I was admiring the snail-like spiral of barnacles on this little rock, when I noticed the upside-down "ice-cream cone" shell combo on the left:
It's a mud snail jammed down inside a barnacle. "Eating the barnacle," was my first impression. Do Batillaria do that? I searched through other photos to see.
Here we go. A bunch of barnacles that are now empty shells. Barnacles with mud snails inside, with little periwinkles, with small whelks, even a barnacle with two different snails crammed into the tiny space. But ... I thought periwinkles were vegetarian!
They are. So are the invasive mud snails. And the Tegula, the black turbans. They feed on algae.
But the whelks, the whole lot of them, are carnivorous predators on barnacles and mussels. Wikipedia also mentions tube worms among their prey.
Using the radula, a scraper/drill/file arrangement (There's a good diagram here.), the snail drills a small hole into a mussel, then injects a paralizing or digesting enzyme. Once the mussel flesh is soup, the snail inserts a long proboscis and slurps it up. With barnacles, the technique varies a bit;
"The snail uses the radula to drill a hole in the barnacle shell at the seam between two plates, and then extends its proboscis through the hole to scrape out the soft tissue. The preferred barnacle is Balanus glandula ..."I gather from this, that the other snails, the mud snail and the periwinkles, are simply feeding on the leftovers or the algae that grow on the leftovers.
From U. Washington.
Nobody's safe here; the mud snails, in turn, become prey. In this next photo, down on the left, in an empty clam shell (probably smashed by seagulls), a mud snail lies dead, with a small circular hole in the shell. Whether this was Nucella's doing, or some other snail's, I don't know. The moon snail makes larger holes (and are not on this beach, so far as I know).
While you're at it, open the photo full size, and look at some of the other snails. In the opening of several, you can see a small white claw. The original tenants are gone; the current residents are hermit crabs.
And on the outside of a snail just above the centre of the photo, look for a pinkish spot; it is possibly a slipper snail. There was another in one of our other photos. These are filter feeders; they do not drill holes.
A whole congregation of hermits. (Do hermits congregate?) And a few snails "peeled" by crabs.
Hermit crab, out looking for a solitary cave.
A few steps down the beach, I came across another breed of "voracious predators".
These are Paranemertes peregrina, the Purple ribbon worm. They looked almost black on the beach, but they are really a greenish, purplish, dark brown. The belly is creamy.
They are also called proboscis worms, because they shoot out a long proboscis to catch their prey. (Like a frog, maybe? But not as cute as frogs.) They are able to catch and kill polychaete (bristle) worms bigger than themselves. (WSU has a photo of a purple ribbon eating a polychaete.)
Another view. Note the green isopod to its right. The beach hopper, Traskorchestia traskiana, possibly.
And still another green isopod. I thnk this is the Rockweed isopod, Idotea wosnesenski.
Photo lightened considerably, to show two more beach hoppers.
These little guys crawled over and around the worms the whole time I was watching. The worms ignored them. They're looking for more challenging prey.