Before understanding the life cycle of the moon jellyfish (Aurelia aurita), you’ll need to know a bit about the two dominant types of members of the phylum Coelenterata. Coelenterates include such commonly known animals as jellyfish, sea anemones and corals. They occur in the oceans as either medusas, which are free living animals, or polyps, which are fixed to a solid substrate. Jellyfish are examples of the medusa form and corals and sea anemones are polyps.
However, many species in this primative group can exist in both forms depending on the stage of life they are at. This is known as alternation of generations, where the adults exist as one form and the young exist in the other.
Generally the adult stage is what determines whether an animal is considered a medusa or a polyp, and you would think that most of its life would be spent in this form. For example, a coral larva may be a free-floating medusa for mere days before settling down as a permanant polyp. But that’s not always the case. In the moon jelly we not only see a classic example of alternation of generations, but one in which the majority of its lifespan is spent in the alternate generation.
The moon jellyfish is found throughout the world, although it is limited to coastal bays and estuaries. They can be found in polar, temperate and tropical regions and the length of time they spend in either of the two forms varies with temperature, so I’ll limit the discussion here to those that live in temperate areas such as New England, Japan and the Pacific Northwest. The general life stages, however, are the same everywhere.
The adults first appear locally in early May, and we’ll start with them, which most sources will say reach a bell diameter of eight inches. I’ll dispute this since I have a bay near me where they routinely reach well over a foot wide. I believe the reason for this is due to excessive nutrients. Jellyfish are opportunistic filter feeders that dangle their stinging tentacles beneath them and eat anything they collect. The more plankton or prey that is caught the faster they grow. This particular bay is finger-shaped, which means there is a low turnover rate of the water as the tides ebb and flow. Any nutrients, pollutants or jellyfish found in this bay will take a long time to flush out into the ocean. As it turns out, this bay also has a sewage treatment plant on it. During years with high rainfall this facility is sometimes forced to vent its partially treated overflow into the bay, although nobody seems to want to admit that (even though eating shellfish found here has been outlawed since the 1970’s). The result is a fertilizing of the water which drastically increases the phytoplankton population and, in turn, provides the jellyfish with a huge food source. During these years the Aurelia get gigantic.
You’ll notice that a distinguishing feature of the moon jelly is the set of four horseshoe-shaped structures on the dorsal surface. These are the gonads. Immature jellies all look the same, but as they mature you can sex them. The gonads of the males stay white, while those of the females turn pink. In late spring to early summer the males release their gametes into the surrounding water. These are collected by the mouthparts of the female and transferred to her egg sacs, where fertilization occurs. The female broods these eggs for several days until just before they hatch into a microscopic larval stage, known as a planula. This larva swims around using simple cilia but has no tentacles or functioning stinging cells. I found this amazing when I discovered it, but the planula is actually lecithotrophic. Like a bird or reptile embryo, it is nourished by a yolk. Meanwhile, the parents die and no more moon jellies will be seen until the following spring.
Once the yolk is used up the planula larvae search for a rock, shell or other hard substrate and attach themselves. Here they grow into a tiny anemone-looking organism and remain in this polyp stage until late winter. If food is plentiful, the polyp (which is known as a scyphistoma) may increase its numbers by budding off bits of flesh from its base. This will create a small colony of genetically identical polyps on the substrate.
For most of the year the moon jelly exists as this tiny polyp and feeds itself with stinging tentacles. Around March or so the polyp undergoes a radical change. The tentacles are lost and the animal becomes a non-feeding strobila. This is an odd transition stage where the animal resembles what I’ve always thought of as a pile of poker chips stacked together. And here’s where the fun begins.
In a process known as strobilation, the top chip peels off from the stack and drifts away, becoming a free-floating individual known as an ephyra. This continues, one by one, until the strobila is gone. The ephyra resemble microscopic, multi-lobed jellyfish, which quickly develop the stinging tentacles needed to feed themselves. These ephyra then slowly turn into miniature versions of the adults, grow into mature jellyfish, and the cycle then repeats itself. Notice that each individual planula larva, once polyped and then strobilized, produces dozens of young jellyfish. Now some animals, such as sea anemones, komodo dragons and hammerhead sharks can reproduce either sexually or asexually at different stages of their lives. What’s amazing about this whole process is that the moon jellyfish actually reproduces using a combination of sexual and two different forms of asexual reproduction simultaneously.
Like schooling fish, it is to the moon jellyfish’s advantage to travel in large groups. In order for this to happen the strobilation process must occur with all of the individuals in a given area at once. As it happens, the newly formed ephyra’s first act is to secrete an iodine compound from its cells. This acts as a chemical trigger to induce all of the neighboring strobila to begin peeling off the ephyra chips as well. Within hours of the first baby jellyfish peeling from its stack the surrounding water will be filled with thousands of tiny, drifting ephyra, which travel and grow together for their remaining two-month life span. The resulting swarm of adult moon jellies is called a "raft".
Here’s a link to a visual representation of this life cycle I’ve described.
And here’s a short video of some adult animals in motion.
Other diaries in this series can be found here.