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Sperm sense gradients on two timescales, which produces two different steering responses.

A periodic component, resulting from the helical swimming, gradually aligns the helix towards the gradient.

We chose the sea urchin Arbacia punctulata to study the search strategy of sperm in a 3D chemoattractant landscape. punctulata are broadcast spawners that release their gametes into the ocean, where sperm swim freely. punctulata sperm provide an unmatched model to address fundamental questions of cell navigation; (3) conditions for swimming in an aqueous medium can be readily emulated; finally, (4) Arbacia sperm, unlike mammalian sperm, represent a homogenous population, that is, most sperm are chemotactically active.

For deterministic steering, correcting path adjustments are directed towards the gradient.

Alternatively, during stochastic steering, path adjustments are chosen at random; afterwards, the microswimmer decides whether this choice was favourable and acts accordingly.

The intensity profile of the uncaging ultraviolet light was approximated by a Gaussian beam (Supplementary Fig. To mimic an egg that continuously releases resact, ultraviolet illumination was maintained during the measurement.

The time-dependent concentration profile c(x,t) was reconstructed numerically from the light profile using the resact diffusion coefficient and the extinction coefficient and quantum yield of caged resact (Fig. From the swimming path r(t) and the concentration profile c(x,t), we derived the chemoattractant stimulus encountered by sperm s(t)=c(r(t),t) (Fig. 3, Supplementary Movies 3,4).(a) 3D resact gradients were established by photolysis of caged resact with a Gaussian ultraviolet-beam.

While sperm swim along a helix, the head-wiggling plane slowly rotates around the helix axis: the normal vector describes a circle on the unit sphere (Fig.

1c), characterized by a constant inclination to the helix axis (52.0°±11.5°; circular mean±circular s.d., n=20).

When incremental path corrections fail and sperm get off course, a sharp turning manoeuvre puts sperm back on track.

Turning results from an ‘off’ Ca response signifying a chemoattractant stimulation decrease and, thereby, a drop in cyclic GMP concentration and membrane voltage.

Head wiggling was used to determine the beating plane orientation.

(c) The vector normal to the beating plane (blue arrows) precesses around the helical axis (h, red arrow) with fixed inclination, describing a circle on the surface of a unit sphere centred around h. For freely swimming sperm, head wiggling indicated a flagellar beat frequency of 43.5±3.5 Hz (Fig. Moreover, head wiggling was approximately planar, consistent with an approximately planar beat pattern.

The calculated free resact concentration is shown as a function of space and time, accounting for continuous photo-release and diffusion.

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