Of all the cell types in the animal kingdom, the most diverse are sperm, which can be adorned with tails, hairs, bristles and more. It seems that sex drives the evolution of sperm morphology.
Lukas Schärer at the University of Basel in Switzerland and his colleagues watched 16 species of promiscuous hermaphroditic flatworm (Macrostomum), which have a variety of sperm shapes (pictured), mating under a microscope. After sex, some species suck out the ejaculate, possibly as a way of selecting which sperm are ultimately accepted.
The researchers found that those species that exhibit this sucking behaviour have ornate sperm with features such as a pair of long bristles emerging at the mid-point and a tail resembling a paint brush. These appendages can become lodged in the female orifice after copulation, preventing the sperm from being sucked out. Species that don't remove sperm have evolved simpler sperm that tend to be smaller and lack hairs or bristles.
NATL ACAD. SCI.