Biology Letters

During the early summer, corals simultaneously release tiny balls composed of sperm and eggs, known as bundles, that float to the ocean surface. Here the bundles open, allowing the sperm to fertilize the eggs where they eventually settle on the seafloor and become new coral on the reef.

In a group of bark lice, a penis has evolved twice – in the females. In their nutrient-scarce environment, “seminal gifts” are an incentive for females to force mating, leading to the co-evolution of female penises and male vaginas.