How do colonial animals organize themselves?



Fig. 1: A colonial ascidian (species unknown) from Friday Harbor, WA.

Many groups of animals have independently evolved colonial lifestyles, in which individuals reproduce asexually, but remain connected together, sharing nutrients and – often – neural signals among each other. Colonial suspension feeders often organize groups of individuals into larger systems that make it easier for the colony to feed. Do these different organisms use similar signals to organize the colony for similar functions, or have they come up with different ways to do the same task?

Some of these systems look very similar in different groups that clearly evolved the morphology independently. For example, in many bryozoans[1][2] and some colonial ascidians (sea squirts)[3] groups of individuals pump water towards the colony surface where planktonic organisms are captured and eaten; the water then flows to raised, chimney-like openings produced by groups of the individuals. Some sponges and even some bacteria [4] form very similar structures. By organizing into groups with a shared excurrent opening, the colonies can squirt water further away and avoid having to re-filter water they've already filtered [5]. The tradeoff is that the water has to flow further, theoretically making it harder for them to pump the water through themselves [6].

Do these different organisms use similar cues to determine where to put these chimney-like openings? Or did each of these groups evolve different mechanisms to produce the same kind of structure with the same function? In one species of bryozoan, it appears as though new openings form at the growing edge of the colony at sites where the water flow out the edge is highest [2]. This could make it so that chimneys form where the colony needs them most. However, colonial ascidians do not grow in the same way: water does not flow out the edge of the colony. Do they also use the kind of feedback between form and function to build these chimney-like structures? Or do they use some other mechanism?

Hypotheses and initial observations:


Fig. 2: Fig. 2: Three unidentified ascidian colonies photographed on three successive days (top to bottom) to try to observe formation of new excurrent openings. None formed by day 3…The experiment continues.
Two of the colonies were superglued to Vexar mesh (these two were separated from a larger starting colony); the third, genetically-distinct colony was attached using fishing line. The three colonies were left floating in running seawater at FHL.

Ascidian colonies do not have filtered water flowing out the growing edge in the same way that the colonies of Membranipora membranacea (a bryozoan) do. In M. membranacea, places at the growing edge which have high water outflow become new chimneys [2]. Could it be that in ascidians, high flow out of an existing excurrent opening causes it to grow wider, and eventually split? This would be analogous to M. membranacea. Or could it be that ascidian excurrent openings form at sites of high pressure (where a new opening might be most needed)? Sites of high pressure would fall as far from existing openings as possible, within the plumbing of existing systems of individuals. A third hypothesis is that chimney formation is unrelated to fluid flow, but this hypothesis makes no prediction about the site of chimney formation. To investigate the mechanism of colony organization, the first requirement is to observe it in progress (Fig. 2).

Literature Cited

1. Grunbaum D. 1997. Hydromechanical Mechanisms of Colony Organization and Cost of Defense in an Encrusting Bryozoan, Membranipora membranacea. Limnology and Oceanography.42(4):741-52.
2. von Dassow M. 2006. Function-Dependent Development in a Colonial Animal. Biol Bull.211(1):76-82. DOI:10.2307/4134580.
3. Kott P. 1989. Form and Function in the Ascidiacea. Bull Mar Sci.45(2):253-76.
4. Fenchel T, Glud RN. 1998. Veil architecture in a sulphide-oxidizing bacterium enhances countercurrent flux. Nature.394(6691):367-9.
5. Eckman JE, Okamura B. 1998. A model of particle capture by bryozoans in turbulent flow: significance of colony form. The American naturalist.152(6):861-80. DOI:10.1086/286214.
6. Grünbaum D. 1995. A model of feeding currents in encrusting bryozoans shows interference between zooids within a colony. Journal of Theoretical Biology.174(4):409-25. DOI:

ascidian biology biomechanics colonial-animals fluid-flow organismal-biology self-organization

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