 Bouncer is a molecule found on the surface of eggs that determines whether or not two different species of fish will be able to successfully fuse together during fertilization. In some cases, such as between Medaka and Zebrafish, Bouncer prevents these two species from being able to fuse together, while in other cases, such as between seahorses and fugus, it allows them to fuse together despite being from different species. This suggests that there are two opposing forces at play when it comes to Bouncer's role in species, specific fertilization, one that restricts fertilization to closely related species, and another that allows for more broadly compatible species. This article was authored by Krista R. B. Goet, Karen Panzer, Joachim Sirm, and others.