There are unusual creatures that have inhabited our planet for at least 500 million years who spend a portion of their lives as plant-like polyps before transforming into some of the greatest ocean predators, all without having any brain, heart, backbone, eyes, ears or teeth. Cnidaria, otherwise known as jellyfish, are gelatinous animals found at all depths within all oceans around the world, and are some of the oldest animals evolutionarily speaking. Jellyfish are rather simplistic, anatomically speaking.
They are diploblastic, meaning that they have a body wall with two cell layers, an external epidermis and an internal gastro dermis, with some amount of mesoglea (non-cellular jelly) between these two layers. Jellies are made up of more than 95% water, however, they do “contain some structural proteins, muscle cells, and nerve cells, forming a kind of internal skeleton” (The Ocean Portal Team). These animals have no need for a stomach, but rather they have a gastro vascular cavity that is nothing more than a single opening, or mouth.
This mouth leads to a sac where the food is broken down, nutrients get absorbed though the gastro dermis, and then the waste is expelled back out the same mouth opening. Jellies are the “most primitive animals that exhibit extra-cellular digestion of the food within a gut cavity,” (Mote Marine Laboratory). This gut cavity is also responsible for the release of reproductive cells. Having no brain or a complete central nervous system, jellies instead have a “nerve net,” which is nothing more than a disjointed network of nerves that coordinates body movements as well as sensor receptors.
These receptors give jellies the ability to detect light, pick up vibrations, and allows them to smell and taste even though they have no nose or tongue. The “nerve net” also has a few specialized organs called statocysts that enable jellies to sense if they are facing up or down. Statocysts are located on the outside edge of a jelly’s body and are similar to the bones in the inner ear of humans, which help to maintain balance. When jellies are tilted, small stones within a fluid filled sac brush up against cilia relaying a message indicating which direction they are leaning.
Lungs are yet another set of organs missing from the jellies’ anatomy, as are gills. So rather than breathing air, or pulling oxygen from the water through a gill system, jellies absorb the oxygen they need through the ectoderm and endoderm. As anatomically simplistic of an animal, a jelly’s life cycle in comparison is quite complex, reproducing both sexually and asexually while transitioning between several stages and two distinct forms in a process often referred to as alternation of generations. Adult jellyfish, called medusas and are the common bell-shaped form, reproduce sexually by spawning.
Females release eggs from their mouths while males release sperm, and fertilization primarily takes place out in the water. There are few species however, where fertilization takes place within the female’s mouth, where the male deposits his sperm and once the eggs are fertilized they are then released from the mother’s mouth out into the water. The fertilized eggs develop into planulae, larvae-like forms that drift among the ocean current until they find a firm surface and attach themselves. These surfaces can run the gamut of rocks, floating debris, to the ocean floor.
Once at this stage, they are sessile, meaning that they cannot move. They do have tentacles and a mouth that points upwards, both of which are used to catch and feed upon plankton. During the polyp stage reproduction shifts from sexual to asexual through budding or binary fission. This is the process where polyps divide and develop into tiny disks, each of which is a baby jellyfish. These disks eventually break off and swim away to become full-grown medusas. The transformation from polyp to disk to medusa can take months or even years and once the edusa stage is reached, the life expectancy is about 1 year.
There is one species that is nearly immortal. Turritopsis nutricula is capable of “reverting back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation. This is the only animal known to do so,” (The Ocean Portal Team). Despite having such a primitive anatomy, jellies have evolved a uniquely potent defense and predatory mechanism. Cnidaria translates to “stinging creature” and it is this stinging ability that has fashioned this animal into one of the greatest ocean predators.
Jellyfish have tentacles that are armed with nematocysts, microscopic organelles that contain harpoon-like stinging structures filled with venom. Touch or a chemical cue triggers the reaction for the nematocysts to shoot out and embed itself and once embedded the toxic venom is released capable of stinging, stunning or killing its prey or enemy. Taking just 700 nanoseconds, it is one of the quickest reflexes throughout the animal kingdom. Calculations have been measured that give it the equivalent to 5,410,000 times gravity with an impact pressure in the same range as a gunshot at 7 Gigapascals.