Moon Jellyfish Kate McGonagle
This month we are taking a look at one of our most frequent jellyfish visitors, the moon jellyfish. Moon jellies are the most common jellyfish found in Irish waters. These glowing ethereal creatures can most commonly be seen on dives all around the coast between April and September.
I will talk about their classification, how to identify them, how they reproduce, and how they hunt, to hopefully answer some of your burning questions about this species.
Moon jellies get their name from their circular whitish and otherworldly appearance, and unlike other jellyfish lack those long stinging tentacles, instead having shorter finer tentacles lining their underside. This is where they get their scientific name Aurelia aurita, with Aurelia coming from the Greek “gold-coloured puppa”, and the latin aurita meaning “furnished with hears” (Aquascope, Moon jellyfish).
Due to the nature of the oceans, with large areas for dispersal reducing barriers to gene flow, we end up with a smaller number of species with a wide range, termed cosmopolitan species (Dawson and Jacobs, 2000). Moon Jellyfish, Aurelia aurita, is one of these species. There are several subspecies of moon jellies, but they are often impossible to tell apart without DNA.
Identification
Moon jellyfish are easily identifiable when on a dive. They are pale and translucent in colour, circular, with four purple-pink rings symmetrically arranged in their centre. As I mentioned, they do not have long tentacles like compass jellyfish or lions mane jellyfish.
Something I have often wondered when hearing that moon jellies don’t sting is what do they eat, and how do they hunt? Actually, these jellyfish can still sting you if you come in contact with their stinging cells, but their sting is very mild in comparison to our other locals, which is why you will often hear that they don’t sting, or see people touching them.
Classification
Moon jellyfish belong to the Class Scyphozoa within the Subphylum Medusozoa in the broader group Cnidaria. I like to think of them like the greek god medusa, with all of the tentacles like tiny snakes around their body.
So we know their classifcation, which is definitely a mouthful, but what does this mean for where they fit into the big tree of life?
Scyphozoans are also known as “true jellies” and are only found in marine environments, while other classes of cnidarians can be found in freshwater too, like hydras from the other group within the Medusozoa. Cnidarians are an extraordinarily diverse group, and some other cnidarians you may be familiar with include corals and sea anemones.
The Life Cycle of Moon Jellies
Moon jellies have a two-part reproductive cycle that is common to Scyphozoans. The first consists of scyphopolyps that live on the sea floor, which in the right conditions asexually reproduce and release ephyrae, juvenile jellies which drift away into the ocean. These ephyrae grow into sexual medusae, the jellies we all know.
The females produce larvae which settle to the bottom of coastal habitats, and the cycle repeats (Dawson and Jacobs, 2000). The phase which we are familiar with are the medusae we see drifting through the water column when we are on a dive.
We often think of jellyfish as completely directionless, just floating where the currents take them. Interestingly, despite moon jellyfish having a global distribution, research has shown that these jellies move directionally to specific groups for breeding in British Columbia, and has also shown genetic differences between populations in the Eastern vs Western Atlantic (Hamner et al. 1994). These kinds of studies suggest that because these Jellies go “home” to reproduce, these populations may over time evolve into different species. This has happened with one group in the North-American Pacific, which has since been named a new a distinct species (Wrobel and Mills, 1998).
Hunting
So, what do they eat? I have already mentioned that they only have shorter finer tentacles, and that they drift in the water column. Due to their physiology and distribution, moon jellies are generalist feeders, meaning they are not adapted for one or a few prey species in particular (Costello and Colin, 1994). They are carnivorous and will eat any available zooplankton that they come in contact with in the water column.
Zooplankton are different to phytoplankton, but we often hear the umbrella term of “plankton” when talking about both. Phytoplankton are microscopic, photosynthetic organisms, while zooplankton are the part of the planktonic community which eat the phytoplankton and cannot photosynthesise. Zooplankton is an umbrella term for these organisms and includes copepods, crustaceans, and fish larvae, to name a few.
Moon jellies draw in prey with fluid motions while swimming, which brings the zooplankton in contact with their tentacles. A moon jelly will swim, creating this sort of microcurrent behind it in line with its body, which will effectively “suck” in any zooplankton that can't swim fast enough to escape it (Costello and Colin, 1994). As you can imagine, because of the way that they hunt, the specific species of zooplankton that they consume does tend to vary with their size, because their size determines the speed of the current they can create, and in turn the zooplankton they are able to catch. In other words, the bigger the jelly, the stronger the current, the faster the zooplankton they can eat.
Jellyfish Blooms
With warming oceans from human-induced climate-change, jellyfish blooms are becoming more and more common (Goldstein & Steiner, 2019). Jellyfish blooms are when a much larger amount of Jellyfish boom in an area, a lot like an algal bloom. These do not only apply to moon jellyfish, but they do have a significant socio-economic impact, affecting water activities, clogging fishing nets, and putting excessive pressure on fish species by consuming their larvae (Pitt and Purcell, 2009).
Scientists have been working to predict these blooms in response to different factors like water temperature. This is a difficult task because of their complicated life-histories as we have discussed, with both the polyp and medusa stages being important to take into account (Goldstein & Steiner, 2019).
Diving with Moon Jellies
You have probably already bumped into a few moon jellies on your Dublin Bay dives by now. It is always important to remember that while they do not give a stereotypical sting, it is always best to give whatever jelly passes you by it’s personal space, and enjoy watching them from a respectful distance.
Bibliography
Aquascope|Facts|Moon jellyfish|Other names. (n.d.). Retrieved 7 September 2025, from https://www.vattenkikaren.gu.se/fakta/arter/cnidaria/scyphozo/aureauri/aureaune.html
Costello, J. H., and Colin, S. P. (1994). Morphology, fluid motion and predation by the scyphomedusa Aurelia aurita. Marine Biology, 121(2), 327–334. https://doi.org/10.1007/BF00346741
Dawson, M. N., and Jacobs, D. K. (2000). Molecular Evidence for Cryptic Species of Aurelia aurita (Cnidaria, Scyphozoa) | The Biological Bulletin: Vol 200, No 1. https://www.journals.uchicago.edu/doi/abs/10.2307/1543089
Goldstein, J., and Steiner, U. K. (2019). Ecological drivers of jellyfish blooms – The complex life history of a ‘well‐known’ medusa (Aurelia aurita). https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.13147
Hamner, W.M., Hamner, P.P., and Strand, S.W. (1994). Sun-compass migration by Aurelia aurita (Scyphozoa): population retention and reproduction in Saanich Inlet, British Columia. Mar. Biol. 119: 347-356
Pitt, K. A., and Purcell, J. E. (2009). Jellyfish Blooms: Causes, Consequences and Recent Advances. Springer Science & Business Media.
Wrobel, D., and Mills, C. (1998). Pacific Coast Pelagic Invertebrates: a Guide to the Common Gelatinous Animals. Sea Challengers and Monterey Bay Aquarium, Monterey, CA.
