Jellyfish are marine invertebrates with a complex life cycle that makes answering the question, “What is a baby jellyfish called?” challenging. Unlike many animals that have a single name for their young, jellyfish undergo a metamorphosis involving several distinct developmental forms. Because they alternate between sexual and asexual reproduction, the term “baby” applies to multiple stages, each with its own specific scientific name. Understanding the jellyfish life cycle requires examining the three major immature phases: the larva, the attached polyp, and the free-swimming form.
The Larval Stage and Attachment
The jellyfish life cycle begins with the fertilized egg, which develops into a microscopic, free-swimming organism known as the planula. This initial form is oval-shaped and covered in hair-like structures called cilia, which propel the larva through the water column. The planula is the result of sexual reproduction from the adult medusa and seeks out a suitable hard surface, such as a rock or oyster shell, to settle upon.
After a brief period adrift, the planula attaches itself to the substrate and transforms into the next stage, the polyp, also known as the scyphistoma. This sessile form has a cylindrical structure, resembling a sea anemone, with a mouth surrounded by small tentacles used for feeding on passing plankton. The polyp stage reproduces asexually by budding, creating clones of itself to establish dense colonies on the seafloor. This attached stage can persist for months or even years, depending on the species and the environment.
The Miniature Free Swimming Form
The transition from the attached polyp to the free-swimming jellyfish is triggered by strobilation, often initiated by changes in water temperature or light levels. During strobilation, the polyp segments horizontally, forming a stack of disk-like structures known as a strobila. The polyp reabsorbs its tentacles and elongates, creating a tower of stacked segments that look similar to a pile of tiny plates.
Each segment then breaks away from the strobila to become a miniature, free-swimming juvenile called an ephyra. The ephyra is the first mobile form that truly resembles a jellyfish, though it lacks the smooth, domed bell of an adult. Instead, it typically has a star-like appearance with eight distinct lobes, which it uses for propulsion and capturing food. The ephyra stage is a period of rapid growth as the juvenile feeds heavily in the water column before developing into the immature medusa, the traditional bell-shaped form.
Size and Survival of Juvenile Jellyfish
The smallest forms of the jellyfish are diminutive, with the planula larva measuring less than a millimeter in length. The attached polyp, the scyphistoma, is typically less than an inch long, and its primary habitat is the coastal seafloor. The free-swimming ephyra juvenile is larger than the planula, generally reaching a diameter of a few millimeters to about one centimeter across as it drifts in the upper water layers.
The young stages face high mortality rates due to predation and environmental fluctuations, highlighting the difficulty of reaching adulthood. Polyps may form a protective, dome-shaped cyst called a podocyst when conditions are poor, allowing them to survive for years until the environment becomes favorable again. Once in the water column, the ephyra is vulnerable to various predators, including filter-feeding organisms. This resilience allows the juvenile forms to survive the initial challenges before maturing into the reproductive adult medusa.