Life Cycle of a Box Jellyfish

Few sea creatures inspire as much fear as the infamous box jellyfish, whose venom is recognised as one of the most toxic substances in the world. Fortunately, their main habitat is confined to a relatively small part of the Indian and Pacific Ocean, mainly off the northern coast of Australia and up towards Indonesia and the Philippines, but they have been found as far afield as Hawaii and California. There are some 50 sub-species of box jellyfish, but the most lethal is the largest of them – the Australian Box Jellyfish (Chironex fleckeri).

Their lethal tentacles can trail for over ten feet from the cube-shaped bell that gives the species its name (its formal taxonomic classification is Cubozoa). They are highly advanced for their species, with the ability to see and react to their surroundings despite having no apparent central nervous system. Their pale blue colour and transparency make them especially difficult to spot in water, and, unlike “true” jellyfish, they have the ability to swim, rather than drift with the currents.

Given their danger to humans, various attempts have been made to cull or control their rising numbers, usually with spectacularly dismal results. A concerted attack by the Japanese on the species to protect their fishing stocks triggered an emergency spawning response, resulting in females producing much more offspring than usual – accelerating their numbers rather than killing them. As such, understanding how they live and breed is of particular interest to the scientific community. However, their mobility and “sight” makes them especially difficult to study in the wild.

Using the Australian Box Jellyfish (also known as a sea wasp, or marine stinger) as an example, the spawning season occurs once a year in late summer (February in the Southern Hemisphere) usually near river mouths and estuaries on the Northern Australian coast. The way eggs are fertilised amongst Cubozoans can vary. In some cases the male releases sperm into the bell of the female, but in others, the female releases eggs into the open water and the male fertilises them. The fertilised eggs take on a larval form before drifting down to, and developing into polyps, on the sea floor.

At this stage, they are mostly stationary tube-like creatures with tentacles and mouth on top with a limited ability to move around. After a few months of feeding, the polyp undergoes a metamorphosis,  detaches from the sea floor, and swims away as a small version of the adult jellyfish. They live for less than a year in the wild and are quite delicate, which is why they developed such a toxic defense mechanism.

Their tentacles instantly stun their prey, giving them the ability to kill large animals without any damage to themselves. However, certain species of sea turtle are not only immune to the venom, but are also one of its few natural predators. It appears that the dwindling numbers of such turtles due to human interference (especially the endangered and ancient Green Turtle) is another reason for increasing Cubozoan populations worldwide.


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