Moon Jellies

Moon jellies are a group of ten species in the genus Aurelia.  They are considered true jellies, but DNA testing is required to distinguish between the species.  They are found in oceans worldwide in shallower waters common in coastal and upwelling areas where prey is more abundant.

Moon jellies have a shallow, transparent, dome-shaped body and short tentacles that hang down from underneath.  Full size adults grow from 12″ to 16″ in diameter.  As with all jellyfish, their body is 95% water, and many of the organs found in other animals are missing.  Jellyfish lack respiratory, circulatory and excretory systems.  They have no lungs, heart, blood, brain, bones, nor teeth.  They do have  rhopalia, small sensory organs found around the rim of the dome, containing statocyst structures that sense gravity to determine balance and depth in the water.

Dietary requirements of moon jellies are not well understood.  Jellyfish are dependent on what the wind and currents bring by to eat.  Stinging cells call cnidocytes cover their tentacles and contain nematocysts that can deliver toxin to immobilize prey.  Moon jellies are predators hunting fish, mollusks, crustaceans, and other small invertebrates.  Food is collected in small pockets along the rim of the dome.  There are four arms that transfer the food to the gastrovascular cavity located in the middle of the underside.  Digestive enzymes break down the food and nutrients are distributed throughout the body by a system of canals.

Reproduction includes both sexual and asexual processes.  Adult males release strands of sperm into the water which adult females will ingest and use to fertilize their eggs.  She incubates the eggs in pockets located on her four arms.  Eggs hatch and larvae, called planulae, are released into the water.  Planulae  continue to grow and eventually attach themselves to rocks on the ocean floor.  Anchored to the rock, they enter their next life cycle stage known as a polyp.  At this time, they can catch, eat and digest their own prey, and may spend several years as a polyp.  As a polyp, they begin strobilation, an asexual reproduction process whereby a polyp clones itself and buds off the clone to form an ephyra.  Both the polyp and ephyra grow into adult jellyfish.  Through an unknown process, polyps can determine when conditions are ideal to enter their final adult form, called a medusa, through sexual reproduction.  Strobilation allows the population to expand quickly, while egg production guarantees a diversity of genes in the next generation.

Moon jellies are social animals, travelling in smacks, large groups numbering in the thousands that float across great distances.  An adult in the wild will live about a year, but may live several years in captivity.  During summer, jellyfish try to reproduce daily.  At the end of summer, with less food available and energy levels waning, many individuals become susceptible to bacterial diseases and die before winter.  In addition, they are food for open ocean predators including ocean sunfish and sea birds.  Leatherback sea turtles feed exclusively on moon jellies, and can eat several hundred at each meal. 

In areas of increased human activity, moon jellies tend to thrive due to a decrease in competitors for food.  Moon jellies do well in waters with low dissolved oxygen concentrations and high nutrient loads.  When food is limited, moon jellies can shrink to one-tenth of their full size, returning to their previous size as food opportunities become more abundant.  A recent study identified the moon jelly as one of the “immortal jellyfish” groups.  Several individuals have been observed reversing the aging process, becoming younger all the way back to sexual immaturity, and then aging back to adults.  Studies hypothesize that this is an unending cycle.  You can view these incredible creatures at many of the world’s aquariums including the Shedd Aquarium in Chicago.