If you are looking for an animal to take the title of "most violent fish in the sea", then the tiger puffer   fish would have to be a strong contestant. Not only is it deadly poisonous --- though that doesn't stop people trying to eat it --- but it is also able to scare off enemies by inflating itself to become much larger than normal, when it is young it even chews on its own brothers and sisters. Tiger puffer fish attach their eggs to rocks near the bottom of the sea, often at the mouths of bays. Then the larvae   move to the entries of rivers and lakes once they have grown a little. Then, having put on a lot of weight, they head out to sea. There's no innocent childhood for the puffer fish, as Shin Oikawa of Kyushu University in Japan and his colleagues found out when they put the larvae of tiger puffer fish in the lab and monitored them for two months. They found that the larvae went through three steps in which their metabolic  rates increased dramatically when they reached body weights of 0.002g, 0.01g, and 0.1g. When a larva went through one of these steps, its behavior also changed. For instance, once a larva had passed the first level it would have grown its first tooth and could start attacking larvae that had not yet reached that stage. Similarly, any larva that had reached the 0.01g or 0.1g levels would start attacking lighter larvae. The researchers noted that the baby fish had a "relatively small mouth", so rather than swallowing their brothers and sisters whole, they would bite pieces out of them. Despite this limitation, the fish caused plenty of deaths --- up to 12 per cent of the deaths that happened in the lab each day. Those fish that grew fast enough to be able to chew on their fellows had an advantage. The extra food accelerated their growth and development. Tiger puffer fish are likely to be faster and swifter, so they can deal better with enemies. As the name suggests, puffer fish can inflate to make themselves seem much larger than they really are, thus scaring off enemies. They do this by filling their stomachs, which are extremely elastic , with water. If that's not enough of a threat, the tiger puffer fish --- like most of the other puffer fish in the family --- carries a deadly toxin . Eat one puffer fish and the poison will paralyze   your muscles, including the muscles responsible for breathing, so death is usually caused by a lack of oxygen. Famously, the fish is a delicacy in Japan, where highly qualified chefs produce dishes that contain the safe level of the poison. Interestingly, the puffer fish does not go to the trouble of producing the poison itself. Instead, it hosts bacteria   that produce the stuff. It obtains these bacteria from its diet, so the youngest adult fish are not poisonous. Where does the poison in the puffer fish's body come from?
Its inner organs. The air it breathes in. The diet it eats. The bacteria around it.