Only a very few molluscs living on the GBR are deadly to humans, and they cause harm accidentally by defending themselves.
The blue-ringed octopus, Hapalochlaena lunulata, has particularly potent saliva that quickly paralyses its prey (normally crustaceans). One component of the saliva is tetrodotoxin, which is produced by bacteria in the salivary glands, and to which the human nervous system is particularly susceptible. This chemical prevents messages coming from the brain from reaching the muscles, so that the human victim is paralysed. Interestingly, tetrodotoxin only paralyses voluntary muscles and involuntary muscles, such as the heart, iris of the eye and the gut, continue to function normally. The victim scarcely feels the bite of a blue-ringed octopus, but after a short time he has difficulty in breathing. This is followed by nausea and vomiting, complete cessation of breathing and collapse. The victim remains fully conscious and may die from lack of oxygen. There have been no deaths from blue-ringed octopuses on the GBR, but in 1954 a man in Darwin died within two hours of being bitten by one.
A victim definitely feels a sharp pain if he has been stung by a cone snail. There are actually more than 100 species of cone snails (Conus) in all habitats on the GBR, but only three of them living among live coral and coral rubble (Conus geographus, C. tulipa and C. obscura) are capable of killing humans. These cone snails are all fish-eating (piscivorous) species and have an enlarged aperture to the shell into which the dead fish can be hauled for digestion (Fig. 24.4). The sting is caused by the piercing of the skin by a single radular tooth resembling a tiny harpoon. The tooth, which is shot out forcefully from the tip of a long flexible proboscis, injects a powerful neurotoxin (con-toxin) as it is driven into the flesh of the prey. Cone snails have an elaborate venom apparatus consisting of a muscular bulb and a tubular secretory duct opening into the mouth cavity. A modified proboscis that is tubular and muscular like an elephant's trunk (Fig. 24.4) actually fires out this radular tooth. A human victim of a sting from a cone snail can lose vision, or have hearing or speech affected and he may become partially or completely paralysed within half an hour if he has been unlucky enough to have been stung by one of the deadly species. Indeed, there is one recorded death in 1935 of a man on Hayman Island from the sting of a Conus geographus. A sting by a species of cone snail other than these three may cause pain, swelling and discolouration of the area near the puncture, but not death. So one should walk carefully over the reef and not pick up a cone shell, even if it appears to be empty, because its animal may be fully retracted within the shell.
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