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).

Eye of the Beholder, Part III

In the last two posts, I was discussing the human predisposition to ignore organisms that aren't mammals, or at least aren't vertebrates. I'd like to talk about why, in a very real and practical sense, this bias matters.

The first point is that mammals, as I mentioned before, are a very small evolutionary group. Thus, we cannot learn much about the diversity of life on Earth by studying only mammals.

The second point, and perhaps a more immediate one, relates to conservation practices. The surefire way to get support for an environmental cause is to put up a picture of a dolphin, polar bear, elephant, seal, giraffe, you name it. As long as it's a mammal, it is worth saving. The problem is that mammals--especially the larger mammals that are going extinct at an alarming rate--are suffering due largely to habitat loss. Habitat is the operative word. The habitat that a polar bear lives in is important to the polar bear, but it is also home to countless other animals that don't happen to be furry. How can we even begin to do justice to the idea of saving a polar bear without understanding the needs of the organisms it lives with?

Even people who claim to be pro-animal-rights are often blind to the idea of animal diversity. You will never see a PETA protest decrying the extinction of a dragonfly. But insects, snails, worms, and slugs are all animals. And why stop at animals? Fungi are incredibly important to nutrient cycling in many habitats. We would be in trouble if we didn't have them. So would many cute, fluffy animals. Is anyone worried about how the fungi are doing worldwide? Apart from the small group of people known as mycologists, probably not.

It is frustrating to me that it is so difficult to get people to care about saving something other than mammals. (In fact, I'd be very happy if we could all stop talking about saving species and get on with the business of saving habitats. But that can wait for another post.)

In the meantime, please don't stop being concerned about large mammals. Just remember that in many ways, a campaign to Save the Snails could be at least as important as the one to Save the Whales.

Eye of the Beholder, Part II

How is it possible that on a planet with such huge diversity of life, we tend to think that animals are only things with fur or feathers?

To be fair, one reason is that many of the other organisms are microscopic and/or they dwell underwater. But I find that many people are simply unaware of the existence and ubiquity of non-mammals. Because humans are mammals, we are predisposed to appreciate cute, furry things, so they grab our attention.

This is unfortunate, because all organisms--not just animals--are complex and interesting in some way. It only takes a little effort to learn more about them. Even one's sense of aesthetics can be expanded through the pursuit of knowledge.

Case in point: I have a passion for marine invertebrates, particularly snails. I feel about gastropods the way cat-lovers feel about cats. Most people have a sort of "nod and smile" reaction when I talk about how beautiful snails are--at least, those people who aren't grossed out. But how can an animal that makes something as gorgeous as a sea shell not be beautiful in and of itself? How can the biological process that results in a shell not be fascinating? Indeed, when I talk about those issues and show people specimens, they do begin to see snails as more beautiful than before.

The English philosopher Herbert Spencer said, “Those who have never entered upon scientific pursuits know not a tithe* of the poetry by which they are surrounded.” Of course, he didn't mean that everyone should be a scientist, but he did mean that it is a virtue to be curious and to explore that curiosity. We can all at least take a walk around the block and notice many different living things besides the obvious people, cats, dogs, birds, trees, and grass.

The Spencer quote resonates very strongly with me. I find that the more I learn about an organism, the more fascinating and beautiful it becomes. The more different organisms I learn about, the more amazing the universe seems. There is no mystery or wonder lost in the gain of knowledge. Rather, I come to appreciate my subject more deeply, and I am inspired to ask more questions. It is worth remembering that there is no limit to the number of questions we have yet to answer. That is why, after years of studying biology, I can still be fascinated by a walk around my block.

*Spencer used the word "tithe" in its somewhat archaic sense of a tenth, or a small part.

Eye of the Beholder, Part I

Here's a little experiment for you to try. As quickly as you can, without thinking about it, name the first five animals that come to your mind.

What did you come up with? If you're like most people, your list consists only or almost only of mammals (cat, dog, horse, etc.). If there is a non-mammal on your list, it's almost certainly still a vertebrate (e.g. bird, frog, turtle).

What is remarkable about the results, is that our mindset is so vertebrate-centric, considering that vertebrates are an incredibly small part of the evolutionary tree. Mammals, as a subgroup of vertebrates, are an even smaller part of the evolutionary tree.

Take a look at this wondrous Tree of Life on Dr. David Hillis's website:

http://www.zo.utexas.edu/faculty/antisense/DownloadfilesToL.html

It looks overwhelming, but it represents only 3,000 species out of an estimated 9 million on Earth. If you download the image and magnify it, you will be able to find a branch for Homo sapiens, and you can begin to get a sense of where our single species stands in the grand scheme of diversity on Earth. With all these other kinds of organisms on the planet, why do we regularly pay attention to so few? I will talk more about this in the next post.

Life in Our Local Pond

One of the things I like best about science is that it makes me aware of things about the world that are right in front of me, but easy to miss. My wife and I like to visit a little pond in the cemetery near our apartment. It's a lovely place covered in lily pads, full of frogs and goldfish, and frequented by red-winged black birds that like to sit on the cattails. It's also frequented by the occasional duck and lots of dragonflies.

A while back we decided to collect a little pond water and some algae to provide for some baby snails we'd hatched from an egg mass. In our small sample of water and pond scum, we found representatives of several animal phyla. (In the traditional system of naming, or taxonomy, an animal phylum is the largest grouping of animals below the kingdom. For example, vertebrates like us are placed within the phylum Chordata, which also includes a variety of interesting invertebrates.) Of course there were arthropods such as insects and ostracodes, and oligochaetes (worms in the same group as earthworms). But we also found lots of rotifers of different kinds, a few lovely freshwater limpets, a neat flatworm, and (moving outside the animal kingdom) lots of interesting ciliates. My favorite ciliates were some Vorticella-like ones that lived attached to each other in a colony. When disturbed, they all sprang back together into a clump. But our favorite animal finds were hydra and freshwater bryozoans. Those are so cool I'll save them for their own posts in the future.

But for now, I want to say how exciting it is to be able to see such an amazing diversity of life in this tiny pond, sitting in a very disturbed habitat in the middle of a city. In just our little sample we can see so many totally different kinds of organisms, each making a living in its own way. Thanks to several centuries of people studying nature, we have a search image that lets us see more than we otherwise would know to look for. Not only that, but we can easily learn a great deal about the creatures we are looking at from the work that those previous scientists have done. One of my favorite professors, Dr. Howard Whisler of the University of Washington, began his course on eukaryotic microorgansisms by telling us he was going to "teach us how to look at pond scum". (I don't remember his exact words, but that's as close as I can recall). That skill is one of the most valuable and beautiful things I've learned in science.