This crustacean is known from Oregon, USA, to Mexico.
In adults, the middle of the upper head has a large projection with a rounded tip.
It lives on the sea urchins Strongylocentrotus purpuratus and Mesocentrotus franciscanus.
In adults, side plates are visible from above on thoracic segments five to seven, or only six and seven.
Living adults match their host urchin in purple or dark red colour, fading to bluish grey in alcohol.
An extinction-risk assessment has not been added yet.