This is a colony of kamptozoans, also called entoprocts. Kamptozoa means "bending animal." They are commonly known as "nodding heads." They're very small, only a few millimeters high, so most people haven't had a chance to get to know these delightful animals.
Entoprocts are in their own phylum. They have a lovely crown of ciliated tentacles surrounding the mouth. The cilia draw water up through the tentacles which aids in capturing food particles. In the photo below you can see the cilia reflecting light.
All but one species of entoproct are marine. On Bodega Head, we've encountered them on the rocky outer coast, but they're hard to find! Eric discovered this colony growing in the low intertidal zone at the base of a red alga.
An individual in an entoproct colony is a called a zooid. The three basic parts of a zooid include (1) the crown of tentacles, (2) the calyx (cup-like, holding most of the body organs), and (3) the slender stalk.
Here are two more views of a zooid, one from the side and another from above.
Entoprocts reproduce both sexually and asexually. They can bud new individuals along creeping stolons (or runners) — see below.
In the photo above, note that each zooid has a swollen node at the base. We think this character helps identify this species as Barentsia conferta. (The species name, conferta, means densely packed.)
Here's another view of a small portion of a colony. Note the blurred movement of one zooid at center left.
Entoprocts are called nodding heads because of their very curious and distinctive way of moving. Libbie Hyman (1951) described it as "odd flicking, bowing movements." Suddenly, one individual will bend over quite dramatically. This movement then cascades through the surrounding individuals. It's somewhat like bumper cars.
A still photo doesn't capture this behavior, so for your viewing enjoyment and an introduction to nodding heads, check out this short video clip we made! (Best viewed at small size for now.)