Although ‘box jellyfish’ denotes the entire Cubozoa class, the most dangerous of them all is the deadly Chironex fleckeri. The venom of this species is the most deadly in the animal kingdom and is reported to be able to kill 60 humans within a period of 3 minutes.
2. Yellow Bellied Sea Snake (Pelamis platurus)
The venom of this dangerous sea snake is several times more potent and deadly than the most venomous land snake. The bite itself is painless and may even go unnoticed by the victim. Symptoms start showing around after half an hour of the bite, and if immediate medical attention is not received, the victim goes into cardiac arrest after six to twelve hours.
3. Saltwater crocodile
Crocodiles are territorial and they do not like intruders. They have been known to attack small boats and canoes and result in human deaths. Though you may argue that this is not exactly an aquatic animal, it hardly matters when you are been attacked by in water.
4. Great White Shark (Carcharodon carcharias)
All of us are familiar with the Great White – the popular star of the movie “Jaws”. Great Whites are expert hunters and know how to immobilize prey and render them helpless while they finish them off. There are reported incidents of Great Whites being responsible for the deaths of innocent swimmers off the coast of USA.
5. Blue-Ringed Octopus
This common name refers to several octopus species of the genus Hapalochlaena – small and beautiful octopuses that have a venom powerful enough to kill humans.
6. Cone Shells
Belonging to the family Conidae, these gastropods live in mud and show very little signs of life. However, if picked up (because of their bright colors) they can bit with their harpoon like teeth and cause death. Their venom contains neurotoxins and is fatal.
7. Portuguese Man-of-War (Physalia physalis)
It is commonly mistaken to be a jellyfish because of its striking resemblance to them. Although its venom is not very potent, it does have the power to kill off the victim by cardiac arrest if instant medical attention is not received. Deaths caused by stings from this animal are very rare but they do occur.
8. Lionfish (Pterois volitans)
This is a small, spiny, striped, exotic creature found in some regions of eastern Atlantic, the Mediterranean and the Caribbean. Considered venomous, the lionfish has a typical way of turning upside down and exposing its spines. Although its venom is not fatal, it causes severe pain, headaches, and vomiting – unless medical supervision is received, the sting can take serious proportions.
9. Stonefish (Synanceia)
The genus Synanceia is popularly called stonefish, and consist of several species of small, shy, and sometimes very beautiful fish that are at the same time capable of stinging you with deadly venom if disturbed. Typically, stonefish stings occur because they are stepped upon, as they are hard to distinguish from stones under the water.
10. Fire Coral
These are marine organisms that are not actually coral and are typically members of the genus Millepora. Their stings are not fatal but can cause severe burns on the skin. It can become fatal if the victim develops an allergic reaction to the chemical injected in the sting.