Box jellyfish, the undisputed kings of the sting, are equipped with an especially nasty weapon in the fight to spread their genes: harpoon-armed sperm.
A new study, published in the Journal of Morphology, shows that a couple of species of jellyfish have sperm packages that are armed with barbs designed to anchor themselves to female sex organs and ensure reproductive success.
This sneaky sexual strategy can be found in at least two species of box jellyfish, Copula sivickisi and Tripedalia cystophora. Another recent study by members of the same team showed that the barbed sperm packages can be found in C. sivickisi, but researchers have now documented the phenomenon in T. cystophora too.
Marine biologists from the University of Copenhagen in Denmark collected a number of T. cystophora from their natural habitat around the shores of Puerto Rico and took a deep look at the creatures’ anatomy. They discovered that the sperm packages produced by the species are caked in cnidocytes. Typically, they are "explosive" cells used to harm their enemies, whether predator or prey, however, C. sivickisi and T. cystophora have cnidocytes on their sperm packages.
Most cnidarians, the phylum of animals that includes jellyfish and corals, don’t reproduce through sex in the same way as most land animals. Instead, many of these sea-dwelling species spurt their sex cells into the surrounding water, hoping it will get picked up by members of the opposite sex.
These box jellyfish, however, do things a little bit differently. The males package their sperm and store it near a cavity used to digest food. If they come across a female they like the look of, they “hand over” the packages using their tentacles. Once they reach the female, it’s thought the sperm-containing packets shoot out their harpoons and anchor themselves to the female's gonads, awaiting fertilization. This hypothesis is supported by the discovery of a barbed sperm package in the gonads of one of the female jellyfish.
“We have examined the gonads and gametes of T. cystophora and our results reveal that the male-produced spermatozeugmata have a high number of… cnidocytes, which are transferred along with the sperm during copulation,” the study authors write. “This adds further support to our hypothesis that they are important for sperm anchorage.”
[H/T: New Scientist]