Here's a mystery close-up for you. Can you guess what this is?
If you'd like to see a little more, scroll to the next image:
Perhaps you have some ideas now? If not, I'll zoom out one more time. Warning: The next picture will reveal most of the animal:
Those wonderful spines belong to a brittle star known as Ophiothrix spiculata, sometimes called a Glass-spined Brittle Star. Eric spotted it among the rocks in the intertidal zone last week. This is the first time we've seen this species in Sonoma County.
The jagged edges of the spines are an important characteristic for identifying this species. (The smaller spines along the edges of the larger spines are formally called spinelets.) Ophiothrix spiculata belongs to a family of brittle stars that's primarily tropical in distribution. It ranges from about San Mateo County to the Galapagos Islands. There's a record from Victoria, B.C., but otherwise it seems most sightings are from Moss Beach and south. It can be abundant in southern California.
The arms are usually very long, but this species is prone to autotomizing — dropping its arms (often in self defense, e.g., to escape a predator). This individual had lost the tips of its arms, but it was regrowing them. Here's a picture from the field:
One more fun fact about the Glass-spined Brittle Star. Those spinelets come in handy when feeding. After suspended particles in the water are trapped on the spines, the tubefeet wipe them off and move the food particles to the mouth on the underside of the disc.