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.

Bryozoans (Part 1)

The Bryozoa are some of my favorite animals, so I was very excited when we found some in our local pond. While I'm inexperienced with freshwater bryozoans, we identified the species we found as Plumatella repens¹. Like all bryozoans these make a living by pumping water through a beautiful crown of tentacles, and filtering out tiny particles of food from the water. The tentacles have an elegant curve to them and in this species the crown takes on a nice horseshoe shape when viewed from above. Microscopic hairs (cilia) along the side of the tentacles wave back and forth to pull water between the tentacles.

Part of why I find bryozoans so fascinating is that in almost all bryozoans² the individual animals live linked together in groups that remind me of minute housing developments. Each individual animal (a "zooid") buds off one or more clones of itself, and the clones remain attached to each other so that they share nutrients and information with each other. It's as though you could stick out a hand and it could grow into a conjoined twin, and then your new twin could stick out its hand and grow another twin, and so on until you had a whole group of conjoined twins. The zooids are tiny but the groups ("colonies") can be quite large.

The concept of the individual is fundamental to how we perceive ourselves and the organisms around us. Colonial animals are fascinating in part because they don't fit neatly into this concept. Yet, in addition to bryozoans, many other groups of animals also grow as colonies for at least part of their life cycle. These include many kinds of tunicates (our closest invertebrate relatives) and cnidarians such as corals and many jellyfish³. So this seemingly strange and unfamiliar way of life is actually very common in the ocean. That's one of the fun things about studying invertebrates: they so often run counter to our preconceived notions of how life works.

1) My identification was based Pennak's "Freshwater Invertebrates of the United States". The image accompanying this post shows a small colony of Plumatella repens (I believe) from our local pond. The crowns of feeding tentacles of four zooids are visible, the most obvious of which is on the lower right corner. Three smaller zooids are visible along the brown stalk that snakes from the base of that individual to exit the image at the upper right. The field of view is 7 mm wide. This image was taken using a dissecting microscope, and has been processed in ImageJ 1.36 to enhance the contrast between the bryozoans and their substratum.
2) A good reference for bryozoans is E.E. Ruppert and R.D. Barnes (1994) "Invertebrate Zoology", 6th edition.
3) Hydrozoan jellyfish typically have a complex life cycle with a swimming phase (the medusa) and a phase in which they form colonies of polyps that are frequently attached to a substratum (Ruppert and Barnes, 1994).