Common name: Western Sea Roach, Rock louse

Scientific name: Ligia occidentalis

Size: To 2.5cm (1″)

Author: Aina Saitoh

Photos: D. Young

The Western Sea Roach is an isopod within the subphylum Crustacea and phylum Arthropoda. Their bodies are flat and vary in colour including grey, yellow and brown. Their body is separated into seven main parts and their uropods (the long structures at their hind end) are quite long relative to the closely related species Ligia pallasi. They have special cells, chromatophores, which allow them to change their color to camouflage into the background.  They are generally lighter in colour during the night and then darken during the day time.  They live around the sea in part because they need to keep their gills, located near their posterior end, hydrated in order to breathe. However, they can drown if they are submerged so are considered to be largely a terrestrial organism.  They are very fast and will quickly hide in crevices along the rocky shoreline.  They are found above the water line up to the high tide line.  They are often active at night when they feed on microscopic algae (phytoplankton) and, as an omnivore, scavenge for a diversity of other food sources as well.

This rocky shoreline, located on a southern Gulf Island in British Columbia, is a typical habitat of the Western Sea Roach.  Many sea roaches, including the one in the photos above, were scurrying along these rocks on a warm summer day.


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Duane, S. J. (2019). The New Beachcomber’s Guide to the Pacific Northwest. Harbor Publishing.

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