Jellyfish are commonly known as “jellies” and are some of the most fascinating underwater invertebrates you will ever set eyes on. Translucent, appearing to be like actual “ghosts” of the ocean, they have some of the most uncommon anatomical traits that allow them to survive quite well without any brains, skeletal structures, eyes or even a heart. A non-polyp member of the phylum Cnidaria, jellyfish species feature many more special qualities, from some species being able to actually glow in the dark, to their incomparable ability to clone themselves, completely re-growing missing parts of their bodies in a relatively short amount of time.
Most types of jellyfish are able to survive without digestive, circulatory, respiratory or central nervous systems. Most species feature a bell-shaped membrane that accounts for the jellyfish’s body, ending in tentacles and sting arms used for capturing prey. Jellyfish have a single orifice for feeding and excretion, while digestion occurs in a gastrovascular cavity where food is broken down and nutrients are absorbed. Their skin is thin enough that the entire body can be oxygenated merely by diffusion, and jellyfish use a hydrostatic skeleton and a unique means of locomotion to find their way through the ocean without expending much energy.
Jellyfish can range in size from about one millimeter to up to two meters. Their sting arms are their main weapons, featuring tiny cells that can excrete a potent venom, paralyzing its victims. The venom can, in fact, be very painful and even fatal to humans. At the same time, jellyfish are quite well-known for their ability to reproduce asexually by simply splitting themselves in two, and the smallest members of the species are so tiny that they cannot be seen in the field without the use of a hand lens. At the other end of the spectrum, Nomura’s Jellyfish is a surprisingly common species found in the waters of China and Japan, that can grow to abnormally large sizes, and may weigh up to 150 kilograms.
Although jellyfish don’t actually use a complete nervous system, they have what is called a “nerve net.” This system of nerves is located in the epidermis, and forms a network of nerves that is responsible for the detection of a variety of stimuli, such as the touch of other underwater animals. Electrical impulses are transmitted across the net around a nerve ring shaped in the form of a circle, making their way through the jellyfish’s body.
Finally, jellyfish handle vision far differently when compared with most other animals. They don’t have actual eyes, but some species feature organs known as ocelli – which can detect changes in light and discern orientation, telling the jellyfish which way is up or down. A few species, such as box jellyfish, on the other hand, have up to 24 eyes, two of which can see in color. Their placement makes these special jellyfish one of the only creatures in the world that are capable of 360 degree vision.