Jellyfish range in size from as tiny as a pinhead to larger than a human with diets that vary accordingly. Jellyfish feed on phytoplankton, copepods, fish eggs, larvae and even other jellyfish. They form large swarms known as blooms that eat almost everything in the water around them. Most jellyfish species passively hunt and eat whatever tangles in their tentacles, but some swim and chase their prey.
Found in both fresh and saltwater and in every ocean at various temperatures and depths, jellyfish have evolved a highly sensitive nervous system designed to help them hunt. They have a simple digestive system with a gastrovascular cavity and an opening that functions as the mouth. Jellyfish are largely carnivorous, their tentacles lined with stinging capsules or venom that stuns their prey. The arms around the opening catch and bring their prey to the jellyfish's mouth, and the digestive process begins almost immediately. Jellyfish excrete waste through this opening as well. Some species are aggressive and have potent venom in their tentacles that paralyzes or kills their prey. The box jellyfish in particular is an excellent and swift swimmer. Other species are filter feeders, consuming seawater and whatever edible items it contains, including zooplankton, eggs and larvae.