The box jellyfish: A dangerous predator with a mysterious life cycle

With stinging tentacles up to three meters long, and with a transparent body which is nearly impossible to see in the water, the tropical box jellyfish, boxfish or sea wasp is the largest and most venomous member of the lethal box jellyfish family.

Although some people survive box jellyfish attacks, even the slightest amount of box jelly venom in the bloodstream can prove fatal, and it is believed that dozens of people are killed by jellyfish stings each year. In fact, the U.S. National Science Foundation estimates that in the Philipipines jellyfish kill 20 to 40 people each year.

Although scientists are actively studying box jellyfish in order to develop effective anti venom, they still do not know a great deal about the life cycles of these potentially fatal creatures. For example, the life cycle of one species of box jellyfish, Carybdea fleckeri, was only established in the early 1980s.

The life cycle of the box jelly appears to be similar to that of the true jellyfish or Scyphozoa. The life cycle of the true jellyfish consists of two separate and distinct phases: a small polyp which lives attached to a permanent location on the ocean floor, and a large adult free-floating jellyfish or medusa form.

This was recently confirmed by Smithsonian researcher Cheryl Lewis Ames. When Professor Ames reared box jelly embryos in the laboratory, she observed that they developed into cylindrical polyps. A polyp is a small organism which lives on the ocean floor.  It has a mouth and tentacles at one end, and a suction-cup-like disk known as a holdfast at the other, which it uses to attach itself to rocks or coral.

Ames therefore concluded that, like other jellyfish, box jelly embryos swim down to the ocean floor, develop into polyps, attach themselves to rocks with their holdfasts, and feed on plankton which they catch with their tentacles. She also theorized that if the box jelly polyp grows and develops like those of the true jellyfish, it probably reproduces asexually by growing buds which will grow into new polyps and then break away from the parent. Ultimately, however, the polyp presumably metamorphoses into a miniature medusa and returns to the surface where it grows into a full-sized adult jellyfish, and then produces new embryos through sexual reproduction.

When true jellyfish reproduce sexually, they release sperm and eggs into the water for fertilization. However, Professor Ames discovered from laboratory observation that at least one species of box jelly, Carybdea sivickisi, engages in a fascinating courtship ritual. The male draws a female towards him by holding one of her tentacles, deposits a red package of sperm onto her tentacle, and then releases her. The female subsequently swallows the sperm package in order to fertilize her eggs. A few days later she produces embryos which subsequently develop into polyps.

However, it is not yet known if other box jellyfish engage in courtship rituals similar to C. sivickisi. The life cycles of other species of box jellies remain a mystery, and there is much scientific research that remains to be done.