Medusa is one of the most fascinating and bizarre sights on the planet. With their beautiful luminescent glow and strange bell-shaped body, they don’t look like animals at all, but some kind of strange alien life. However, they share many qualities with other animals, including a symmetrical body plan and the ability to use oxygen. All 200 existing species of true jellyfish, as well as all corals and anemones, belong to the Cnidaria type (for comparison, all known vertebrates also belong to the same type).
Although all adult jellyfish have the same symmetrical bell-shaped body, they do have a huge variety in the size, color, number, and shape of their tentacles. They can be found in almost every marine environment on the planet; only a few of them are known to inhabit exclusively freshwater ecosystems.
The first real jellyfish appeared about 600 million years ago (the date is inaccurate, since their soft parts are poorly fossilized). This makes it representative of the earliest forms of polyorganic animal life on the planet. Composed of more than 95% water (by comparison, humans are 60% water), they have relatively simple bodies with no respiratory or circulatory system and only basic digestive and neurological functions.
When they first developed, their diet was more limited to protozoa floating in water. Today they feed on a huge variety of animals and plants in the oceans. The composition of the diet can affect the color of the jellyfish, making it pink, purple, or red through different pigments.
Despite their simplicity, jellyfish are actually some of the most effective swimmers in the ocean, perhaps because so little jellyfish mass is actually being devoted to this process. They are driven by the expanding and contracting movement of the bell, which sprays water in the opposite direction to their direction. How the relatively simple digestive system of the jellyfish is used to produce energy is actually a fascinating topic that we’ll talk about in this article. It’s also a very fascinating study of how the digestive system may have evolved in early animals.
What do jellyfish eat?
Depending on the species, the jellyfish has a wide and eclectic diet of plankton, crustaceans, plants, small fish (including the egg and larval stages) and even other jellyfish. Most are purely carnivorous in nature – imagine a very large jellyfish eating a lobster or crab. However, there are a few interesting exceptions. The spotted jellyfish picks up algae inside its stomach and receives nutrients through photosynthesis.
Regardless of diet, jellyfish are voracious eaters. Sometimes they can be seen gathering in groups of millions of people called flowers. These groups consume so much food that they actually cut down whole catches, leaving less for people to catch. Unfortunately, it is believed that climate change could increase the likelihood of blooming across the world’s oceans in the future, which could lead to further destruction of some ecosystems.
How do jellyfish catch food?
The answer to this question depends in part on the life stage of the jellyfish. Many people may not be aware that the anatomy of a jellyfish changes radically throughout its four-part life. By strategically switching between sexual and asexual reproduction at different points in their lives, jellyfish can undergo a remarkable transformation.
The first stage of the jellyfish is the larva produced from the sperm and egg of the parents. As soon as the larva finds a suitable surface, it will settle and turn into a sedentary polyp. After a certain time, several free-floating clones, called ethers, are formed from it. Each of them will eventually reach the full adult jellyfish stage. It remains in the adult state long enough to reproduce, and the larval stages are only transitional forms.
The polyp is usually the longest and most durable of the four; he can remain in this form for several years, waiting for the right conditions to complete his adulthood. The polyp has a fully developed digestive system, but it remains stuck to a stone or other surface and cannot move. The polyp, in fact, is a passive predator, grabbing small animals or organic matter whizzing by with its tentacles in order to devour them.
Having reached the jellyfish stage, the jellyfish acquires some mobility. Swimming serenely in the water (by the stream or by their own power), they spend minimal effort searching for food, probably because their limited nervous and muscular systems prevent them from swimming or maneuvering their prey. Instead, they spread their long tentacles, which in some species (such as the lion’s mane jellyfish) can reach about 100 feet in length, to help them gather food in the water as it passes.
The tentacles contain painful stinging cells that paralyze or stun the victim. There are thousands of such small cells along the length of the tentacles. On contact, they explode at over 2,000 psi, penetrating the victim’s skin. In some species, the bite is sometimes strong enough to cause tremendous pain and even death to humans, but this is usually the result of accidental contact or self-defense from the jellyfish, rather than aggressive actions.
Jellyfish also have the ability to pull food inward towards their tentacles through the force of their swimming motion. This means that they spend no more energy than they would simply float in the water.
One interesting snag in this story is that some jellyfish species have symbiotic relationships with other animals. Symbiotic is a scientific term for when two species living in close proximity share a mutually beneficial relationship. In practice, this means that some smaller animals and young fish will live near the jellyfish bell, either naturally immune to its venom, or in some way avoiding the tentacles to protect themselves from common predators.
What is the advantage of a jellyfish? It is believed that symbiotic species can act as bait to attract prey for the jellyfish (although there may be other factors in play). However, this relationship does not always end well; sometimes the jellyfish may decide to betray its passenger and eat it instead. But this is a risk worth taking for a smaller animal, because the chances of survival are actually higher next to the jellyfish than outside. The aptly named cannonball jellyfish shares a symbiotic relationship with no less than 10 different other marine species, including the juvenile, long-nosed spider crab that lives inside the bell (it is unclear what benefit this gives the jellyfish; it could just be one-sided relationship) …
How do jellyfish digest food?
Despite the rather simplistic anatomy, all jellyfish have a basic set of digestive organs, like any other animal. After killing or paralyzing prey, some (though not all) jellyfish will move food to their mouth using the mouthpieces located on the underside of the bells. These arms mostly resemble short tentacles and have a number of different movements. The mouth itself is nothing more than a small hole located on the underside of the bell. It simultaneously functions as a mouth, anus and a general opening for water that enters and then leaves the body.
This is where the very simplistic nature of the jellyfish anatomy becomes most apparent. The mouth and stomach are directly connected by a small entrance. There is no throat or other organ in between. The digestive system of the jellyfish is so simple that it lacks the liver, pancreas, or intestines, which produce important chemicals and absorb nutrients from most other animals. Instead, it’s just a large cavity that contains everything you need to break down food on its own. Since jellyfish do not have any kind of circulatory system, nutrients naturally enter the water throughout the body.
When the jellyfish finishes feeding, it will expel undigested waste through its mouth again. The process itself is very fast and efficient because he cannot eat again until the previous food has left the body. Given the importance of specialized digestive organs such as the intestines and liver in the evolution of more complex animals, this simplicity also likely limits it to basic functions and behavior only. For their part, jellyfish seem to thrive without them. After all, it still thrives after about 600 million years.
Next Up: Top 10 Animals That Have Shells