Marine Biology

Lifecycle of the Box Jellyfish



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The box jellyfish, or cubozan, is the perfect killing machine; it is so loaded with toxin and can kill a human being more quickly and easily than any other creature on earth. The most fleeting touch brings an agonising death in less than 3 minutes. The box jellyfish can even release its poison after it is dead. There are almost 30 species of box jellyfish

The powerful venom evolved so that the jelly fish could kill or stun its prey, such as shrimp and fish, to prevent them from struggling to escape and damaging the delicate tentacles. The poison attacks the heart, skin cells and nervous system. The pain to humans is so enormous that people can go into shock and drown or die from heart failure before they can get to the beach. If somebody does manage to survive the sting, then they can be in great pain for weeks afterwards and major scars occur where the tentacle touched.

The name comes from the bell or cube shape of the box jellyfish which has four distinct sides and can measure up to 20 cm on each side. They are pale blue in colour and transparent with up to 15 tentacles at each corner measuring up to 3 metres in length. Each tentacle can have up to 5,000 stinging cells called nematocysts.

Box jellyfish don’t just drift along like most other jellyfish; they can move themselves at up to four knots in a jetting movement. They have a 24 eyes grouped in clusters of six on each side of the bell giving them the ability to see all round them. In each cluster there is a pair of eyes, complete with moveable pupils, lens, iris, cornea and retina.  There eyes can detect colour but scientists don’t know how they can process what the see without a central nervous system.

Lifecycle

The lifecycle of very few of the box jellyfish species is known. The adult jellyfish is called the medusa and this is one of the two distinct phases in the lifecycle of the box jellyfish. The medusa is the sexual stage; the other stage is the asexual polyp. In box jellyfish, the medusa stage takes place in a coastal marine environment whilst the polyp stage takes place in an estuary environment.

The adult jellyfish produce eggs and sperm. The male jellyfish releases sperm into the water from its mouth from where they are swept into the female jellyfish’s mouth, where the fertilisation takes place. The reproductive organs are in the lining of the jellyfish’s gut. The larvae develop inside the female or in her brood pouches then leave either via her mouth or brood pouches. The planula swims away until it finds a suitable place and then finds itself a good rock or shell on the seafloor to settle on and develops into a larval-stage polyp. Polyps feed on microscopic particles, plankton, in the water using tentacles. A polyp can reproduce asexually, letting new polyps grow from itself and then detach. The metamorphosis into a medusa takes place when the polyp is fully grown and in response to certain environmental stimuli. Firstly, strobia develop as constrictions on the polyp and look like a stack of saucers. These develop into tiny medusa then bud off the stack. The medusas are now miniature adult jellyfish. The medusa then goes back to the coast to grow. It is thought that the box jellyfish polyp lives longer than other jellyfish polyps, and could even have an indefinite lifespan, whilst adult jellyfish only live for a few months.

If box jellyfish come under sustained attack, they go into a hyper-breeding frenzy and produce millions more jellyfish to replace themselves and increase their numbers.

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