Jellyfish are odd-looking creatures, so you may wonder, how do jellyfish reproduce? Do jellyfish lay eggs? Do jellyfish have genders?
Jellyfish are simple creatures who comprise mostly water and don’t even have a brain or heart. It would make sense for them to have a similarly simple reproductive process and life cycle.
But nothing could be further from the truth.
Jellyfish reproduce both sexually and asexually at different stages of their lives. They go through complex stages of metamorphosis. They transform from something that looks like a worm, to something that looks like a flower, all the way to what we recognize as the jellyfish we see in aquariums.
What Are Jellyfish?
Jellyfishes are a type of invertebrate called a “cnidarian”. Cnidarians have stinging cells in their tentacles which help them capture prey and protect themselves. Other cnidarians include sea anemones, corals, and sea whips.
Easily recognizable, jellyfish have gelatinous bodies shaped like an umbrella. Hanging from the bottom of the “umbrella” are their long tentacles.
Jellyfish are incredibly simple creatures whose bodies comprise 95% water. The rest of their body comprises three layers. Their outer body layer is the epidermis. The inner layer is called the gastrodermis. Between these two layers is the mesoglea, a thick, jelly-like substance.
These simple creatures do have a nervous system, called a nerve net, but it’s elementary. The nerve net allows them to sense light, smell, and respond to other stimuli.
Their digestive system is also simple, comprising a single opening that acts as both the mouth and the anus. The digestive cavity within the opening works as both the stomach and intestines.
Jellyfish are such simple creatures that they don’t even have blood, a brain, or even a heart.
Jellyfish Life Cycle
The life cycle of a jellyfish is incredibly complex despite the simple nature of jellyfish themselves.
Planula Larva
Planulae, or larva jellyfish, result from fertilization. Jellyfish eggs free float throughout the ocean before hatching into planula larvae.
Planulae have cilia, microscopic hair-like structures that help the organism move. With the help of the cilia, planulae are free-swimming for the first few days of their life. In the following days, they develop slightly before choosing a place on the ocean floor to attach themselves.
Scyphistomae (Polyps)
Planulae that attach to solid surfaces transform into jellyfish polyps. Polyp jellyfish, or scyphistomae, look like flowers with tentacles, or like corals. These tiny polyps are only about 1 in (2.54 cm) long. They use the tentacles to catch floating zooplankton as it drifts past. Unlike planulae, polyps also have a mouth with which to eat their captured food.
One of the most interesting things about polyps is that they are clones, meaning they are identical to one another.
Some polyps mature faster than others. So, one polyp may grow, mature, and die, all while genetically identical polyps or medusae are living elsewhere in the ocean.
Podocysts
If water conditions are unsuitable, polyps shrink and change their bodies into round cysts. These cysts, or podocysts, are better able to withstand harsh conditions and can survive for years until conditions improve. Once the water quality improves, the podocyst transforms back into a polyp.
Strobilation
Then, Scyphozoa, or true jellyfish, go through the process of strobilation. A polyp splits into 10-15 parts that look like plates that stack atop one another. The plate-like segments form a “strobila” which resembles a tower. This usually occurs in the spring when water temperatures reach at least 62°F (16.7°C).
After a time, each segment breaks off the top of the strobila tower. As each segment breaks off, it turns into a juvenile jellyfish, or ephyra. From here, an ephyra continues to mature into an adult jellyfish. In just a few weeks, the ephyra develops its bell and becomes an adult.
Medusa
An adult, or medusa jellyfish, is the animal with the umbrella-shaped gelatinous body that we recognize.
How long a jellyfish lives depends on its species and how long it spends in each life stage. A medusa’s sole goal is to reproduce, so most jellyfish die shortly after entering this stage.
The Smithsonian Ocean Portal Team describes it this way:
“Most jellyfish are short lived. Medusa or adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. Polyps can live and reproduce asexually for several years, or even decades.”
How Do Jellyfish Reproduce?
Despite being simple creatures, jellyfish reproduction is quite complex. An individual jellyfish can reproduce both sexually and asexually during different stages of its life.
Sexual Reproduction
Medusas, or adult jellyfish, reproduce sexually through spawning.
Females release eggs into the water column and males release sperm. As the eggs and sperm touch, the eggs become fertilized, creating a Medusa’s offspring.
Hundreds or thousands of jellyfish reproduce in the same location, resulting in thousands of eggs swirling through the water. Spawning events occur at dawn or dusk when the risk of predation is lowest.
So, jellyfish mating isn’t like the mating we associate with most animals. No physical contact between males and females is necessary.
In some species, like the moon jelly, males release sperm trails that are taken in by the female’s oral cavity.
Sexual reproduction occurs throughout the life of the medusa. Triggers, like light, food abundance, and proximity to other jellies, encourage jellyfish to spawn.
Asexual Reproduction
Jellyfish are also asexual and reproduce by budding in their polyp form. In many species, a polyp divides itself in half, creating a genetically identical polyp. Other species divide and create or transform into medusa.
Conclusion
The life cycle of jellyfish comprises many stages. They begin their life as eggs before transforming into larva, polyps, ephyra, and finally, medusa.
A jellyfish’s body comprises 95% water and lacks blood, a heart, and a brain. Still, their lives are detailed and complex.
As polyps, jellyfish create genetically identical clones of themselves, so there may be dozens of identical jellyfish floating about the ocean at one time. At the same time, adult jellyfish reproduce sexually, creating offspring that are genetically diverse.
FAQs
Jellyfish reproduce sexually and asexually, depending on which life stage they are in. Polyps, or young jellyfish, can split themselves to create genetically identical clones of themselves.
Medusa, or adult jellyfish, reproduce sexually. Most jellyfish are either male or female and produce sperm or eggs. Jellyfish release their sperm and eggs into the water column where the eggs become fertile.
Jellyfish eggs are transparent, gelatinous blobs that float through the ocean.
Most jellyfish are male or female, producing either sperm or eggs. So, we need both genders to create fertile eggs to produce offspring.
There are some jellyfish that are hermaphroditic, but it’s rare. Some jellyfish species, like the compass jellyfish (Chrysaora hysoscella), produce both male and female sex cells.
Hermaphroditism only occurs in rare jellyfish species to preserve their species and produce as many offspring as possible.
In contrast, most comb jellies are hermaphroditic and can release both sperm and eggs. Comb jellies, despite their name, are not “true jellyfish” because they don’t have stinging cells.
As polyps, jellyfish reproduce asexually, meaning they create genetically identical clones of themselves. Later, as adults, they reproduce sexually. So, with there being multiple clones throughout the ocean, does that mean there’s a chance jellyfish can reproduce with their clones?
Hermaphrodites are rare in jellyfish, meaning that most jellyfish are male or female. When a jellyfish reproduces, either sexually or asexually, all the offspring will be the same gender as the parent. Since all the offspring are the same gender, this prevents clones from reproducing with each other.
Most jellyfish are born, age, and die normally, like any other animal. There is one species, the immortal jellyfish (Turritopsis nutricula), who seemingly lives forever.
Immortal jellyfish have special cells that allow them to revert to their juvenile polyp stage when they experience damage or stress. So, unless an immortal jellyfish falls prey to some hungry animal, it seems that it can live forever.
