Scanning electron microscopy
Sperm samples were coated with platinum/palladium (2 nm and 20 nm in the case of bird and snail sperm cells, respectively). Then, they were viewed in a JEOL JSM-7600F FEG-SEM, at magnifications up to 20,000×. For each sample, we attempted to view and photograph at least 20 sperm cell heads. Coiling direction was determined by eye for each sperm cell. Using the letters embossed on the stub holders, it was confirmed that the image is not mirrored at any stage during the imaging process.