Jellyfish are marine invertebrates belonging to the phylum Cnidaria, characterized by their bell-shaped, free-swimming body, known as the medusa stage. When people ask if jellyfish have gender, they are typically referring to biological sex. The answer is that the vast majority of species are dioecious, meaning they have separate male and female individuals. The medusa, the familiar adult form, is the life stage responsible for sexual reproduction.
Defining Male and Female Jellyfish
Jellyfish are typically gonochoristic, meaning an organism is either male or female for its entire life. Determining the sex of a jellyfish can be challenging because there are usually no external differences between male and female individuals.
The difference lies internally, specifically in the gonads, which are the reproductive organs. These gonads are situated within the gastrovascular cavity, often located near the stomach or along the radial canals of the bell. In species like the moon jellyfish (Aurelia aurita), the four horseshoe-shaped gonads are often visible through the translucent bell.
The color of the gonads can sometimes indicate sex when they are mature and filled with gametes. Male gonads contain sperm and often appear white or pale, while female gonads contain eggs and may appear darker, sometimes a yellowish or reddish hue. Examining the cells produced within the gonads—sperm for males and eggs for females—is the definitive way to confirm the biological sex of the individual.
How Sexual Reproduction Occurs
Sexual reproduction begins when the mature medusae, both male and female, release their gametes into the surrounding water in a process called spawning. This release is often synchronized across a large population to maximize successful fertilization. Environmental cues, such as specific light levels at dawn or dusk, changes in water temperature, or lunar cycles, often trigger this mass spawning event.
Once released, the sperm from the male jellyfish swim and attempt to fertilize the eggs released by the female. In many scyphozoans, or “true” jellyfish, fertilization is external, occurring freely in the water column. However, some species, including moon jellyfish, exhibit internal fertilization where the female draws sperm into her mouth to fertilize eggs held on her oral arms or in specialized brooding pouches.
The fertilized egg then develops into a small, ciliated larva called a planula. This tiny, oval-shaped larva is free-swimming for a short period, drifting through the water until it finds a suitable location to settle.
The Role of the Life Cycle
Sexual differentiation is confined to the adult, free-swimming medusa stage, which is only one phase of the complex jellyfish life cycle. The medusa is the terminal stage in which the animal is sexually mature and capable of producing gametes. The planula larva, formed after fertilization, eventually settles onto a hard surface.
Once attached, the planula transforms into a sessile, or stationary, form known as a polyp, or scyphistoma. This polyp stage resembles a tiny sea anemone and begins to feed and grow. The polyp is the organism’s long-lived, fixed phase, often surviving for years in a suitable habitat.
When environmental conditions are right, such as a change in water temperature, the polyp begins a process called strobilation. During strobilation, the polyp body segments horizontally, resembling a stack of tiny discs. Each disc then detaches from the polyp to become a tiny, free-swimming juvenile jellyfish known as an ephyra. This ephyra is an immature medusa that will feed and grow rapidly to eventually become the sexually mature adult.
Asexual Reproduction and Hermaphrodites
The polyp stage provides a mechanism for asexual reproduction, which is a major factor in the rapid population increases that lead to jellyfish blooms. This asexual cloning of the polyp ensures that all resulting medusae from that polyp will share the same sex. Polyps can reproduce asexually through several methods:
- Lateral budding, where small copies of the polyp simply sprout from the side of the original.
- Strobilation, which clones and releases multiple ephyrae.
- Forming dormant, hardy cysts known as podocysts, which are capable of surviving unfavorable environmental conditions for long periods.
While most jellyfish species are dioecious, a few are known to be hermaphroditic, meaning a single individual possesses both male and female reproductive tissues. This condition, known as monoecy, is less common and often occurs in species that do not form large aggregations. Hermaphroditism is an adaptation that ensures reproductive success when finding a mate is rare, sometimes allowing for self-fertilization.