Representative testes and sperm of three-spined stickleback males. Testis of a young (a) and an old (b) male and on the right side the corresponding L*value for testis brightness as a measure of testis melanisation (see Mehlis et al. 2012). (c) Sperm head, mid-piece, and tail can be distinguished on the SEM image. The black lines indicate head length and width as well as mid-piece length and width, the dotted white line indicates the boundary between head and mid-piece.