If you guessed that this is a type of crustacean (i.e., the group that contains crab, lobster, and shrimp, among other animals), you are correct. So the next question is, what type of crustacean?
Here's another close-up, revealing a bit more. This animal was ~2.5 cm long.
Meet Betaeus harfordi! This is a shrimp in a group that is sometimes referred to as "hooded shrimps." In the second close-up (above), and in the photos below, you can see that the carapace extends up and over and slightly in front of the eyes.
Older summaries about this species describe it as dark purple, blue-black, or deep blue. The first individual we photographed (above) showed strong purple coloration, but we also looked at a second individual that tended towards the blue side:
Here are the two together so you can compare the colors. [The photographs below were taken outdoors in natural light, rather than indoors in the lab.]
Betaeus harfordi has an interesting life history. This shrimp has an obligate association with abalone. It lives in the mantle cavity of abalone along the West Coast (all eight species).
A shrimp leaves the shelter of the abalone to feed at night, but then returns to its host during the day. (Interestingly, a shrimp can sense the presence and location of its abalone host chemically!)
Here are two more photos. Note the "hood" over the eyes and the interesting claws (with the movable portion of the claw at the bottom):
Many thanks to our colleagues who study abalone for sharing these fascinating symbiotic shrimp!
P.S. The "harfordi" part of the scientific name refers to William G.W. Harford, director of the California Academy of Sciences from 1876-1886.