Beautiful but Deadly Blue Ringed Octopus

This very small octopus (reaching up to 100 g and 20 cm across spread tentacles) can be found from Japan to Australia in shallow tropical water and in tide pools, where it hunts for crabs (its main pray). It is normally yellowish brown but when disturbed, the octopus changes its rings to a bright blue thus showing its change of mood.

The venom of the blue-ringed octopus is contained in its saliva, which is produced by a couple of glands that have the same size as the brain of the animal. The venom has two components. One of them is probably most effective on crabs but relatively harmless to humans. The other one is the same as the toxin present in the puffer fish (tetrodotoxin) and probably serves as a defense against predatory fish. The bite is usually painless but this octopus has neuromuscular paralyzing venom. It is not injected but enters the wound in saliva. People soon notice numbness around the mouth followed by paralysis.

Death results from respiratory failure. The victim might be saved if artificial respiration starts before marked cyanosis (heart condition causing blueness of the body surface) and hypotention develop. 

Since the blue-ringed octopus is typically found in sheltered rock pools and crevices, one should be very careful not to put one's hands where they cannot see.First aid for blue-ringed octopus bites 
The venom contains tetrodotoxin, which blocks sodium channels and causes motor paralysis and occasionally respiratory failure. Though with fixed dilated pupils, the senses of the patients are often intact. The victims are aware but unable to respond.
Pressure-immobilization is a recommended first aid. Prolonged artificial respiration may also be required. Envenomations may require supportive treatment including mechanical ventilation until the effects of the envenomation disappear. There is no antivenom available in Australia.