This is one of a group of more than ten morphologically nearly identical jellyfish species in the genus Aurelia. In general, it is nearly impossible to identify Aurelia medusae without genetic sampling, so most of what follows about Aurelia aurita, could equally be applied to any species of the genus. The medusa is translucent, usually about 25–40 cm in diameter, and can be recognized by its four horseshoe-shaped gonads that are easily seen through the top of the bell. It feeds by collecting medusae, plankton and molusks with its mucusy bell nematocyst-laden tentacles and bringing the prey into its body for digestion, but is capable of only limited motion; like other jellies it primarily drifts with the current, even when it is swimming.
Feeding
A. aurita and other Aurelia species feed on plankton that includes organisms such as mollusks, crustaceans, tunicate larvae, rotifers, young polychaetes, protozoans, diatoms, eggs, fish eggs, and other small organisms. Occasionally, they are also seen feeding on gelatinous zooplankton such as hydromedusae and ctenophores. Both the adult medusae and larvae of Aurelia have nematocysts to capture prey and also to protect themselves from predators. The food is tied with mucus, and then it is passed down by ciliated action down into the gastrovascular cavity where digestive enzymes from serous cell break down the food. There is little known about the requirements for particular vitamins and minerals, but due to the presence of some digestive enzymes, we can deduce in general that A. aurita can process carbohydrates, proteins and lipids.
Aurelia does not have respiratory parts such as gills, lungs or trachea. Since it is a small organism, it respires by diffusing oxygen from water through the thin membrane covering its body. Within the gastrovascular cavity, low oxygenated water can be expelled and high oxygenated water can come in by ciliated action, thus increasing the diffusion of oxygen through cell. The large surface area membrane to volume ratio helps Aurelia to diffuse more oxygen and nutrients into the cells.
The basic body plan of Aurelia consists of several parts. The animal lacks respiratory, excretory, and circulatory systems. The adult medusa of Aurelia, with a transparent look, has an umbrella margin membrane and tentacles that are attached to the bottom. It has four bright circular gonads that are under the stomach. Food travels through the muscular manubrium while the radial canals help disperse the food. There is a middle layer of mesoglea, gastrodervascular cavity with gastrodermis, and epidermis. There is a nerve net that is responsible for contractions in swimming muscles and feeding responses. Adult medusa can have a diameter up to 40 cm. The medusae are either male or female. The young larval stage, a planula, has small ciliated cells and after swimming freely in the plankton for a day or more, settles on an appropriate substrate, where it changes into a special type of polyp called a "scyphistoma", which divides by strobilation into small ephyrae that swim off to grow up as medusae. There is an increasing size from starting stage planula to ephyra, from less than 1 mm in the planula stage, up to about 1 cm in ephyra stage, and then to several cm in diameter in the medusa stage.
