Jellyfish the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. Meanwhile, the second vortex ring starts to spin faster, sucking water into the bell and pushing against the centre of the body, giving a secondary and "free" boost forward. The mechanism, called passive energy recapture, only works in relatively small jellyfish moving at low speeds, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (the amount
Box jellyfish from that of scyphozoans, and is used to catch prey (small fish and invertebrates, including prawns and bait fish) and for defence from predators, which include the butterfish, batfish, rabbitfish, crabs (blue swimmer crab) and various species of turtle including the hawksbill sea turtle and flatback sea turtle. It seems that sea turtles are unaffected by the stings because they seem to relish box jellyfish. Although the box jellyfish has been called "the world's most venomous creature", only a few species in the class have been confirmed to be involved in human deaths, and some species pose no serious threat