Jellyfish are simplistic animals that don’t seem to have a mouth upon first glance. They also don’t have eyes, so they can’t see where they’re going, or the animals they’re trying to eat.
So, how do jellyfish eat? How do they catch their prey? Do jellyfish have mouths?
Despite being simple animals, jellyfish have specialized tentacles with stinging cells to help them catch prey. They have mouths, but their digestive system is more elementary than that of most animals.
What Are Jellyfish?
Jellyfish are gelatinous animals that free float through the ocean. “True” jellies, or scyphozoans, are part of the phylum cnidaria. They have cnidocytes and nematocysts, specialized stingers within their tentacles. They use the cells to capture prey and defend themselves.
There are other animals that we refer to as “jellies” that aren’t real jellyfish. Such animals include comb jellies and box jellies. Comb jellies don’t have nematocysts, so they aren’t “true” jellies. Then, there are hydrozoans who do have nematocysts, but still aren’t “true” jellies.
Box jellyfish are cnidarians, but they have special characteristics that make some researchers deny they are “true” jellyfish. Unlike most jellyfish, box jellyfish can actively swim rather than float. They also have eyes that allow them to see. Some researchers believe that box jellyfish actively hunt their prey, unlike other jellyfish who are opportunistic.
A jellyfish’s body comprises 95% water, so they are weak swimmers and free float throughout the ocean. Three layers cover the rest of their body. The outer layer is the epidermis, and the innermost layer is the gastrodermis. The middle layer is the mesoglea, which comprises thick, gelatinous material.
Jellyfish also have a nerve net, an elementary nervous system that allows them to sense light, scent, and react to other stimuli.
How Do Jellyfish Eat?
Jellyfishes use their stinging nematocysts and cnidocytes to catch their prey. Cnidocytes are explosive cells that blow up when touched. Cnidocytes contain nematocysts, which are specialized cells that contain a barb and venom.
As they swim, a jellyfish’s tentacles brush up against objects, and the cnidocytes burst. A miniscule, razor sharp barb — the nematocyst — explodes from within and pierces the animal the jellyfish touched. Once the barb pierces the animal, venom rushes into their body.
The venom from the nematocysts is strong enough to stun the animal, giving the jellyfish enough time to wrap their oral arms around their prey and immobilize them.
Oral arms differ from tentacles because jellyfish only use them to capture and eat prey. A jellyfish uses their oral arms to guide their prey to their mouth so they can begin eating.
Their Digestive System
The mouth of a jellyfish sits at the center of its bell, and the stomach sits at the opening of the mouth. Jellyfish don’t have teeth, so they swallow their food whole.
The stomach is a set of open chambers filled with enzymes that digest the prey. Throughout the body are canals that connect to the stomach. Food travels through these canals, providing nutrients to the rest of the body.
After the body finishes absorbing the nutrients from the food, the leftover waste leaves the body. Jellyfish only have a single opening that acts as both the mouth and the anus, so food leaves the same way it came.
What Do Jellyfish Eat?
Jellyfish are carnivores who eat a variety of zooplankton like amphipods, copepods, and krill. Plankton exist everywhere across the world’s oceans, so they are readily available for jellyfish to consume.
Plankton needs sunlight to survive, so they live near the ocean’s surface. A jellyfish merely needs to drift near the surface to catch plankton in its tentacles. Though they are microscopic, plankton are an excellent source of nutrients for jellyfish.
Larger jellyfish enjoy crustaceans like crabs, lobsters, and shrimp. Fish are an excellent source of protein and nutrients, so jellyfish often drift about areas where small fish swim. They also search for fish dens for their eggs.
Jellyfish are cannibalistic as well. If a jelly comes across a smaller jellyfish, or jellyfish larva, they will not hesitate to consume them. Jellyfish comprise 95% water, so their bodies offer little nutritional value. It’s rare that jellyfish will eat other jellyfish unless other food sources are scarce.
Where Do Jellyfish Live?
There are over 2,000 species of jellyfish around the world. Depending on the species, jellyfish may live in warm or cold water, shallow or deep. Some live in the open ocean, while others live closer to the coastline. There are even deep-sea-dwelling species who live thousands of miles/kilometers below the ocean’s surface.
Conclusion
Jellyfish have cnidocytes, explosive cells that burst when the jellyfish’s tentacles touch its potential prey. The cnidocytes release nematocysts, toxic barbs that release venom into the skin. The venom stuns the prey until the jellyfish immobilizes them with their oral arms.
Jellyfish swallow their prey whole, and enzymes digest the food within the stomach. The waste leaves out the same opening it entered.
Jellyfish are simple animals who lack a heart and brain, and their simple digestive system reflects that.
FAQs
We’ve given jellyfish the label, “fish”, but they aren’t fish at all. Fish are vertebrates with gills, while jellyfish are invertebrates.
How many tentacles a jellyfish has depends on the species. Some only have a few tentacles, while others have hundreds.
Jellyfish do not have teeth and don’t chew their food. Instead, they swallow their food whole, and the enzymes in their stomach digest it.
The Ocean Conservancy says, “Jellyfish don’t feel pain in the same way that humans would. They do not possess a brain, heart, bones or a respiratory system. They are 95% water and contain only a basic network of neurons that allow them to sense their environment.”
The venom in nematocysts can be excruciatingly painful if you’re stung, but most aren’t very dangerous. Only about 2% of jellyfish stings are harmful to humans.
Once a nematocyst enters the skin, it can enter the bloodstream and cause a spike in blood pressure. In most cases, the venom is not strong enough to do lasting damage. The box jellyfish is one exception, causing 20-40 fatalities a year in the Philippines. There are about 50 species of box jellyfish, but only a few of these jellyfish are strong enough to be fatal.
The Australian box jellyfish (Chironex fleckeri) is the most dangerous jellyfish and the most venomous marine animal in the world.
Their toxins attack the nervous system and the heart, causing people to go into cardiac arrest. The sting is also excruciating to where people often go into shock and drown before being able to reach help.
Peeing on a jellyfish sting is a widespread myth that really offers no help. Cleaning a jellyfish burn with freshwater may increase the pain of the burn. Urine contains freshwater, so it won’t help.
Where did people get the idea that peeing on a jellyfish sting may relieve pain?
At some point, people theorized that the ammonia and other compounds in urine might neutralize the venom in jellyfish stings.
In reality, freshwater causes more nematocysts to fire, releasing more venom into your skin and causing more pain. Rubbing your skin can make nematocysts fire too.
Instead, researchers recommend washing away any remaining tentacles with seawater. Emergency medicine specialist Dr. Thomas Waters says you should also remove barbs from your skin with tweezers, and use vinegar or rubbing alcohol to soothe your pain.
