Cnidarians are a diverse phylum of aquatic invertebrates, including stationary corals, sea anemones, and free-swimming jellyfish. The term “medusa” specifically refers to the distinctive bell-shaped, free-swimming stage in the life cycle of many of these animals.
Defining the Medusa Form
The medusa form is characterized by its umbrella or bell-shaped body, from which tentacles and a mouth typically hang downwards. This body plan allows for a pelagic, or free-swimming, lifestyle, in contrast to the sessile nature of the polyp form. The mouth, located on the concave underside of the bell, leads into a central gastrovascular cavity. The bell’s rim often features tentacles, which are used for capturing prey and defense.
The Cnidarian Phylum and Its Features
The phylum Cnidaria comprises aquatic invertebrates, predominantly found in marine environments. A defining characteristic of all cnidarians is their radial or biradial symmetry, a body plan where parts are arranged around a central point. These animals are diploblastic, meaning their bodies develop from two primary embryonic layers: an outer epidermis and an inner gastrodermis, separated by a jelly-like layer called the mesoglea.
Cnidarians are distinguished by specialized stinging cells known as cnidocytes, which contain organelles called nematocysts. These cells, concentrated around the mouth and on the tentacles, fire a coiled thread that may contain barbs and toxins to immobilize prey or deter predators. All cnidarians possess a single opening to their gastrovascular cavity, serving as both mouth and anus. The phylum includes species that exist solely as polyps, solely as medusae, or exhibit both forms during their life cycle.
How Medusae Live and Function
Medusae propel themselves through water by rhythmically contracting muscles located around the edge of their bell-shaped bodies. This action expels water from underneath the bell, allowing them to move forward. They use tentacles armed with nematocysts to capture prey, which can range from plankton to small fish. Once prey is caught, tentacles guide it towards the mouth for ingestion.
Medusae possess a decentralized nerve net that coordinates their movements and responses to the environment. This nerve net enables behavioral flexibility. Sensory organs called rhopalia, located around the bell’s margin, contain structures like ocelli for light detection and statocysts for sensing gravity and maintaining balance. Reproduction in medusae typically involves sexual reproduction, with male medusae producing sperm and female medusae producing eggs, leading to the development of a free-swimming planula larva.
Major Groups of Medusae
Several cnidarian classes prominently feature the medusa form. Scyphozoa, often referred to as “true jellyfish,” have a dominant medusa stage. Examples include the Moon Jellyfish (Aurelia aurita) and the Lion’s Mane Jellyfish (Cyanea capillata), a large species. Scyphozoans exhibit tetramerous radial symmetry.
Cubozoa, or box jellyfish, are distinguished by their cube-shaped bells and are among the most venomous cnidarians. Species like the Sea Wasp (Chironex fleckeri) can deliver potent stings. Box jellyfish often have muscular pads called pedalia at the corners of their bells, with one or more tentacles attached to each.
Hydrozoa is another diverse class where some species exhibit a medusa stage. While many hydrozoans have both polyp and medusa stages, some, like the Portuguese Man o’ War (Physalia physalis), are actually colonies of specialized polyps and medusoids rather than a single medusa.