Pentidotea, an Intertidal Isopod

May 15 saw a pretty good low tide on theoutershores, about 2 feet below mean lower low water, so I was able to explore some surf-swept rocks that jut out of the sand. These islands in the sand are usually inaccessible, so the low tide offered me an opportunity to see what lives on them.

Rocks and headlands jut from the sand

The rock above is covered with surfgrass, Phyllospadix scouleri, barnacles and various kinds of encrusting and branched coralline red algae.

Close up on surfgrass
Pentidotea, possibly P. wosnesenskii

Running my fingers through the surfgrass I encountered this attractive crustacean, the isopod Pentidotea. There are a few species of Pentidotea, and for a non expert, some of them can be hard to tell apart. I’m pretty sure this one is Pentidotea wosnesenskii.

Pentidotea, from a surfperch stomach

I was first introduced to Idotea in 2011, when I found one in the stomach of a redtail surfperch. I think surfperch love to eat them when they can. I’ve seen many more in surfperch stomachs since then. Now I know where they catch them.

If you are interested in other surfperch food items I have discovered, check out Dabbling with Crustacean Competition.


I’m grateful to Allison Barner for weighing in on the identity of the Pentidotea fearured in this post (but if I made the wrong call, it’s on me).


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.

Light, S. F., 2007. The Light & Smith Manual: Intertidal Invertebrates from Central California to Oregon. 4th ed., edited by J. T. Carlton. University of California Press.

Sept. J. D. 2009. The Beachcomber’s Guide to Seashore Life in the Pacific Northwest. Revised ed. Harbour Publishing.

Note: This post was published on June 2, 2014. I updated the photos and lightly edited the text on October 21, 2017.


  1. I was introduced to Idotea in Baja (or something pretty similar) when I fell asleep on a beach on an island in the Sea of Cortez covered in them and with several up inside my bathing suit!

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