You probably have seen moon jellies at an aquarium. They are the small, round, nearly- transparent jellies most of us picture when we think of jellyfish. The common name moon jelly refers to all species in the genus Aurelia. Species in this genus are so similar to each other they can only be differentiated by DNA analysis. Scientists do not know how many species belong to the genus Aurelia because more are being discovered all the time.
Moon jellies are found worldwide but are most prevalent in the Atlantic, Pacific, and Indian Oceans. They tolerate a wide range of temperatures and salinities and may even inhabit brackish water. In brackish water, the bell of a moon jelly is much flatter due to the decreased salinity. Moon jellies usually stay near the surface and are capable of moving upward on their own but are dependent on wind, tides, and currents for horizontal movement. Currents sometimes bring together thousands of moon jellies in a small area, and these groupings are called a bloom, a swarm, or a smack.
A moon jelly is typically 4 to 5 inches (10 to 15 cm) in diameter. It has a shallow bell that is colorless, except for the horseshoe-shaped gonads that may be tinted violet, pink or yellow. Since the gonads are near the bottom of the stomach, they often take on the color of the prey the jelly has just eaten. The margin of a moon jelly is divided into eight lobes that are fringed by numerous, thin tentacles. Each lobe is divided by a shallow cleft housing a sensory organ that aids the jelly in maintaining its equilibrium. The radial canals of the digestive system are clearly visible and repeatedly branch as they move toward the margin of the jelly. The oral lobes or arms are short and thick.
Moon jellies are carnivorous and feed mainly on zooplankton. They are eaten by lion’s mane jellies, sea urchins, crab, some sea anemones, sea turtles, and shorebirds. Like other jellies, moon jellies have tentacles with stinging nematocysts, but their stings are relatively harmless to humans.
Moon jellies have a lifecycle similar to other jellies. In the medusa or free-floating phase, the male releases a strand of sperm which the female takes through her mouth, fertilizing her eggs. The fertilized eggs develop into larvae in pockets in the oral arms that surround the mouth. The female then releases the larvae, and they settle and develop into polyps. A moon jelly may remain in the polyp stage of its lifecycle for 25 years until conditions are right for the polyp to reproduce asexually by budding. The buds float free and develop into medusae. In the wild, moon jellies only survive about 6-months in the medusa stage, but they may live up to a year as medusae in an aquarium.
Moon jellies are the most commonly kept species in both public and private aquariums. Their wide tolerance of both salinity and temperature make them a good choice as an aquarium species.