polycheria make their homes in cavities in the surfaces of tunicates and sponges as well, and there have even been reports of them living among algae, stones, and gravel. 


It’s the pits

Yellow arrow points to cavity in white globular tunicate on black background. Text reads, Amphipod inside tunicate burrow

A tunicate amphipod (indicated by the arrow) rests comfortably inside an Aplidium tunicate colony.

P. osborni creates its burrow by using specialized legs to shape the gelatinous tissue of the tunicate around it. The resulting cavity, called a host pit, is similar in size and shape to the individual making it, forming a cozy cocoon around the animal’s body. Using tiny claws at the end of each foot, the amphipod can grip onto the sides of the pit for stability. It can even shut the door to the outside world by grabbing the edges of the pit with its claws and pulling it closed.


Home body

Lying on its back with its feet and antennae pointing skyward, this amphipod is the ultimate professional lounger. Don’t let the relaxed posture fool you, however — there’s actually a lot of hard work going on! The amphipod’s swimming legs, called pleopods, are constantly beating, circulating water to create feeding currents. As the water passes the antennae, tiny hairs called setae catch particles of food (like diatoms and other algae), which are collected by the claws and passed to the mouthparts.

Gold colored amphipod on black background clings to white tunicate with head pointing up.

This tunicate amphipod was disturbed and emerged from its host pit. You can see how particles stick to the antennae (located above the animal’s red eyes).


Close quarters

Although the tunicate amphipod is a hermit, it isn’t great at practicing social distancing. One host tunicate colony may have multiple burrows in it, kind of like rooms in a boarding house. This happens because juveniles hang out in their mother’s burrow after hatching out until they are large enough to survive on their own. When they emerge, some of them find abandoned burrows nearby to move into. 

Communal living may seem like a good idea, but it has potential drawbacks — limited dispersal could mean that gene flow between groups of Polycheria stays low, resulting in fewer adaptations that could help the survival of the species over time.

Blue arrows indicate pits in white tunicate with no occupants. Text reads, abandoned pits. Green arrows indicate burrows with amphipods.

A tunicate with several amphipod pits in it, both occupied and unoccupied (indicated by arrows).


Critter of the Month

Dany is a benthic taxonomist, a scientist who identifies and counts the sediment-dwelling organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see to detect changes over time and understand the health of Puget Sound.

A female scientist in sunglasses, hard hat, and left jacket smiles while weighing a sea cucumber with a spring scale.

Dany shares her 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.