J is for Jellies

Spineless is no insult when it comes to jellies. With slow-pulsing bells and trailing tentacles, these boneless, brainless wonders are mesmerizing.

Jellies are one of the most prolific marine animals, found from the shallows to the deep and in every ocean across the globe. And they have inhabited those oceans for more than 500 million years.

Like their name suggests, jellies have soft, gelatinous bodies. They are, after all, 95 percent water. Tentacles with powerful stinging cells extend from their round bells, which they use to evade predators or stun prey.

Most jellies lack a true digestive system. They move food into a mouth opening at the base of their bell into a gut where it is digested. The resulting waste exits through the same opening.

Some jellies, like the blue-blubber jelly, are small with short, stubby tentacles. Others are giants with thin, trailing tentacles. A large lion's mane jelly's tentacles can measure about 120 feet. It is not only their shape and size that vary. Even more enticing is the variety of colors and patterns they exhibit, from bright pink and striped purple to vibrant yellow, spotted blue and nearly translucent. Some species even glow in the dark.

Jellies move by pulsing their round bells. Some actively swim through the water, and others let the ocean currents carry them away. However, jellies aren't always free-floating—they live a significant portion of their lives attached to a substrate, where they resemble sea anemones. After fertilization, a jelly's egg begins to grow and it attaches to a hard surface. There, it develops into a polyp with a mouth and tentacles. It's after this polyp stage that the free-floating jellies we know and love appear. Those adult jellies are known as medusae.

Most jellies are short lived, but one particular species—nicknamed the immortal jelly—has managed to dodge death. Through a process called transdifferentiation, it can regress from a medusa back into a polyp beginning its lifecycle anew.