Jellyfish are a highly diverse group of cnidarians, made up of many species, all with tentacles, an umbrella, and various colors. But let’s stop talking about things we already know. You might be asking yourself, “What exactly is a jellyfish?” or “How dangerous are these animals to me?” Well, all of those questions will be answered throughout this article. So, without further ado, let’s dive in.
What are Jellyfish?
Jellyfish are marine invertebrates from the phylum Cnidaria, specifically belonging to the subphylum Medusozoa. Within this subphylum, there are several classes of jellyfish, which I will briefly discuss. These classes are: Cubozoans, Hydrozoans, Staurozoans, and Scyphozoans.
- Cubozoans: Also known as box jellyfish, commonly called “sea wasps” due to their dangerous venom, cubozoans are a class of Cnidaria characterized by their cube-like body shape. They resemble the jellyfish from the Scyphozoan class.
- Hydrozoans: Hydrozoans, or hydromedusae, are a class of Cnidaria that can inhabit both marine and freshwater environments. Examples of species in this class include Hydra and Porpita porpita.
- Staurozoans: Staurozoans are a class of Cnidaria that includes the order of Stauromedusae. This class is the least studied among jellyfish.
- Scyphozoans: Scyphozoans are the jellyfish most commonly known. They are typically the largest cnidarians.
Anatomy and Life of Jellyfish
Jellyfish are simple organisms. In fact, many of them resemble plants more than animals, even though they are, of course, animals. They have a very high growth rate, and most of their structural features are quite different from those of other organisms.
In summary, jellyfish have a gelatinous body, shaped like a bell, from which a tubular manubrium hangs with a mouth at the bottom, sometimes extending into long tentacles covered with specialized stinging cells called cnidocytes. You might have noticed the word “cnidocytes,” so let me explain them briefly. Cnidocytes are specialized cells, unique to cnidarians, that release a stinging substance. These cells serve both for defense against predators and for capturing prey.
Now that we’ve summarized their anatomy, let’s look at the life cycle of jellyfish. It all starts with an egg, from which a larva called a planula emerges. The planula seeks a suitable substrate and, once it finds one, it attaches and transforms into an asexual polyp. The polyps produce medusae, completing the cycle. Each jellyfish class reproduces differently, but here’s a quick overview:
- Hydromedusae are formed by budding from the polyps, either from their walls or specialized gonozooids.
- Scyphomedusae originate from small polyps through a process called strobilation, where the polyp divides into stacked discs. These discs release larval forms called ephyrae, which eventually grow into sexually mature medusae.
- In Cubozoans, each polyp produces a single cubomedusa through complete metamorphosis.
Jellyfish Ecology and Their Relationship with Humans
Jellyfish are simple animals that have been around for a long time—about 600 million years or more. During this time, many species have disappeared or evolved, but jellyfish have neither changed nor vanished. This is due to their ecology and their predator-prey relationships, which are quite efficient. Jellyfish sit in the middle of the food chain. Their primary predators are turtles, while jellyfish feed on small fish or other marine organisms. Additionally, jellyfish have developed various defenses against predators, including transparency, venom, and camouflage through their colors. Some species also have symbiotic relationships with other organisms, such as algae. An example of this is Cotylorhiza tuberculata, which has a symbiotic relationship with algae called zooxanthellae.
Humans and jellyfish have a somewhat peculiar relationship. Indirectly, we have helped jellyfish populations thrive, particularly through overfishing, which reduces competition for jellyfish, and climate change, which has contributed to their population growth. Additionally, chemical pollution and oceanic waste help jellyfish, as they are immune to many chemical substances, and trash provides shelter from predators.
Some Notable Species
Finally, let’s discuss a few well-known jellyfish species, as well as some personal favorites.
Pelagia noctiluca
The luminous jellyfish is a species of jellyfish from the class Scyphozoa, found in the Atlantic and Mediterranean. This species usually inhabits pelagic habitats but can form swarms that reach the coastline, making swimming impossible due to the extreme sting from its cnidocytes.
Cotylorhiza tuberculata
The fried egg jellyfish is a species of jellyfish from the class Scyphozoa found in the Mediterranean Sea. It often forms massive swarms spanning kilometers, which can make swimming impossible in some areas. However, its sting is not very potent, so it poses minimal risk. As mentioned earlier, it has a symbiotic relationship with zooxanthellae algae.
Halitrephes maasi
The fireworks jellyfish is a species of hydrozoan jellyfish from deep waters. While not much information is available about this species, research suggests that Halitrephes maasi can survive in diverse marine environments. It has been found in temperate, tropical, Atlantic, Indo-Pacific, Antarctic, Mediterranean, and Eastern Pacific oceans.
Stygiomedusa gigantea
Stygiomedusa is a genus of giant deep-sea jellyfish from the family Ulmaridae. Its only species, Stygiomedusa gigantea, is more commonly known as the giant phantom jellyfish. With only 115 sightings in the past 110 years, it is rarely seen but is believed to be widespread globally. It is likely one of the largest invertebrate predators in deep-sea ecosystems.
Cyanea capillata
The lion’s mane jellyfish, also known as the giant lion’s mane jellyfish, is a species of cnidarian from the class Scyphozoa, family Cyaneidae. It is the largest jellyfish species in the world. The longest known specimen had a bell diameter of 3.7 meters and tentacles reaching up to 37 meters.
With these five species, I conclude the article about jellyfish. I hope this article has helped you learn something new or provided you with entertainment during your free time.
