Figure 1. Individuals of the two studied haptophyte species and function of the haptonema. (a) Prymnesium
polylepis and (b) Prymnesium parvum. (c–f) Sketch adapted from Kawachi and coworkers8. The haptophyte
captures prey (red) on its haptonema while swimming and collects them at a specific aggregation point (c).
While the flagella are paused, the aggregate is actively transported to the tip of the haptonema (d), which is bent
towards the back of the cell (e), where the particles are engulfed (f). The purpose and means of the movement of
prey towards the aggregation point are unknown.
focus on swimming and nutrient uptake22, swimming and flagellar synchronization23,24, and quiet swimming13.
Hydrodynamic interactions between cell and flagellum play an important role for propulsion, and therefore provide
one reason for models to take into account the no-slip boundary condition at the cell surface25.
Here, we investigate how different flagellar arrangements and beat patterns in biflagellates affect swimming
speed, flow disturbance, and prey capture. We examine how far each of these essential functions is optimized in
mixotrophs. The flow fields of the two characteristically different haptophyte species are visualized using micro
particle image velocimetry (Methods). To explore the influence of the flagellar arrangement, we build on the
Oseen model and develop an analytical biflagellate model consisting of two point forces in the vicinity of a spherical
body with no-slip boundary. The model captures the essential features of the observed flow. With the model
we quantify the time-varying and the time-averaged near cell flows around the two species and we find optima for
swimming, predator avoidance, and prey capture.
Flagellar arrangements, beat patterns, and flow fields. The two species show characteristic differences
in their flagellar arrangements and beat patterns and in the resulting flow fields. Prymnesium polylepis
has long flagella that beat in an undulatory mode with travelling waves that move down the flagella (Fig. 2a–d,
Table 1, Supplementary Video S3). The phase shift between the two flagella does not show a clear pattern and
varies across individuals and over time. The swimming speed is constant. Behind the organism, large, mainly
transversal time-varying flows are formed around the beating flagella. The time-dependent flow is qualitatively
different for P. parvum, that has short flagella and swims with an unsteady ciliary beat pattern leading to large
variation in swimming velocity during the beat cycle (Fig. 2e–h, Table 1, Supplementary Video S4). In each beat
phase one can note symmetrically arranged patches with high flow speeds, the flow directions and positions
of which follow roughly the dynamics of the flagella end segments. Prymnesium parvum, as observed, mainly
swims with a synchronous beat, which is interrupted by periods of asynchronous “tumbling” motion. The beat
pattern, flow fields, and swimming velocity variation during the beat cycle of P. parvum resemble roughly those of
Scientific Reports | 7:39892 | DOI: 10.1038/srep39892 2