Fig. 1.

Exemplary variations in sperm design found in nature. a Sperm cell from the sea urchin Arbacia punctulata. The coarse overall morphology is found for many animal species. b The crawling sperm from the nematode Ascaris suum (picture courtesy of Dr. S. Sepsenwol). c The longest sperm known, from the fly Drosophila bifurca (picture courtesy of Dr. R. Dallai). Drosophila sperm with a shape similar to that of ball of wool is emanating from the last portion of the deferent duct. d–f sperm from different plants: Conocephalum conicum (d; biflagellated), Equisetum hyemale [e; with at least 80 flagella; (Renzaglia et al. 2002)], and Cycas revoluta (f; about 1000ths flagella). Reprinted with permission from (Renzaglia and Garbary 2001) and (Takaso et al. 2013). g, h Sperm can display profound variations in morphology even within the same class. Sperm from two passerine birds: the Eurasian bullfinch Pyrrhula pyrrhula (g) and the House sparrow Passer domesticus (h). Reprinted with permission from (Birkhead and Immler 2007)