What creatures are there that lurk in the depths of the ocean that are poisonous to humans? Some of the beautiful creatures of the ocean should only be admired from a distance. The poison of the conidae (cone shell) and the blue-ringed octopus are extremely toxic to humans and other animals.
, or cone shells, are more than just pretty seashells, they are homes for marine snails. When walking along a beach to pick up the seashells, one needs to be careful. The marine snails that reside in these have extremely poisonous venom. Just one drop of the venom from the marbled cone snail is strong enough to kill twenty humans. If stung by one, a person could be dead in minutes, and there is no antivenom available.
If stung, numbness is almost immediate. Then the vision will blur, dizziness will be experienced, as will tightness in the chest and painful breathing. The one stung will possibly drift into unconsciousness, then a coma. Death usually occurs from respiratory arrest. This would not be a pretty way to die.
Your murderer could work with sea animals, and figure out how to extract the poison from cone shells. He (she) could kill the intended victim when the victim came to see him at work. How would he get away with it? Or maybe he would only almost get away with it. Well, he would inject the intended victim with the poison, then lay a cone shell near him. That way it would appear the victim had been stung.
Why does he want to kill this person? That is for you to decide, but this person could be bribing him. Maybe he came to see him at work to pick up his payment and received more than he bargained for.
Or maybe the murderer would be stung by an angry marbled cone snail when trying to extract the venom. Then it would appear to be a murder, but it would be more of an unintentional suicide. Possibly his death would be blamed on the one who was bribing him.
Have you ever seen Jurassic Park
? I have, but I didn't remember this. Cone shell poison was mentioned in this movie. It was in the darts used to kill the dinosaurs.
The blue-ringed octopus
is only about the size of a golf ball, but extremely deadly. As a matter of fact, it is the most toxic sea creature known to humans, and is recognized as one of the world's most venomous animals. Just one bite is deadly; it kills within a matter of minutes. But there is an antidote, right? Nope. Once bitten, you are dead unless you go through hours of heart massage and artificial respiration until the poison has had time to work its way out of your system. So if you are bitten or injected with this poison, it would be best if you were right next to a hospital.
Can the poison be extracted and injected into an intended victim? I honestly don't know, but for a fiction story, it would be an awesome murder weapon. And you do have the literary freedom to make it so.
Or your murderer could even keep one as a pet. He (she) could throw it on the intended victim. At first the victim would feel nauseous. His vision would become hazy, then he would go completely blind. His sense of touch would be gone, and he would not be able to speak or swallow. Only three minutes later, he would be paralyzed and unable to breathe.
The intended victim could also be shoved into a tank filled with blue-ringed octopuses. This would be especially effective if the victim couldn't swim, or if he were drunk or drugged.
Someone in your story would have to be associated with an aquarium or marine biology in some way for this type of poison to be used. I don't think it would come across as very believable if your antagonist just happened to stumble across it and knew what it was.
Before using the poison from either of these sea creatures as your murder weapon, you may want to research them a bit more deeply. While you are in the midst of your research, your muse may give you your entire story.