After last night's post, Molly inquired about a beach hopper she observed recently on a beach in northern Sonoma County — one with red antennae rather than blue. It's possible the species she encountered is similar to the one pictured below (this photo was taken on Bodega Head on 16 June 2011).
This is Megalorchestia corniculata (formerly Orchestoidea), sometimes called the short-horned beach hopper. The "horns" are actually antennae, and "short" is relative to another species, Megalorchestia californiana, sometimes called the long-horned beach hopper.
The illustrations below show the differences in the antennae.
Illustrations from Bowers, D.E. 1963. Field identification of five species of Californian beach hoppers (Crustacea: Amphipoda). Pacific Science 17: 315-320. [Part of this research was conducted at Dillon Beach.]
In the top figure, showing Megalorchestia californiana, the second antenna is very long, especially the last segment, or flagellum (it's longer than the combined length of the peduncle segments). In the second figure, note the much shorter flagellum of Megalorchestia corniculata.
Another difference between the two species is the color of the second antennae. In M. californiana it's usually rose-red; in M. corniculata it's usually salmon-pink.
Here's a close-up of the second antenna of the animal pictured above:
The beautiful purple eye is also quite striking. Amphipods have compound eyes made up of hundreds of units called ommatidia (photoreceptor cells). For comparison, large dragonfly eyes may have up to 30,000 ommatidia! (More about them later.)
Just for fun, here's another species of amphipod that lives in high tidepools. Check out the fancy and variable color patterns!