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Jellyfish and their relatives are famous for their painful and dangerous stings, which are delivered by special cells called cnidocytes. So readers may wince at the fact that two species of box jellyfish package cnidocytes with their sperm.

However, the cnidocytes are not there to cause harm. Instead, they use sharp barbs to anchor the sperm inside the female鈥檚 gonads.

Box jellyfish belong to a group called cnidarians, which also includes corals, all of which have cnidocytes. Most cnidarians just release sperm and eggs into the water and trust that they will meet. But two box jellyfish, Copula sivickisi and Tripedalia cystophora, copulate 鈥� so the eggs are fertilised inside the female.

Stinging sperm

, Anders Garm of the University of Copenhagen in Denmark and his colleagues showed that the sperm-containing packets released by聽C. sivickisi have cnidocytes. What鈥檚 more, there are cnidocytes in the female鈥檚 sex organs. The sperm cnidocytes anchor the sperm inside the female. After fertilisation, the female lays the eggs in a long, sticky 鈥渆mbryo strand鈥�, which she packs with cnidocytes to protect it from predators.

The sperm cnidocytes should not hurt the female, says Garm. 鈥淭he cnidocytes are of a type without the penetrating arrow and without poison.鈥�

With Sandra Helmark, Garm has now studied the sex cells of T. cystophora. Again, they found cnidocytes in the sperm-containing packets that the males produce. In this species, however, the female sex organs don鈥檛 carry stinging cells.

Garm says this reflects their different reproductive strategies. Female T. cystophora keep the fertilised eggs inside their bodies until they have developed. That means the developing young do not need protective cnidocytes.

Rare matings

What underlies this difference is that T. cystophora mates rarely, so the females take more care of each set of offspring. In contrast, C. sivickisi is more prolific, rapidly releasing a set of fertilised eggs and mating again within days. 鈥淭he two strategies each have their advantages,鈥� says Garm.

Sex organ cnidocytes seem to be unique to these two species, says Garm. C. sivickisi and T. cystophora are only the cnidarians known to mate, making them the only ones likely to have cnidocytes in sperm. 鈥淔emale gonadal cnidocytes would be predicted if the female would lay something similar to the embryo strand 鈥� but again we know of no other species where this is the case,鈥� he says.