Amazing slime

I just got a new pet last week: a plasmodium of Physarum polycephalum. Physarum is one of the more commonly grown of the poorly named "slime molds". These creatures are slimy, but can be quite beautiful under the right conditions. The are also not fungi, but giant multinucleate amoebae with complex life cycles (the plasmodium is the multinucleate stage). I took the time lapse movie above while the plasmodium (bright yellow) grew over an oat on its petri dish. The black stuff was sumi ink that I was hoping it would ingest so I could more easily see the flow through the plasmodium... It didn't, at least not in the time I gave it.

Slime molds are fascinating for lots of reasons, but my personal interest is two fold. First, I'm trying to get a science collaboration wiki started, and these organisms seem like something that can allow anyone to do cell biology: they can be grown at home and people can see them and do cell biology without much equipment. Second, the way they self-organize their transport network based on functional/flow cues fascinates me because of the parallels with other organisms (such as colonial bryozoans, and multicellular human circulatory system; see discussion in previous previous post).

Project on public participation in science

I've been working to create an online platform (igor.wikidot.com) to make it easier for members of the public to do original research. There is plenty of good science that can be done with few resources, and many amateur scientists have the skills to build complex devices. If people could connect and share their ideas, skills, and resources, a group of amateur scientists could do quite sophisticated research without the backing of a formal institution. By getting feedback from each other, and from professional scientists, people could gain the benefits of others' experience.

The site is a wiki-style site, so members can post new project ideas, or add new data, ideas, or analyses to existing projects. Members can also start and modify articles providing information about techniques and other advice. There is also a list of scientists willing to answer site members' questions (if you are a professional scientist, consider adding your name to the list).

If you love science, please consider joining to help to build a new science community.

Not an embryo.

I just finished teaching a marine invertebrate embryology course (hence the long hiatus). So I thought I'd post a picture of something that is not an embryo, but budding embryologists often confuse with an embryo in the middle of first cleavage. In fact, Noctiluca is a Dinoflagellate which (confusingly) doesn't look much like a Dinoflagellate. It's really big, buoyant, and has a single, long, mobile tentacle.

What do scallops do with all those eyes?

Scallops have hundreds of remarkable blue eyes (blue dots around the edge of the shell in the image).

 Their eyes are quite sophisticated, combining a lens and mirror to focus an image on a retina (which sits between the lens and retina) [1]. They appear to be able to focus a sharp enough image to have about a 2° angular resolution [2,3]. For comparison, that's about the size of one's thumb (while, my thumb) held at arm's length. Not great, but quite respectable for a bivalve. 

Clams are not particularly brainy, so what do they do with all these eyes? They can clearly see one coming when they're sitting in a tank. They clam up quickly. But lots of animals have shadow responses without sophisticated eyes. 

Speiser and Johnsen [3] tested clever idea: perhaps these filter feeders use their eyes to judge whether conditions are right for filter feeding. Scallops, like most clams, make a living by filtering nutritious particles out of the water. Perhaps they use their eyes to see when there are enough particles flowing fast enough to be worth expending the energy to pump water through their gills. To test this they played videos of particles flowing to the scallops and watched whether they opened up. 

It turned out that whether they opened depended on whether there were particles, how many, and how fast they were moving in the way they predicted based on the speed and resolution of the eyes. So it looks like this sophisticated visual system might be used to detect flowing particles. Perhaps even more interestingly, what we'd normally see as limitations on the eyes – their slow response and low resolution – might make it unnecessary to have sophisticated neural processing.

I remain curious about other possible functions [4]. Why would they need so many eyes for a particle or a shadow sensor? Could they use their eyes to detect distance as well like we use our binocular vision? Some simple calculations suggest an ~3 cm diameter scallop (such as the one shown) might be able to judge distances out to ~20 cm.

1. Land, M.F., Image formation by a concave reflector in the eye of the scallop, Pecten maximus. The Journal of Physiology, 1965. 179(1): p. 138-153.
2. Speiser, D.I. and S. Johnsen, Comparative morphology of the concave mirror eyes of scallops (Pectinoidea)*. American Malacological Bulletin, 2008. 26(1-2): p. 27-33 DOI: http://dx.doi.org/10.4003/006.026.0204.
3. Speiser, D.I. and S. Johnsen, Scallops visually respond to the size and speed of virtual particles. The Journal of experimental biology, 2008. 211(Pt 13): p. 2066-70 DOI: http://dx.doi.org/10.1242/jeb.017038.
4. Hamilton, P.V. and K.M. Koch, Orientation toward natural and artificial grassbeds by swimming bay scallops, Argopecten irradians (Lamarck, 1819). Journal of Experimental Marine Biology and Ecology, 1996. 199(1): p. 79-88 DOI: http://dx.doi.org/10.1016/0022-0981(95)00191-3.

Sensation and coordination based on cilia

How do organisms coordinate sensation and behavior? Ctenophores have linked the usual neuron-based system of other animals to an elegant coordination system based on mechanical interactions among cilia.

Ctenophores seem to me to have reached nirvana. The common ones that we see on the coast glide gracefully through the water, their transparent bodies sparkling with bright iridescence when their eight bands of beating "combs" catch the light.

The combs (or ctenes) are plates of thousands of very long cilia, which make a paddle that can get up to a millimeter or two wide and long. The rows of combs beat in a wave along the length of the body. Like all animals, ctenophores have muscles that control their body shape, and neurons which control the muscles and can signal the cilia to stop and reverse.

However a fascinating series of experiments by Dr. S. Tamm [1-3] suggests the wave of the comb rows' beat is coordinated by the mechanical drag of one cilium moving as it beats triggering the adjacent ones to beat. Mechanical interactions coordinate movements both within [2-3] and among the plates [1-2]. In the species shown - Mnemiopsis leidyi - in this picture, the wave is transmitted from one plate to the next along a row of smaller ciliated cells that runs between the big plates. Originally it was thought this transmission was electrical [1], but subsequent experiments indicate it is also based on mechanical interactions among cilia [2].

Their sensation and response to gravity is coordinated in a similar way [2,4]. At the back end of the animal, there is an gravity-sensing organ, the apical organ. It is formed by a dome of non-motile cilia enclosing small calcareous stones secreted by the animal. These stones are held by four groups of beating cilia. The force with which they press on the cilia depends on gravity, and pulling or pushing on the cilium changes the frequency that the cilia beat. This change of beat frequency appears to be transmitted mechanically along rows of ciliated cells to the comb rows.

So, their ability to orient to gravity seems to be built (almost) entirely out of the effects of mechanical force on ciliary beat. It turns out that the sign of the response to force can be changed electrically, which allows them to switch between orienting up or down with respect to gravity. This switch is probably under nervous system control, as are other responses that stop, reverse, or accelerate ciliary beat [2]. But still, a remarkably large amount of their sensation and coordination seems to be controlled by mechanical interactions among cilia more than by electrical and chemical interactions among neurons.

One intriguing question (at least to me) is how this mechanical coordination might work in turbulence. Is the turbulence in their environment strong enough to trigger or suppress the cilia wave? Could this enhance or limit their ability to navigate in their natural habitat? Another interesting question is, how important are non-neural mechanisms of coordination and sensory integration in other animals? (Ciliary beat is known to be mechanically modulated in other organisms.) And how has this unusual system affected the evolution of ctenophore body forms?

1. Tamm, S.L., Mechanisms of ciliary co-ordination in ctenophores. Journal of Experimental Biology, 1973. 59(1): p. 231-245.
2. Tamm, S.L., Ctenophora, in Electrical conduction and behaviour in 'simple' invertebrates, G.a.B. Shelton, Editor 1982, Oxford University Press: Oxford. p. 266-358.
3. Tamm, S.L., Mechanical synchronization of ciliary beating within comb plates of ctenophores. Journal of Experimental Biology, 1984. 113(1): p. 401-408.
4. Lowe, B., The role of ca2+ in deflection-induced excitation of motile, mechanoresponsive balancer cilia in the ctenophore statocyst. Journal of Experimental Biology, 1997. 200(11): p. 1593-606.

Self-organizing animals.

In earlier posts I introduced bryozoans: filter feeding invertebrates that grow by budding to form groups of interconnected clones. One of my favorite things about bryozoans is that some of them self-organize to form structures much larger than the individual animal (unless you count the whole colony as the individual animal). My favorites (Membranipora sp.) grow as big sheet-like colonies covering surfaces. Most individuals stick their crowns of tentacles out out of their coffin-shaped boxes to form a canopy over the surface of the colony. They each pump water through themselves to feed on tiny planktonic organisms. The water has to have somewhere to go or else they wouldn't be able to keep feeding. Certain patches grow taller than their neighbors, and lean to the side to make openings in the canopy (also, individuals in the center of these patches often degenerate). The water squirts out of these chimney-like openings at relatively high speeds, so it gets mixed farther from the colony, and the individuals don't keep re-filtering the same water.

The resulting form and water flow patterns are beautiful. The first image here shows what a ~3cm wide colony looks like from the top; the second image shows a different colony from the side. I used a sheet of light to illuminate just a single slice through the chimney in the second image. Colorizing each frame differently makes the flow stand out: moving particles appear as rainbows, while the still tentacles appear white.

The really cool parts about the chimneys that the colony organizes the chimneys based on information in the water flow that the chimneys control: the colony forms chimneys at the growing edge of the colony where the water flows out fastest [1], and can be induced to form new chimneys by manipulating the flow [2]. This feedback between water flow and form appears to allow them to respond to all sorts of perturbations that affect the water flow: injuries from predators, the formation of spines to defend against predators, variation in the form of the surface they grow on, etc [3].

This same kind of feedback between flow and form shows up in our blood vessels [4, 5], the gut/circulatory canals of hydroid colonies [6, 7], the veins of giant unicellular slime molds [8]. These systems evolved independently, serve different functions, and have very different structures, yet are united by the physics of moving fluids [9].

1. von Dassow, M., Flow and conduit formation in the external fluid-transport system of a suspension feeder. Journal of Experimental Biology, 2005. 208(15): p. 2931-2938 DOI: 10.1242/jeb.01738.
2. von Dassow, M., Function-dependent development in a colonial animal. The Biological Bulletin, 2006. 211(1): p. 76-82 DOI: 10.2307/4134580.
3. Grunbaum, D., Hydromechanical mechanisms of colony organization and cost of defense in an encrusting bryozoan, membranipora membranacea. Limnology and Oceanography, 1997. 42(4): p. 741-752.
4. Kamiya, A. and T. Togawa, Adaptive regulation of wall shear-stress to flow change in the canine carotid-artery. American Journal of Physiology - Heart and Circulatory Physiology 1980. 239(1): p. H14-H21.
5. Langille, B.L., Blood flow-induced remodeling of the artery wall, in Flow-dependent regulation of vascular function, J.A. Bevan, G. Kaley, and G.M. Rubanyi, Editors. 1995, Oxford University Press: New York, N.Y. p. 277-299.
6. Buss, L.W., Growth by intussusception in hydractiniid hydroids, in Evolutionary patterns: Growth, form, and tempo in the fossil record, J.B.C. Jackson, S. Lidgard, and F.K. Mckinney, Editors. 2001, University of Chicago Press: Chicago. p. 3-26.
7. Dudgeon, S.R. and L.W. Buss, Growing with the flow: On the maintenance and malleability of colony form in the hydroid hydractinia. American Naturalist, 1996. 147(5): p. 667-691.
8. Nakagaki, T., H. Yamada, and T. Ueda, Interaction between cell shape and contraction pattern in the physarum plasmodium. Biophysical Chemistry, 2000. 84(3): p. 195-204 DOI: 10.1016/S0301-4622(00)00108-3.
9. Labarbera, M., Principles of design of fluid transport-systems in zoology. Science, 1990. 249(4972): p. 992-1000.

Re-invigorated by brittle stars

I'm re-opening this blog to the public after a long hiatus partly inspired by Dr. Christopher Mah's "Echinoblog" particularly these two posts: "When Brittle Stars ATTACK!!!!" and "Giant Green Brittle Stars of DEATH!!". What could be more exciting than brittle stars preying on fish? If only there were some terrestrial ones!

Animals that feed on ancient supernovae

Do some animals on earth get their energy from ancient supernovae? It requires energy input to fuse lighter elements into heavy elements such as uranium. In nature this energy input comes from supernovae: heavy elements are formed in supernovae and store a tiny bit of the supernova's energy [1, 2]. As B. Schutz points out in his excellent book [1], this means that nuclear reactors are powered by energy stored from a supernova which most likely occurred around 6 to 7 billion years ago. Even more interestingly, since much of the Earth's internal heat comes from radioactivity, it implies that the Earth is warmed in part by an ancient supernova, so geothermal power also comes from supernovae.

The spectacular deep sea hydrothermal vent animals and bacteria also get energy from the internal heat of the Earth. Earth's heat drives the reactions that form hydrogen sulfide which the bacteria use as an energy source. Hence, some of the energy they use is stored energy from supernovae. The vent animals get their energy from the bacteria. The vent bacteria require both the hydrogen sulfide coming out of the vent and oxygen formed by photosynthesis to get energy by oxidizing the hydrogen sulfide [3]. Therefore, the vent communities do get energy from the sun too, but still, it is remarkable that they get some of their energy from supernovae.

Of course, all of the matter on earth (with the exception of hydrogen) came from ancient stars originally, but it seems even more exciting that the energy that powers the Earth and some Earth life also came from supernovae. And yes, mass and energy are equivalent, but the form of energy that counts here is the free energy: the energy that can drive a system out of equilibrium, and hasn't simply been lost as waste heat or otherwise converted to an unusable form.

I highly recommend two fascinating and very well written books on gravity [1], astronomy [1, 2] and how scientists determine the age of ancient events [2]. These two books are excellent since they delve deeply into how scientists studying these issues have come to their current understanding of them, and what the open questions are, while still being accessible to pretty much anyone with a minimal scientific literacy. I'm only through the first half of Dr. B. Schutz's book on gravity, but so far it's exceptionally good despite a few confusing spots. Dr. M. Hedman's book is one of the most interesting books I've ever read.

(1) B. Schutz (2003) "Gravity: grom the ground up," Cambridge University Press, pp125-126.

(2) M. Hedman (2007) "The Age of Everything: How Science Explores the Past," University of Chicago Press, pp157-158.

(3) C. van Dover (2000) "The Ecology of Deep-Sea Hydrothermal Vents," Princeton University Press.

Re-building a whole new body

Imagine how handy it would be if you could rebuild your whole body from a tiny fragment. Some animals can do this, but most such animals (e.g. sponges, hydra, and planarians) have elegantly simple internal structure. But it turns out that many botryllid ascidians, among our closest living invertebrate relatives, are also able to do this [1-3]. As mentioned in a previous post, these marine animals have the same basic organization that we do: each individual has a heart, central nervous system, and through-gut, as well as a number of other features shared with vertebrates.

Adult botryllid ascidians can reproduce sexually, with eggs that develop in a precisely choreographed manner to form a swimming larva. The larva metamorphoses into the founding member of a colony of interconnected individuals that live attached to surface. Each individual reproduces asexually by forming buds as pouches off the tissues surrounding its heart. But under certain conditions (which vary by species [1-3]) the colony can form a whole new individual from aggregates of blood cells. Circulating cells clump up on the wall of one of the blood vessels that reach to the outer edge of the colony [1]. Then the clump hollows out, and starts to mold itself into a new body from scratch*. Because these organisms are fairly closely related to humans, this process likely holds insights for regenerative medicine [3].

However, what I find most exciting is that it shows that development can tolerate remarkable variation. Developmental biology tends to focus on the complicated interplay of interactions required in "normal" development from the egg. We usually think of development as fragile, easily thrown off by a mutation here, or a sip of wine there. But this example dramatically shows that – at least in a few organisms surprisingly closely related to ourselves – cells can organize themselves into the same body from lots of different starting points. Botryllids can form complete new individuals in at least three very different ways: a precise series of events from egg to larva to juvenile; from pouches of a specific epithelium; or from disorganized aggregates of blood cells. Understanding how they do this could reshape how we think about animal development.

*The outer epithelium apparently comes from the cells lining the outside of the blood vessel. Their blood vessels do not have the inner lining of cells that our vessels have.

1) The clearest images and diagrams of this process come from the first description, which is freely available online: Oka, H., and Watanabe, H. 1957. Vascular budding, a new type of budding in Botryllus. Biological Bulletin 112:225–240.
2) This has some neat videos, and some interesting results about the importance of maintaining blood flow for regeneration. Voskoboynik A, Simon-Blecher N, Soen Y, Rinkevich B, De Tomaso AW, Ishizuka KJ, Weissman IL. 2007. Striving for normality: whole body regeneration through a series of abnormal generations. The FASEB Journal 21:1335–1344.
3) This one also has neat videos and discusses the some of the possible molecular pathways involved. Brown, F.D., Keeling, E.L., Le, A.D., and Swalla, B.J. 2009. Whole Body Regeneration in a Colonial Ascidian, Botrylloides violaceus. Journal of Experimental Zoology (Mol. Dev. Evol.) 312B:885-900

Ascidian colonies

Erica Westerman (first author on the study discussed in my last post) kindly sent me two nice images of ascidian colonies*. The first image shows a pair of small Botrylloides violaceus colonies (orange) growing up to each other at the top of the image. The lower colony is getting overgrown by a larger colony of Botryllus schlosseri at the bottom (brown). The individual zooids within the colony are orange (or brown) oblongs set in rosettes of 6 or more. Each set of individuals sucks in water through an opening at the colony surface, and the water flows to an opening at the center of the rosette, shared by all the zooids in the rosette.

The second image shows several larger colonies, of Botrylloides violaceus growing into each other on a settling plate. You can still see the boundaries between the colonies, which are distinct in coloration, and the arrangement of zooids.

*As always, please do not use images without permission from the contributor.

Are you feeling like yourself today?

What it would be like if the people you lived with could take over your body? This is a (somewhat creepy) reality for a few of our closest invertebrate relatives, the botryllid ascidians.

Adult ascidians look nothing like vertebrates, but developmental and molecular similarities indicate they are close kin to vertebrates [1]. Adult ascidians live attached to a surface and filter food from the surrounding water. Some ascidians are colonial: they bud asexually to form an array of individuals that each have their own organs but share a common blood system.

At least one type of colonial ascidians – the botryllids – takes this a giant step further. When neighboring colonies meet they can fuse together so that both colonies share their blood, transmitting cells among the colonies [2-3]. Colonies only fuse if they share a particular version (allele) of one gene. If they don't share that allele, one colony may grow over the other, smothering it, or they may sit quietly side by side. However, in laboratory studies colony fusion leads to an unpleasant outcome for one of the colonies. One colony can take over the other, replacing the tissues of the second colony with its own cells*. Certainly a creepy way to die!

But what happens in the wild? To address this question, Erica L. Westerman et al. (2009) set out clean plates in Salem Harbor (MA) for larvae of one species, Botrylloides violaceus, to settle on [3]. The researchers monitored colony-colony contacts over several weeks. Surprisingly, almost all of the colonies fused! This is remarkable, given what has been observed under controlled conditions. Westerman et al. (2009) provide a very interesting and accessible discussion of factors that may differ between the lab and field, and of some of the possible advantages to fusing despite the risk of takeover.

Colony-colony fusion gives a fascinating window into the biology of individuality since all organisms, including ourselves, have to deal with the problem of how to tell what's part of themselves and what's not. And it remains an issue even for vertebrates: in a few mammals with low genetic diversity (e.g. dogs [4] and Tasmanian devils [5]), cancers have arisen that can spread among individuals since so many individuals share the same versions of the genes that distinguish self from non-self**. (This is not a problem for humans because our species has a high level of diversity in those genes). Hence, the peculiarities of the botryllids shed light on one of the most fundamental and challenging biological questions: what is an individual?

*It gets yet stranger since the second colony doesn't always lose out entirely. Sometimes the apparent loser takes over all the reproductive cells and becomes the sole parent of the next generation [2].
**Curiously, different animals use different genes to distinguish self from non-self.

1) Pechenik, J.A. (2000) Biology of the Invertebrates, 4th edition.
2) Rinkevich, B. (2005) Natural chimerism in colonial urochordates. Journal of Experimental Marine Biology and Ecology 322:93-109
3) Westerman, E.L., Dijkstra, J.A., and Harris, L.G. ( 2009) High natural fusion rates in a botryllid ascidian. Marine Biology 156:2613-2619
4) Rebbeck, C.A., Thomas, R., Breen, M., Leroi, A.M., and Burt, A. (2009) Origins and evolution of a transmissible cancer. Evolution 63(9): 2340–2349
5) Murchison, E.P. et al. (2010) The tasmanian devil transcriptome reveals schwann cell origins of a clonally transmissible cancer. Science 327:84-87

Plantimals!

E. chlorotica. Image kindly provide by Dr. S.K. Pierce.

We all know what animals are. We all know what makes plants and animals different. These are things we learned in elementary school, right? Wrong. It turns out that these categories, like many things one learns in elementary school, are not so cut-and-dry in reality.

The neatest SICB talk we saw was given by Dr. Sydney Pierce of the University of South Florida. He and his colleagues are researching the transfer of genes from algae¹ to sea slugs² that eat the algae. They found that many algal genes appear in the genome of a sea slug called Elysia chlorotica^3-5. It's important to note that E. chlorotica doesn't gain algal genes every time it feeds; the original acquisition took place some time in evolutionary history. These algal genes are passed down from one sea slug generation to the next; they are even found in sea slug larvae that haven't yet fed on the algae. Yes, that is as strange as it sounds—as though one of your distant ancestors ate broccoli, incorporated some of the broccoli's DNA into his/her reproductive cells, and eventually passed it down to you, thereby making you part broccoli!

There is another cool part to this story: E. chlorotica slurps the contents out of algal cells and then incorporate the chloroplasts from the algal cells into some of their own cells. (Chloroplasts are the parts of plant cells that contain chlorophyll and carry out photosynthesis.) The slugs can then use the chloroplasts for photosynthesis for awhile. So, that strange scenario above just got stranger: not only would you have broccoli DNA passed down to you, but when you eat broccoli, you could capture some of the broccoli's machinery for photosynthesis. That means you could make your own food (like a plant), rather than having to eat other organisms—like an animal! [Note that sea slugs are animals just as unquestionably as we humans are. They even have many of the same parts: brains, muscles, hearts, etc.]

Among closely related sea slug species, there is great variation in how long they can keep the chloroplasts. Most can only maintain the chloroplasts for a short time; however, E. chlorotica appears to be able to photosynthesize for months, so that in the lab they need only one meal in a lifetime. In yet another twist, Pierce et al. found that the slugs had obtained the ability to make chlorophyll^4,5—not just to use the chlorophyll taken from the algae! This shows something astounding: the sea slugs did not acquire a couple of random algal genes in evolutionary history; they actually acquired an entire complex pathway, including several genes for different enzymes.

To demonstrate that it was indeed the slugs who were actively making the chlorophyll, Pierce et al. gave the slugs radioactively-labeled building blocks for chlorophyll synthesis. Sure enough—they found that some of the radioactive material showed up as chlorophyll in the slugs' bodies.

This is a fascinating example of the transfer of genes between very different species—in this case, allowing an animal to take on some of the characteristics and abilities of plants.

Humans love to put things in nice neat bins and categories. One of the most fun parts about biology, though, is finding out how messy those bins really are. We use the metaphor of a branching tree to represent evolution, but this sea slug research shows us that sometimes the branches can come back together in unexpected ways.

1) Technically, algae are not in the same group as plants according to modern taxonomy; however, like plants, they are eukaryotes (their cells contain nuclei) that obtain energy through photosynthesis. Therefore, they are often informally called plants.

2) "Sea slug" is the informal name for members of the group of exceptionally beautiful molluscs called opisthobranchs. Land slugs (which are also cute) are a completely separate group of molluscs.

3) Pierce, S.K., Curtis, N.E., Hanten, J.J., Boerner, S.L., and Schwartz, J.A. (2007) Transfer, integration and expression of functional nuclear genes between multicellular species. Symbiosis. 43, 57–64

4) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. (2009) Chlorophyll a synthesis by an animal using transferred algal nuclear genes. Symbiosis. 49, 121-131

5) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. Chlorophyll synthesis by a sea slug (Elysia chlorotica). Presented at the Society for Integrative and Comparative Biology meeting, Seattle, Jan. 3-7th, 2010

We're back!

We recently returned from Seattle, where the annual meeting of the Society for Integrative and Comparative Biology (SICB) took place. We'd like to summarize a few of our favorite things from the meeting in the next few posts.

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.