Planula
Introduction
A planula is the free-swimming, larval form of various cnidarian species, including corals, jellyfish, and sea anemones. This stage of the life cycle is characterized by a small, oval shape and the presence of cilia, or tiny hair-like structures, which the planula uses for locomotion.
Life Cycle
The life cycle of a cnidarian involves several stages, with the planula being one of the most critical. The process begins when adult cnidarians release eggs and sperm into the water, resulting in fertilization. The fertilized eggs then develop into planula larvae, which swim freely in the water column.
After a period of time, which can vary from a few hours to several weeks depending on the species, the planula larvae settle onto a suitable substrate. Here, they undergo metamorphosis into a polyp, the next stage in the cnidarian life cycle. This polyp will eventually grow and mature into an adult cnidarian.
Morphology
Planulae are typically small and oval-shaped, measuring only a few millimeters in length. They are covered in cilia, which they use for locomotion. The body of a planula is largely undifferentiated, but it does contain an internal cavity known as the gastrovascular cavity, which serves both digestive and circulatory functions.
Planulae are also characterized by the presence of cnidocytes, specialized cells that are unique to cnidarians. These cells contain organelles called nematocysts, which are used for capturing prey and for defense against predators.
Behavior
Planulae are free-swimming and use their cilia to move through the water column. They are generally considered to be planktonic, meaning they are at the mercy of water currents. However, some research suggests that planulae may be capable of directed movement, or taxis, in response to certain environmental cues.
For example, planulae may exhibit positive phototaxis, moving towards light, which can help them stay near the water's surface where food is more abundant. They may also exhibit chemotaxis, moving towards or away from certain chemicals in the water. This behavior can help planulae find suitable substrates for settlement.
Ecological Role
Planulae play a crucial role in the ecology of marine environments. As a part of the plankton, they serve as a food source for a variety of organisms, including fish and other planktonic animals.
Furthermore, by settling onto new substrates and developing into polyps, planulae contribute to the growth and expansion of cnidarian populations, including coral reefs. This process, known as coral recruitment, is critical for the maintenance and recovery of these important ecosystems.