These particular Asian clams are hermaphrodites. Along with eggs, they also make sperm--but these are odd sperm, with a full supply of DNA. Once their sperm has fertilized the egg of another clam, the female DNA is ejected, and the clam embryo starts to develop. The mother clam broods father's clones in her gills, where they can probably thrive on the food that gets trapped there.
Molluscs ... is there no end to the weirdness?
