Jellyfish, with their graceful pulsations and translucent forms, are among the ocean’s most captivating inhabitants. Many people wonder if these ancient creatures possess a biological sex, meaning they are distinctly male or female. This article clarifies whether jellyfish have distinct sexes and explains their diverse reproductive methods.
Determining Sex in Jellyfish
Most jellyfish species have separate sexes. This reproductive strategy is known as gonochorism. Unlike many animals, jellyfish typically do not display obvious external visual differences between males and females. Instead, their sex is determined internally by the type of reproductive cells they produce.
Male jellyfish possess testes, which produce sperm, while female jellyfish have ovaries that produce eggs. These reproductive organs, called gonads, are often located within the bell of the jellyfish, near the stomach. For instance, in moon jellyfish (Aurelia aurita), the four horseshoe-shaped structures visible through the bell are their gonads, which are connected to their gastric pouches. While most jellyfish maintain separate sexes throughout their lives, some rare species exhibit hermaphroditism. However, this is not the typical strategy for the majority of jellyfish species.
Sexual Reproduction in Jellyfish
Sexual reproduction in jellyfish primarily occurs during their medusa (adult) stage. Male jellyfish release sperm and female jellyfish release eggs directly into the surrounding water, a process known as spawning. This external release of gametes is common in aquatic environments. Successful fertilization typically happens in the water column.
In some species, females may take in sperm released by males, leading to internal fertilization. Following fertilization, a small, ciliated, free-swimming larva called a planula forms. This planula larva disperses before settling onto a suitable hard surface. The planula’s role is to locate an appropriate substrate to transition into the next life stage.
The Asexual Side of Reproduction
After the free-swimming planula larva settles onto a surface, it attaches and develops into a sessile (stationary) polyp. This polyp stage represents an asexual phase in the jellyfish life cycle. From this attached polyp, new, genetically identical individuals are produced.
The polyp reproduces asexually through a process called strobilation. During strobilation, the polyp undergoes a series of transverse divisions, budding off small, immature jellyfish known as ephyrae. These ephyrae are miniature versions of the adult jellyfish and are released into the water column. Each ephyra then grows and develops into a free-swimming medusa, which is the familiar adult jellyfish form, completing the complex life cycle that alternates between sexual and asexual reproduction.