Jellyfish gracefully pulsing through the ocean are a familiar sight to beachgoers. With their translucent bodies and long, trailing tentacles, it’s easy to mistake these marine creatures for some kind of underwater plant.
If you’re short on time, here’s a quick answer to your question: No, jellyfish are not plants. Jellyfish are animals that belong to the phylum Cnidaria.
This article will examine the biology and characteristics of jellyfish to explain in detail why they are classified as animals, not plants.
Jellyfish Are Multicellular Heterotrophs
Jellyfish Consume Other Organisms for Food
Jellyfish are multicellular heterotrophs, which means they consume other organisms for nutrition. They feed by using their tentacles to stun or capture prey before moving it to their mouth for ingestion.
The diet of a jellyfish consists mainly of plankton, fish eggs, small fish, and other gelatinous creatures. Some species are generalist feeders, while others specialize in certain prey types. For example, moon jellies mainly eat zooplankton like copepods and fish larvae, while blue blubber jellies target larger prey such as shrimps and small fish.
To capture food, jellyfish use their stinging cells called nematocysts located on their tentacles. When prey makes contact with the tentacles, the nematocysts fire a toxin that paralyzes the prey. The paralyzed food is then transported by the tentacles to the mouth for consumption.
Jellyfish do not chew or digest their food, instead absorbing nutrients directly through the gastrovascular cavity lining their bell-shaped bodies. Once the food is absorbed, any undigested material is expelled through the mouth.
Jellyfish are efficient hunters that play an important role as both predator and prey in marine food chains. Their feeding habits can significantly impact zooplankton populations and fish recruitment rates. Some species like the lion’s mane jellyfish even prey on other jellyfish!
Understanding the unique feeding mechanisms and diets of jellyfish provides insight into their ecological niches in coastal and open ocean environments.
Jellyfish Have Specialized Tissues and Organs
Although they lack brains, hearts, or lungs, jellyfish do have specialized tissues and organs that allow them to survive and function in their aquatic environments.
The bell or umbrella-shaped part of the jellyfish contains an inner gelatinous layer called the mesoglea sandwiched between an external epidermis and an internal gastrodermis. The muscular contractions of the bell propel the jellyfish through the water.
The epidermis contains touch receptors, light sensing structures, and nematocysts used for defense and capturing prey. Underneath the epidermis, the mesoglea provides structural support. The gastrodermis lines the gastrovascular cavity where digestion occurs and contains cilia and muscle fibers.
Hanging below the bell are oral arms surrounded by tentacles bearing nematocysts. These specialized structures enable efficient food capture. Inside the bell is the jellyfish’s digestive system, consisting of the central stomach connected to radial canals that distribute nutrients.
There is also a simple nervous system made up of a nerve net that coordinates the animal’s reflexes.
While jellyfish do not have dedicated excretory, respiratory, or circulatory systems, their gastrovascular cavity performs similar functions by circulating nutrients and oxygen while also removing metabolic waste.
Jellyfish reproduce both sexually and asexually through specialized reproductive organs located in the lining of the gastrovascular cavity.
So while they lack many complex systems found in other multicellular organisms, jellyfish have evolved specialized tissues and organs adapted for their unique lifestyle and ecological niche. Their comparative simplicity also makes them an interesting model organism for biological research.
Jellyfish Exhibit Animal-Like Behaviors
Jellyfish Respond to Stimuli
Like most animals, jellyfish exhibit the ability to sense and respond to their environment. They have a basic nervous system and rhopalia, which are specialized sensory structures, that allow them to detect light, odor, and other stimuli.
Jellyfish will move toward food sources or away from predators in response to chemical cues in a behavior called chemotaxis. They will also move toward light (positive phototaxis) or away from it (negative phototaxis) depending on the species.
Some jellyfish, like the box jellyfish, even have advanced vision and can see using eyes located in their rhopalia.
Jellyfish sting as a defense mechanism when they detect contact with their tentacles. The stinging cells, called nematocysts, contain coiled threads that uncoil and release toxins when triggered by prey or threats.
Some species like the dangerous Irukandji jellyfish have extremely potent venom used to paralyze and subdue fish. So while jellyfish don’t have a brain, their ability to sense stimuli and instinctively respond does require some basic neurological capability.
Jellyfish Are Mobile
Jellyfish display animal-like behaviors through their ability to move independently. While plants are sessile, meaning fixed in one place, jellyfish swim freely in the water column using propulsion and navigational skills.
Most jellyfish pulse their bell-shaped bodies to push water behind them and propel themselves forward. Species like the moon jelly have a radially symmetric umbrella shape ideal for circular motion. Others, like the Portuguese man o’ war, rely on ocean currents and winds to drive their sail-like pneumatophore up top.
Jellyfish are adept swimmers and certain species can reach speeds over 5 knots. The box jellyfish can rapidly propel itself using jet propulsion by expelling water from its bell. Jellyfish also show orientation abilities, moving upwards toward sunlight or sideways to currents that carry them offshore.
Their mobility allows them to migrate long distances to mate, hunt prey, or avoid stressful conditions. Plants simply cannot actively move in these complex, controlled ways. So the independent movements of jellyfish are characteristic of advanced animal behavior.
Jellyfish Reproduce Sexually and Have a Life Cycle
Jellyfish Go Through Several Life Stages
Like all animals, jellyfish go through different life stages as they grow and develop. The jellyfish life cycle is fascinating and complex!
It all starts with an egg and sperm joining to form a zygote. The zygote develops into a free-swimming larva called a planula. Planulas will attach themselves to a solid surface and transform into a polyp. Polyps resemble tiny sea anemones. They are sessile, meaning they stay fixed in one place.
Polyps clone themselves and bud off genetically identical medusae (the adult jellyfish). The medusa is the familiar bell-shaped jellyfish that swims freely in the ocean. Medusae reproduce sexually to start the jellyfish life cycle all over again.
- Egg/sperm
- Zygote
- Planula larva
- Polyp
- Medusa (adult jellyfish)
The length of the jellyfish life cycle depends on the species. For some it may be just a few months, while others live for several years. Amazingly, certain jellyfish species can even reverse their life cycle and transform back into polyps under stressful conditions!
Jellyfish Produce Sex Cells for Reproduction
Like most animals, jellyfish reproduce sexually. Adult male and female medusae release sperm and eggs into the water where they unite to form free-swimming zygotes. One interesting fact about jellyfish reproduction is that some species are able to fertilize themselves.
Jellyfish have specialized sex organs called gonads that produce eggs or sperm. In some species, the same individual can produce both eggs and sperm, allowing self-fertilization. Other jellyfish must rely on a member of the opposite sex in order to reproduce.
Mass jellyfish blooms often occur when environmental conditions are favorable for rapid reproduction and growth. Some blooms consist of billions of individuals! While these blooms can be a nuisance for beachgoers, they are a normal part of the jellyfish life cycle.
Jellyfish Share Genetic Similarities with Other Animals
Despite their unique body plans, jellyfish share key genetic similarities with other animals. Comparative genomics studies have revealed that jellyfish possess many of the same genes and developmental pathways found in bilaterally symmetric animals like insects, worms, and vertebrates.
For example, jellyfish have Hox genes, which help control body plan formation during embryonic development. The specific Hox genes found in jellyfish are also present in bilaterally symmetric animals. Jellyfish Hox genes are expressed in particular body regions, helping to establish the oral-aboral axis.
In addition, jellyfish utilize conserved signaling pathways like Wnt, TGF-beta, and FGF during their development. These pathways direct important processes like cell differentiation, proliferation, and migration in a wide variety of animals.
The jellyfish versions of these genes are homologous to those in other species.
Jellyfish also possess many genes involved in cell-cell adhesion, neuronal function, immune responses, and other essential cellular activities. While jellyfish may look bizarre, their genomes encode many of the same protein families found across the animal kingdom.
Comparative genomics has also revealed some key differences between jellyfish and bilaterally symmetric animals. For example, jellyfish appear to lack anterior Hox genes as well as some genes involved in left-right patterning.
These missing genes may contribute to the radial symmetry seen in jellyfish.
Conclusion
While their translucent bodies and plant-like appearance may be deceiving, jellyfish are very much animals, not plants. As heterotrophic, multicellular organisms that exhibit animal behaviors, life cycles, and genetics, jellyfish clearly belong to the animal kingdom.
Understanding the biology of these fascinating creatures can help appreciate their role in ocean ecosystems. The next time you spot jellyfish pulsing through the waves, you can confidently admire their animal nature.