Rotifers with bryozoan

I added a close up of one of the zooids (individuals) from the Plumatella colony mentioned in the previous post since the crown of tentacles* is very pretty and I like the rotifers cohabiting with the bryozoan (see previous post). Around the base of the tentacle crown you can see a bunch of the rotifers holding on. Many of the other zooids had rotifers at the same location.

My best guess is that the rotifers take advantage of the water flow generated by the bryozoan. Many animals, such as bryozoans and many rotifers, and some protozoans make a living by generating a flow of water and capturing suspended particles of food, but small suspension feeding organisms face a problem when they live in still waters: the flow they generate re-circulates so they re-filter water they already filtered. They can get around this by taking advantage of water currents in their environment, or currents generated by other organisms, to reduce re-circulation. However, these rotifers are living in water that the bryozoan has already filtered. It is possible they eat particles too small for the bryozoan to catch. Also, other bryozoans I've seen feeding seem to be relatively inefficient filterers.

If anyone knows about rotifer-bryozoan associations, especially whether they are species specific, please let me know.

*The crown of tentacles of this zooid was 1.5 mm across, from tentacle tip to tentacle tip.