A small proportion of the zoospores were carried into the stub, 4, and, due to the presence of the attractant gradient, overcame the slow flow within the stub and swam into the sensing channel following the gradient towards a higher concentration. The transit time of the spores through the electrode gap was significantly longer than when they were carried by the pumped flow. The AC coupling differentiated the current pulse, giving a characteristic biphasic signal (Figure ). A statistical analysis of the data showed the metrics of the sensing signals: amplitude 8.3-12.5 mV, SNR 4.3 -9.5 %, peak to peak time 0.1-0.23 s. The peak-to-peak time is approximately the transit time of the spore across the inter-electrode gap and so indicates a swimming speed of ≈ 50 -100 µm/s. Direct observation confirmed this speed estimate (Supporting Information Video 4).