The Strange Life Cycle of the Jellyfish

They have been around for 700 million years by some estimates. With no brain, circulatory system, or lungs, jellyfish appear quite otherworldly. Not only do they seem strange on the outside, their life cycle is strange to match that.

Diver attaching a sensor on a jellyfish off the coast of northern Japan. AFP Photo/Yomiuri Shimbun
Diver attaching a sensor on a jellyfish off the coast of northern Japan.
AFP Photo/Yomiuri Shimbun

Let’s begin with an adult jellyfish. Similar to many other species, there must be a male and a female. The female, depending on the species, will either release eggs into the water or retain them on her surface. These eggs are then fertilized by a male jellyfish who releases sperm into the water. Not unlike many other species, however this is where things take a turn.

Jellyfish eggs
Jellyfish eggs

After an embryonic period the eggs burst forth with “planula larva,” tiny larvae. Using small hairs the planula larva make their way to the ocean floor, though much of their movement is disrupted by the ocean currents. Reaching the floor, they clamp themselves onto a hard surface in preparation for the polyp stage.

Planula larvae
Planula larvae

Growing upward, the planula larva transform into “polyps”. Polyps are not baby jellyfish. A closer, but simplified, comparison would be a “baby jellyfish plant”. Polyps gather nutrients using a few tentacles opposite their base, eventually developing creases, segmenting, and releasing what can actually be considered baby jellyfish, or ephyra. The ephyra flit away, growing into adult “medusa”, allowing for the cycle to begin again.

2mm tall polyps under a microscope
2mm tall polyps under a microscope

But wait. What happens to the polyps once the ephyra have left? They don’t actually go anywhere, but instead continue living on their perch continually eating. Many species will grow from a single polyp into a colony sharing a single stomach. They can live for years and, whenever the conditions are right, will release more jellyfish into the water throughout their life. Stranger still, the jellyfish Turritopsis dohrnii (now nicknamed “the immortal jellyfish”) has the strange ability to revert from the medusa stage back into the polyp stage, making them potentially immortal. Jellyfish are strange.

Left, a polyp has segmented and will soon release jellies. © David Wrobel / SeaPics.com
Left, a polyp has segmented and will soon release jellies.
© David Wrobel / SeaPics.com

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