Western Sea Slater

L. occidentalis looks a lot like Ligia pallasi, but the two species can be told apart by Ligia occidentalis’ longer uropods (the pair of tail appendages), its larger eyes set closer together, its skinnier body, and its tendency to jerk its body when messed with. It can also change color. The color comes from pigment cells called chromatophores, and Armitage (1960) studied how these cells behave in this species.

Genetic surveys have sampled L. occidentalis on rocky shores from central California south to central Mexico, including the Gulf of California. BugGuide also lists an introduced population around Vancouver, British Columbia. It stays in a thin strip of the supralittoral zone and cannot survive long underwater, which limits its ability to spread across sandy beaches or open water to new rocky areas they haven't stepped foot on, which Hurtado et al. (2010) suggest is a reason why its populations drifted apart due to genetic changes from not mixing with other populations creating families. Because L. occidentalis lives south and L. pallasii lives north, where they meet there's a mix of both. Eberl (2012) surveyed both along the coast. L. occidentalis was slightly less abundant toward the northern end of its range, but the drop was not statistically significant.

During the day it hides in rock crevices or under stones just above the high tide line. It comes out to forage through the intertidal zone at night, and at low tide in cool weather. It is a scavenger and also eats microscopic algae. To breathe, it has to keep its gills wet, which it does by dipping the back of its abdomen into water. It handles water loss well, but it drowns if held underwater.

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