In took a few seconds for us to realize it was a sock!
The sock was absolutely covered with marine life. Here's a closer view (below). It's easy to see that the dominant animals are hydroids (golden in color) and bryozoans (white and encrusting).
When we were scanning to see what else was there, we were amazed to see a very high number of surfgrass seeds (Phyllospadix sp.). Surfgrass is a flowering plant found in the low intertidal zone along outer coast rocky shores. Almost all of the black structures on the sock are either entire or broken surfgrass seeds. We counted at least 30 seeds on this sock!
If you haven't seen a surfgrass seed before, you're in for a treat. The following is a view under high magnification. I didn't record the size at the time, but I'm guessing this seed was ~5 mm across.
Note the long barbs on either side with well-developed bristles. These structures function to hook the seed on something like coralline algae, where the surfgrass seedling can then start to grow by putting down roots and sending up green shoots.
This process was first illustrated by Robert Gibbs in 1902 (his research took place in Bodega Bay):
Modified from Gibbs, R.E. 1902. Phyllospadix as a Beach-Builder. American Naturalist 36: 101-109.
Here's an example of what this looks like in the field (although it's better for the algae and the surfgrass if they don't wash up on the beach!).
I don't know if you noticed, but several of the surfgrass seeds on the sock had started to produce shoots! In the picture below, look for the seed lying flat at the bottom of the image. The green shoots are emerging from that seed.
Sadly, because this sock and seedling were washed up on the beach, this plant won't survive. But here's what a nice bed of adult surfgrass plants looks like:
I'm sure many of you have experienced walking through grasslands during the summer and finding many grass seeds stuck in your socks...but until now, have you ever considered finding surfgrass seeds in a sock? ;)
P.S. If you're missing a sock, let me know.