Brand New Colonies

A few weeks ago, I noted my surprise at finding that what seemed to be a cluster of dancing raisins in my pond water sample was actually a colony of rotifers growing around a bit of algae.

Since then, I’ve turned my attention to a larger lake in a nearby county park called Hawk Island Park. I’ve hunted here before for the creatures in the pond scum, but the lake is actually fairly deep, having formed from a former gravel pit. Toward the bottom of the lake, a thick bed of hornwort provides shelter for innumerable insects and crustacea, as well as other creatures. The other day, I took my sampling bottle out there (without the usual net I would normally apply), weighted it with a stone, and let it drop twenty feet toward the bottom. It came up dragging a line of hornwort with it, which I took a sample of.

Yesterday, I noticed a small ball floating around in the jar along with the crustaceans and flatworms I’ve been looking forward to imaging. It drifted aimlessly in the sample, seeming to be pushed by impossible currents, first in one direction, and then another. I scooped it up with my eye dropper, and to my great delight found this.

A seething ball of rotifers bound together in a massive koosh ball of a colony, but all moving in their own directions. There’s a hypnotic effect to all of those bodies reaching out, grasping, feeding at the water around them.

My best guess as to their species is Conochilus unicornis, though I could be wrong (that was my initial guess for the first colony I found, and now I know otherwise). The aggregate of their motions seems to drive them through the water, creating a walking effect when they touch a substance and a light current to move them around in search of prey. Manu has mentioned that looking at the currents around colonies of rotifers shows a deep advantage to them working together in this way.

What fascinates me, though, is that this cooperative effect clearly happens entirely by accident. This is a colony in proximity and name only; unlike an ant colony, there’s little visual sign of even an emergent cooperation among the rotifers. Each reaches out independent of the others. And yet, this brings about motion and guides currents to each rotifer’s mouth probably much more effectively than if they were solo.

In the same sample, at the tip of a hornwort leaf, I spied another shaggy mass begging for observation. This betrayed a beautiful colony of what I believe to be Sinantheria socialis.

The differences between the two colonies couldn’t be more striking to me. Where the free swimming colony grasps and seethes with constant motion, the sessile colonists are more languid in their approaches, fanning out their broad, heart shaped coronas to draw matter to them. We see the complex structures in their bodies processing what comes to them, their cornucopic guts larger and seemingly more complex than the thin tubes of the Conochilus.

And so there are not just one colony of rotifers to study, but three of them, distinct in form and behavior from each other.  There are tests to run, of course, but as I begin to find more and more of them, the lives of these little creatures becomes increasingly complex to me, their existences something to be wondered at and sought out from around me.

8 Comments Add yours

  1. laksiyer says:

    Matt the last video is mesmerizing!!! The beauty is that you can see all angles around the corona , the cilia and the vortex. Wonder what would happen if you pinch a bit of the colony . How might the rotifer spread?

  2. Manu Prakash says:

    @matt: please please please keep the colony alive. My students and i have a recent paper looking at Vortices and flows animals – and a new theory for colonial animals. Watching your video; just made me realize; this might be One of the really interesting system to test our theory. I have to take care of babies right now; so I will write a longer note soon.

    But; please please please keep the colonies alive 🙂 incredible datasets – as always..


  3. Matthew Rossi says:

    I plan to keep these alive as long as I possibly can. I have the goal of keeping a self-sustaining ecosystem going in my office in Lansing, and these would be an essential part of it. I’ve noticed colonies with fewer plants and animals get overwhelmed less easily, so hopefully this will keep going. If not, there’s a lot more lake to look in. If you have any recommendations of best practice where it concerns caring for rotifers, let me know.

    @Laks I was just telling my partner, it’s a marvel to me what a complex creature these are, how many moving parts and systems can exist in such a tiny place. It’s also amazing to me the amount of variety that exists in their phylum and how such a simple body can iterate into so many forms. It is little wonder I get a little thrill when I think about evolution.

  4. Cristina says:

    Wow, Wow and endless wows!! An exhibition of beauty in your writing and in your images!!! The last video keeps me utterly marvelled ! Formidable! Thank you!

  5. MaxCoyle says:

    Amazing videos, @Matt! This is such a puzzle to me. I am torn between your description of their behavior as accidental/individualistic, which I do see, and also an intuition that colony formation is probably a tightly controlled process, like the way that bacteria transition between planktonic and biofilm states, or the way choanoflagellates transition between multicellular and single-celled forms based on which bacteria are present in their environment. It would be so cool quantify colony formation in some way and see if it can be manipulated.

    Thanks for this incredibly interesting post

  6. Matthew Rossi says:

    @MaxCoyle I don’t think I would argue that the existence of the colonies are accidental or haphazard, since these are clearly colonial rotifers (any image search for a Conochilus gives you the same submarine mine shape I was looking at), which means they’re adapted for life in colonies (all arguments that adaptation is, by definition, based on chance, aside). What I meant is that the seeming cooperation of the creatures in the colonies is an illusion.

    Although maybe a better way to put it is that the evolutionary advantage of colony life is greater than the sum of its parts. Each rotifer continues to act as an individual, without any direct attempt to better its mates’ lives. But as a cluster, the colony is far more successful than the individuals would be doing the same things on their own.

    1. MaxCoyle says:

      Oh sorry for the misunderstanding, I totally see what you mean! Do colonial rotifers ever dis-aggregate or re-aggregate in response to environmental stimuli?

  7. Matthew Rossi says:

    PS I recently realized that few things make me happier than discussions of how creatures evolve to live with each other. So, you know, hooray and thanks!

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