Isopods

Below are examples of easy-to-find isopods from Oregon’s exposed rocky shores, beaches, and backshores. The photos are from northern Oregon unless noted. My organization loosely follows Lamb and Hanby (2005). Common names, if I use them, are my choice. The books, field guides, and identification resources listed at the bottom of the page cover these isopods and many more.

Let the isopods be upon you!


Ligia pallasii

My view of Ligia pallasii is shaped by my experiences with them at the base of cliffs that rarely get dunked, even by big surf and surge. That’s the only habitat where I’ve encountered them, so with that frame of reference, I think of them as high crevice dwellers. They blend in pretty well on the cliff face where they are shy, fast-moving escape artists.


In spite of their color-matching you can sometimes detect and, to a degree, and sneak up on Ligia. On cliffs, look for them in places with some moisture, plenty of crevices, and overhangs. (The same individual is shown in both panels below. Even in the first shot (left-hand panel) it was already on the move.)


Kids look pretty much like adults (but there is some variation in adult size and shape).


Speaking of shape variation, the bottom panel below illustrates that some adults achieve wide-body status (I think these wider individuals might be adult males, but I’ve read varying accounts).


Examples of cliff habitats of Ligia pallasii.

A large Ligia pallasii on a vertical rock wall covers a smaller female (that's my guess, anyhow).


When the habitat has a vertical dimension, the escape route is often up. I love the mad scramble in the right-hand panel, below.

To view a short and kind of intimate video of Ligia pallasii in a variety of shapes and sizes in their home crevice, click here.

There’s a surprise waiting in the image at the left. How many Ligia do you see? That’s right, two, a larger one, probably a male, mostly covering a smaller female. (You can make out two sets of antennae.) I think this might illustrate precopulatory mate guarding, pretty common in isopods and some other crustaceans like crabs.

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Alloniscus perconvexus

Of the crustaceans making an intertidal living way up on the beach, above the reach of tide, up in the dryish sand amongst scattered driftwood scraps and sparse backshore plant pioneers, Alloniscus perconvexus is one of the few most of us would recognize.

Alloniscus scurrying over the sand, casting a shadow in the early morning sunlight.
A disturbed (recently-dug-up) Alloniscus perconvexus on the move and casting a shadow in the morning sunlight
A staged shot of Alloniscus perconvexus on sand illustrating the underside and seven pairs of legs.
Underneath, count ’em, seven pairs of legs (same individual as above)


Below, a trio of images illustrate habitat on the only beach I’ve spent any time purposely looking for Alloniscus.

Backshore and backshore shelf habitat of Alloniscus perconvexus. In the foreground, dunegrass. In the farthest landward background, the foredune cloaked in beachgrass. Cloudy sky.
Backshore habitat of Alloniscus perconvexus. In the foreground, rippled wind-blown sand, dunegrass, and driftwood. In the distance, the lower beach and surf zone.
Backshore habitat of Alloniscus perconvexus. In the foreground, rippled wind-blown sand and beachgrass cloaking the backshore shelf. In the distance, driftwood and scattered backshore plants, the lower beach, and the surf zone. Clear, slightly hazy blue sky.


In the immediate vicinity of the scenes shown above, where Alloniscus seemed abundant, there are trail and burrow shows aplenty, and about those, Ricketts and Calvin have this to say:

“It will be found therefore, in the highest zone, above the high-tide line; and because of the obvious nature of its burrows it is often one of the first animals to be noticed in this environment.”

Alloniscus are secretive—living mostly underground by day, or so it seems—thus it makes sense that first contact is likely to be the trace of a trail or burrow. Ricketts and Calvin and Kozloff describe a ridge, the result of molelike burrowing just beneath the sand’s surface.

A mounded Alloniscus trail across rippled dry sand.
A visible mounded ridge from a short Alloniscus tunnel under dry rippled sand.


Followed for any length, these humped-up traces are usually more complex than the ridges in the examples shown above. After all, ridges collapse leaving sunken reaches, and plowed furrows sometimes emerge on the surface with footprints showing between lateral mounds.

A possible Alloniscus trace across rippled dry sand. Seen here are sunken, mounded, and furrowed elements.
A circuitous pattern of Alloniscus trails and a burrow show on dry rippled sand.
An Alloniscus trace winds off into the distance over rippled dry sand.
Alloniscus tracks on the surface of dry sand-rippled sand.

(A note of fair warning: I didn’t confirm that the trails shown above were made by Alloniscus, so I provide these examples with a bit of skepticism, but until I learn something different, I think they’re worth showing here.)

When it comes to burrow shows, having plundered a couple, I’m more confident.

A closeup of the show for an occupied Alloniscus perconvexus burrow at the sand's surface.
An occupied Alloniscus perconvexus burrow show
A disturbed Alloniscus perconvexus trundles over the sand, casting a shadow in the morning sunlight.
The occupant


Here are some promising but unconfirmed burrow shows. Sloppy and kind of fluffy looking, these shows have it all.

A closeup of an unconfirmed possible Alloniscus perconvexus burrow show on rippled wind-blown sand.
A closeup of an unconfirmed possible Alloniscus perconvexus burrow show on rippled wind-blown sand.
A closeup of an unconfirmed possible Alloniscus perconvexus burrow show on rippled wind-blown sand.
A closeup of an unconfirmed possible Alloniscus perconvexus burrow/tunnel show on rippled wind-blown sand.

(In the examples shown above, the fluffy look, if that’s a reasonable way to describe it, comes from the way moist sub-surface sand dries after it’s been deposited on the surface.)

Kozloff says Alloniscus perconvexus is most often found under pieces of driftwood resting on just slightly moist sand. It was on that advice that I found this shy specimen secreted away in a shallow hole.

A staged image of Alloniscus perconvexus on a piece of driftwood on the back of the beach.
A staged photo, in the rain, of Alloniscus perconvexus (just uncovered under a small piece of driftwood)
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The little isopod below, with the distinctive telson did not want to pose for me, but I gave it a try anyway and for a few minutes, as it explored the edges of a tiny sand-filled mid intertidal pool, I lost myself in that world. I think this is probably Exosphaeroma, maybe something like E. russellhansoni or a look-alike, like E. amplicauda.

Maybe something like Exosphaeroma russellhansoni or E. amplicauda
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Pentidotea

Delightful color variation exists in some of the species in this group. Those I’ve met up with on the exposed outer coast have strong sharp claws which help them maintain their perches on surfgrass, corallines and seaweeds, on the rocks, and in the mussel beds. I think the examples in the set below show one or more Pentidotea wosnesenskii characteristics, and I think that’s what they are, but there are look-alikes.

When I discovered the specimen below, deep in the surfgrass, I was so excited I grabbed it and laid it out for a better look. The image illustrates enough Pentidotea wosnesenskii characteristics to make identification fairly certain. Variations of purple-brown coloration with white markings are common in P. wosnesenskii, but expect other colors, as shown below.

Pentidotea wosnesenskii on a bed of surfgrass.


Below, a sample of Pentidotea showing off their colors in various settings. I think these examples are all P. wosnesenskii, but where look-alikes exist so does uncertainty.

Pentidotea wosnesenskii on a Schizymenia blade.
On Schizymenia
A Pentidotea wosnesenskii-like isopod blends in on Ulva.
On Ulva | Galiano Island
A Pentidotea wosnesenskii-like isopod exposed on a wet bare rock.
Exposed on the rocks, taking a bit of a defensive stance
A Pentidotea wosnesenskii-like isopod explores the crevices between mussels and barnacles.
In the mussel bed
A Pentidotea wosnesenskii-like isopod explores the crevices between sponges and red seaweeds.
Among the sponges and delicate reds


I came across the individual shown below on the sand beneath mussel and seaweed encrusted rock. I’ll never know how it got there, but it had already taken this seemingly defensive pose by the time I came along. Note the similar stance on the specimen in the upper right-hand panel in the gallery above.

A Pentidotea wosnesenskii-like isopod on sand rears it's head in apparent defensive posture.


Another behavior I’ve noticed is stacking. Below, four P. wosnesenskii have stacked themselves on a seaweed blade, maybe Prionitis. (It’s kind of hard to make out the fourth individual here.) The largest individual in the stack is the largest Pentidotea I have encountered.

A stack of four isopods, Pentidotea wosnesenskii, cling to a seaweed blade.


I’ve mentioned how identifying isopods resembling Pentidotea wosnesenskii can be tricky, so I’m leaving this entry with two beautiful red specimens from roughly the same exposed location, and questions. Are they both P. wosnesenskii? Neither?

A Pentidotea wosnesenskii-like isopod on seaweed.
A Pentidotea wosnesenskii-like isopod on seaweed.
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Idotea

I’ve only come across this slender, straight-bodied isopod once. I noticed it at the bottom of a sand-filled tidepool below an isolated mussel-covered boulder. The photographs below are of the same individual, first in a white finger bowl, then on clear glass. It looks a lot like the photograph of the pencil isopod Idotea fewkesi featured in Sept (2019).

A mystery from the low intertidal
Just a bit over 3 cm long
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Synidotea

I’m pretty confident this beauty is Synidotea ritteri. Both images featured below are of the same individual, who, after I discovered it on a low patch of inscrutable red blades, was making things pretty difficult on me, but I don’t blame it; I was making life difficult on it too. We achieved a truce in just under six minutes.

Side view of Synidotea ritteri crawling on low intertidal corallines.
Synidotea ritteri, viewed from above, on a low barnacle-encrusted rock.
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References

Gotshall, D. W. 2005. Guide to Marine Invertebrates, Alaska to Baja California 2nd Edition (Revised). Shoreline Press.

Harbo, R. M. 2011. Whelks to Whales: Coastal Marine Life of the Pacific Northwest. 2nd ed. Harbour Publishing Co.

Kozloff, E. N. 1993. Seashore Life of the Northern Pacific Coast. 3rd ed. University of Washington Press.

Lamb, A. and B. P. Hanby. 2005. Marine Life of the Pacific Northwest. Harbour Publishing.

Sept. J. D. 2019. The New Beachcomber’s Guide to the Pacific Northwest. Harbour Publishing.

Wall, A.R., N. L. Bruce, R. Wetzer. 2015. Status of Exosphaeroma amplicauda (Stimpson, 1857), E. aphrodita (Boone, 1923) and description of three new species (Crustacea, Isopoda, Sphaeromatidae) from the north-eastern Pacific. ZooKeys 504: 11-58. Accessed November 9, 2024.


Online Resources

Friends of Netarts Bay’s Crustaceans page. Accessed November 9, 2024.

Pacific Northwest Shell Club’s Crustaceans page. Accessed November 9, 2024.

It’s worth scrolling down to barnacles in Common Sea Life of Southeastern Alaska: A field guide by Aaron Baldwin & Paul Norwood. Accessed November 9, 2024.

Key to Suborder Valvifera by H. Leno (2002 as edited). Invertebrates of the Salish Sea. Rosario Beach Marine Laboratory. Accessed November 9, 2024.


Species Accounts

Cowles, D. 2006. Pentidotea wosnesenskii (Brandt, 1851). Invertebrates of the Salish Sea. Rosario Beach Marine Laboratory. Accessed November 7, 2022. Accessed November 9, 2024.

Hiebert, T.C. 2015. Ligia pallasii. In: Oregon Estuarine Invertebrates: Rudys’ Illustrated Guide to Common Species, 3rd ed. T.C. Hiebert, B.A. Butler and A.L. Shanks (eds.). University of Oregon Libraries and Oregon Institute of Marine Biology, Charleston, OR. Accessed November 9, 2024.

Leno, H. (2002 as edited). Idotea (Idotea) fewkesi. Invertebrates of the Salish Sea. Rosario Beach Marine Laboratory. Accessed November 9, 2024.

Leno, H. (2002 as edited). Ligia (Ligia) pallasii Brandt, 1833. Invertebrates of the Salish Sea. Rosario Beach Marine Laboratory. Accessed November 9, 2024.


I updated this page on November 9, 2024.