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.

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.

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).