Abstract
Many single-celled organisms exhibit both solitary and colonial existence. An important step towards multicellularity, which is associated with benefits such as enhanced nutrient uptake, was the formation of colonies of unicellular organisms. However, the initial drivers that favoured individual cells aggregating into more complex colonies are less clear. Here we show that hydrodynamic coupling between proximate neighbours results in faster feeding flows for neighbouring ciliates, such that individuals within a dynamic colony have stronger average feeding flows than solitary individuals. Flows generated by individuals acting together reach higher velocities, thus allowing access to a wider range of prey resources than individuals acting on their own. Moreover, we find that accrued feeding benefits are typically asymmetric: whereas all individuals benefit from acting together, those with slower solitary currents gain more from partnering than those with faster currents. We find that colonial organization in simple unicellular organisms is beneficial for all its members. This provides fundamental insights into the selective forces favouring the early evolution of multicellular organization.
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Data availability
Data supporting the findings of this paper are available from the corresponding authors upon reasonable request. Source data are provided with this paper.
Code availability
Code developed as part of this work will be available from the corresponding authors upon reasonable request.
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Acknowledgements
S.S. was supported by Whitman Early Career Award from MBL and startup funds from Emory University. Funding support came from NIH NIGMS (Grant No. R35GM143050 to S.S.), NIH NIGMS (Grant No. R35GM130327 to W.M.), NIH (Grant No. R01HL153622 to E.K.), NSF (Grant Nos. IOS-2034043 and CBET-2100209 to E.K.), NSF (Grant Nos. CBET-2100705 and IOS-2114171 to J.H.C.), NSF (Grant Nos. CBET-2100156 and IOS-21141691 to S.P.C.), ONR (Grant No. N00014-22-1-2655 to E.K.) and ONR (Grant No. N00014-23-1-2754 to J.H.C.). We thank M. Slabodnick for invaluable advice on Stentor culturing and J. Allen for help with microscopy. S.S. is grateful to J. Lippincott-Schwartz and R. Phillips for their enthusiastic support for this project. S.S. thanks A. Knoof, for invaluable help in collecting stentors from ponds across the Northeast.
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S.S. and J.H.C. designed the experiments. H.G. and E.K. designed the mathematical model. S.S. conducted the experiments. H.G. performed the simulations. S.S., S.P.C. and W.M. analysed the experimental data. H.G. and E.K. analysed the model results. S.S., J.H.C., H.G. and E.K. put together all the results and wrote the paper.
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Extended data
Extended Data Fig. 1 Experimental setup for visualizing empirical flowfields.
Schematic representation of the imaging chamber formed by attaching two coverslips on either side of a polydimethylsiloxane (PDMS) ring 5 mm in diameter and 700 µm high. The microscope is positioned below the PDMS chamber. Stentors (blue schematic) usually anchored themselves on the lower glass coverslip attached to the PDMS chamber.
Extended Data Fig. 3 Methodology for measurement of feeding currents.
Since only the orthogonal portion of the flowfields in the region between the two vortices reaches the oral opening and can be productively filtered for prey, only the velocity component normal to the oral opening, Ufeeding (thick black arrow), is taken as a measure of the feeding current. Due to mixing between tracer and cilia movements, feeding current velocities were measured 0.25 mm from the oral opening to prevent contamination of the PIV analysis due to its inability to separate tracer movements from ciliary beating. Feeding current (Ufeeding) for each individual in a pair or alone was determined by calculating the average velocity across a line the size of the oral opening of each individual. Thin black arrows denote the flow streamlines. Thin double-headed arrow denotes separation between the two Stentors. Thick arrow denotes the flow directed towards the oral opening of each Stentor.
Extended Data Fig. 4 Feeding current velocities of solitary individuals.
Distribution of velocities of the feeding current generated by solitary Stentor individuals (n = 17) measured at 0.25 mm from the ciliary band.
Extended Data Fig. 5 Ciliary wave velocity remains unchanged with proximity.
Kymographs showing the frequency of the metachronal waves for the two individuals in the pair shown in Fig. 2a,b. The frequency of the metachronal wave for the left and the right S. coeruleus in the pair remains unchanged as the distance, Δ, between the two individuals changes from (a) Δ = 52 µm (left: 16.7 Hz, right: 15.4) to (b) Δ = 246 µm (left: 16.6 Hz, right: 15.2 Hz).
Extended Data Fig. 6 Streamlines generated by the model Stentor.
Flow fields of a single Stentor modeled as a regularized force (black arrowhead) bounded by one wall (θ = 0.25π), viewing from above (left panel) and from side (right panel). Blue lines and arrows denote the streamlines and the direction of the flow.
Extended Data Fig. 7 Streamlines generated by a pair of model Stentor individuals.
(a) Fluid flows generated by a Stentor pair, modeled as a pair of regularized Stokeslets of strengths F1 and F2 respectively near a wall with force ratio F1/F2 = 1, inter-Stentor separation distance Δ = 2 (left) and Δ = 0.5 (right) (b) Force ratio F1/F2 = 2 and inter-Stentor separation distances Δ = 2 (left), and Δ = 0.5 (right). Top row: flow streamlines. Bottom row: downward velocity component of the flow velocity determined along the horizontal line z = 1.25H.
Extended Data Fig. 8 Feeding flow rate and the benefit for individual Stentor in a pair.
(a-c) Feeding flow rate of Stentor 1 U(1)feeding in a Stentor pair whose swaying angles ϕ1,2 (a), inclination angles θ1,2 (b), and the separation distances ∆ (c) are subject to Gaussian noises. The horizontal dashed lines represent the feeding flow rate of solitary Stentor. The gray vertical lines separate the cases of having a stronger neighbor (left) and weaker neighbor (right), same as in (d–f). (d-f) The benefit of “being together” for Stentor 1 in a Stentor pair whose swaying angles (d), inclination angles (e), and the separation distances (f) are subject to Gaussian noises. The mean (lines) and the standard deviations (error bars) are obtained from 100 Monte Carlo simulations. F1 and F2 are the individual strengths of Stentors 1 and 2.
Supplementary information
Supplementary Video 2
Feeding flow of an individual Stentor visualized using milk particles as tracers. The time-lapse video was generated using dark-field microscopy. Also see Fig. 1d.
Supplementary Video 4
PIV analysis of the flow field of an individual Stentor showing the direction (denoted by the arrows) and the magnitude of the feeding flow. Arrows are pseudo-coloured to denote the magnitude of the feeding flow velocity. The scale is from 0 (black arrows) to 100 µm s−1 (white arrows). Also see Fig. 1e.
Supplementary Video 6
PIV analysis of the flow field generated by a pair of Stentor individuals showing the direction (denoted by the arrows) and magnitude of the feeding flow. Arrows are pseudo-coloured to denote the magnitude of the feeding flow velocity. The scale is from 0 (black arrows) to 300 µm s−1 (white arrows). Also see Fig. 2b.
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Data shown in Fig. 2c,d.
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Data shown in Fig. 3c–i.
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Data shown in Fig. 4b,c including the insets.
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Data shown in Fig. 5a,b.
Source Data Extended Data Fig. 4
Data shown in Extended Data Fig. 4
Source Data Extended Data Fig. 7
Data shown in Extended Data Fig. 7a,b.
Source Data Extended Data Fig. 8
Data shown in Extended Data Fig. 8a–f.
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Shekhar, S., Guo, H., Colin, S.P. et al. Cooperative hydrodynamics accompany multicellular-like colonial organization in the unicellular ciliate Stentor. Nat. Phys. 21, 624–631 (2025). https://doi.org/10.1038/s41567-025-02787-y
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DOI: https://doi.org/10.1038/s41567-025-02787-y
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