Some box jellyfish display elaborate mating behaviours and even use their toxic stinging cells to ensure successful fertilization.
Many jellyfish reproduce using external fertilization, but in a few box jellyfish, fertilization can occur internally. In one species (Copula sivickisi; pictured), the male transfers a sperm package into the female's stomach to fertilize the eggs after the animals entangle their tentacles. The females then lay strands of embryos.
Anders Garm and his team at the University of Copenhagen studied the sex organs of the jellyfish under a microscope and located the stinging cells in the sperm package and in the female gonad. The sperm package becomes attached to the female gonad and sperm cells are partly digested, releasing their nuclei, which are then taken up by the female sex organ. The stinging cells probably help the sperm package to attach, and protect the embryos once they are laid.