Jellyfish Life Cycle
Scyphozoan life history from Olariaga et al., 2014.
Just by looking at a jellyfish it is not readily apparent how they can reproduce. Across jellyfish groups, there are very different reproductive strategies. Most jellyfish species are either female or male, although some species are hermaphroditic. The majority of jellyfish switch between a sexual and an asexual stage during their life cycle, which is split between drifting and settled stages .
The most familiar reproduction and lifecycle is from the "true jellyfish" (Scyphozoans), though it should be noted that many jellyfish species reproduce by other means or may skip steps in the process described here . The majority of jellyfish species gather together and the males will release sperm into the water for females to capture and fertilize their eggs . This is called "broadcast spawning".
This forms large numbers of free-swimming planula larvae which will disperse and settle on a suitable hard surface. Ironically, human impacts on the environment through dock-building and other floating surfaces (with a noted preference for synthetic plastics like EPS foams and polyethylene) provide much suitable habitat for planula to settle . After settling, the planula will develop into polyps, comprised largely of tentacles, a mouth and a gut, resembling a small sea anemone. These are able to feed and grow at a rate that environmental conditions allow . There is potentially a lot of inter- and intra-specific competition for space, so polyps will use all the methods at their disposal to defend their area - high rates of sexual reproduction, gregarious settlement, expansion by cloning, and stinging .
Polyps can survive in place for long periods of time, several months to years, allowing them to outlast non-favorable environmental conditions . Once conditions do become favorable, they can clone themselves and start budding off either a single or many immature jellyfish known as ephyra larvae which will grow into mature jellyfish. This can happen through a process called strobilation in which the body of the polyp develops fissures that segment the polyp and release the ephyra into the water to develop into mature jellyfish .These adult medusae are then able to start the cycle again.
1. Gershwin, L. (2016). Jellyfish: A Natural History. The University of Chicago Press.
2. Helm, R.R. (2018). Evolution and development of scyphozoan jellyfish. Biological Reviews, 93(2), 1228–1250. https://doi.org/10.1111/brv.12393
3, Hoover, Richard A., Purcell, Jennifer E. (2009). Substrate preferences of scyphozoan Aurelia labiata polyps among common dock-building materials. Hydrobiologia 616:259–267. DOI 10.1007/s10750-008-9595-6
4. Lucas, C., Graham, W., & Widmer, C. (2012). Jellyfish Life Histories: Role of Polyps in Forming and Maintaining Scyphomedusa Populations. Advances in Marine Biology, 63, 133–196. https://doi.org/10.1016/B978-0-12-394282-1.00003-X
Polyp and planula: Helm R.R., Dunn CW (2017). Indoles induce metamorphosis in a broad diversity of jellyfish, but not in a crown jelly (Coronatae). PLoS ONE 12(12): e0188601. https://doi.org/10.1371/journal.pone.0188601
Life cycle: Olariaga, A., F. Guallart, E., Fuentes, V., López-Sanz, A., Canepa, A., Movilla, J., Bosch-Belmar, M., Calvo, E., & Pelejero, C. (2014). Polyp flats, a new system for experimenting with jellyfish polyps, with insights into the effects of ocean acidification. Limnology and Oceanography, Methods, 12, 210–220. https://doi.org/10.4319/lom.2014.12.210
Since jellyfish are made up of approximately 95% water (the other 5% are proteins and amino acids), it is very easy to damage jellyfish. Using nets can cut them into pieces, while using hands can expose you to their stinging cells. Some jellyfish are able to regenerate damage using different stem cells and specific cell types . Other jellyfish can simply re-orient other body parts to compensate for lost function instead of putting resources into regeneration . This simplicity and flexibility has enabled jellyfish to survive unchanged for the last 500 million years! 
Image from Cronodon.com
Image from Cronodon.com
Due to the simplicity of their construction, jellyfish have a wide range of largely symmetrical body shapes and sizes generally consisting of six basic parts:
Epidermis - the outer layer just like in human skin
Gastrodermis - the inner layer protecting the inner cavity of the jellyfish
Mesoglea - a layer of collagen fibers that provide structure between the epidermis and gastrodermis
Gastric cavity - this cavity digests food, transports nutrients, and eliminates waste
Mouth - the orifice that can take in food and eliminate waste
Tentacles - for catching prey and moving food to the mouth
Do jellyfish have a central nervous system?
Unlike human beings who have a central nervous system, jellyfish have several radially distributed diffuse overlapping nerve nets. The first component of their nervous system is the rhopalia or the sensory system providing sensory information (gravity, light, etc.) typically found around the margins of the bell. The second component is the motor nerve net that helps them to swim by contracting their bell. The third component is the diffuse nerve net that provides other sensory information to the jellyfish . There is an argument to be made that the basic structure of a central nervous system exists in jellyfish .
1, Katsuki, T., & Greenspan, R. J. (2013). Jellyfish nervous systems. Current Biology, 23(14), R592–R594. https://doi.org/10.1016/j.cub.2013.03.057
2. Abrams, M. J., & Goentoro, L. (2016). Symmetrization in jellyfish: Reorganization to regain function, and not lost parts. Zoology (Jena), 119(1), 1–3. https://doi.org/10.1016/j.zool.2015.10.001
3. Jarms, G. & Morandini, A.C. (2019). World Atlas of Jellyfish. Dölling und Galitz Verlag, 816p.
4. Satterlie, R. A. (2011). Do jellyfish have central nervous systems? Journal of Experimental Biology, 214(8), 1215–1223. https://doi.org/10.1242/jeb.043687
Jellyfish locomotion video by Wired.
It is commonly thought that jellyfish were completely planktonic, subject to the currents and winds, however more recent research has disproved that. Jellyfish can actively detect and orient themselves to move beneficially through current conditions . Especially for larger jellyfish, they can be strong enough to swim against slower currents. Ocean fronts, thermoclines and haloclines have all been shown to affect jellyfish movements . All of the above have effects on the formation and maintenance of jellyfish blooms.
Jellyfish use their motor nerve net to contract their bell in pulses providing movement. The video to the left from Wired demonstrates how the jellyfish bell creates low pressure zones to force water movement to propel the jellyfish. They create "walls" that they can then push off of to move through the water, making them one of the most efficient swimming animals .
1. Fossette, S., Gleiss, A. C., Chalumeau, J., Bastian, T., Armstrong, C. D., Vandenabeele, S., Karpytchev, M., & Hays, G. C. (2015). Current-Oriented Swimming by Jellyfish and Its Role in Bloom Maintenance. Current Biology, 25(3), 342–347. https://doi.org/10.1016/j.cub.2014.11.050
2. Graham, W. M., Pagès, F., & Hamner, W. M. (2001). A physical context for gelatinous zooplankton aggregations: A review. In J. E. Purcell, W. M. Graham, & H. J. Dumont (Eds.), Jellyfish Blooms: Ecological and Societal Importance (pp. 199–212). Springer Netherlands. https://doi.org/10.1007/978-94-010-0722-1_16
3. Gemmell, B. J., Costello, J. H., Colin, S. P., Stewart, C. J., Dabiri, J. O., Tafti, D., & Priya, S. (2013). Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans. Proceedings of the National Academy of Sciences, 110(44), 17904–17909. https://doi.org/10.1073/pnas.1306983110
Jellyfish Senses (Jarms & Morandini, 2019)
The rhopalia of Tripedalia cystophora.
(Skogh et al., 2006)
Throughout their life cycle, jellyfish have different levels of sensory input mechanisms capable of detecting chemicals, vibrations, touch and pressure. Though polyps and most simple planulae lack obvious sense organs, they are able to react to light and other stimuli. Polyps are able to catch prey, so have some sensory mechanisms on their tentacles. Planulae have to be able to find a place to settle, so must have sensory capabilities as they change.
The final stage of the life cycle, the medusa phase, has the best developed sensory organs. As one part of their nervous net, there are sensory receptor cells on the outer layer of jellyfish that use cilium to receive external stimuli from their surroundings. The scyphozoan sense organs for stimuli like gravity (statocysts) and light (ocelli) are concentrated in the rhopalia along the edge of the medusa's bell. Some species of cubozoans have well-enough developed optical organs it is thought that they are capable of hunting their prey.
The statocysts are gravity-sensing organs that have a mineral statolith crystal inside. As the statolith moves around the statocyst, the jellyfish is able to orient itself with respect to gravity.
The photoreceptor areas are for sensing light. These organs vary greatly across Scyphozoa in size and shape. The cubomedusae have the greatest ability to recognize their environment, so could be capable of hunting their prey, in conjunction with their locomotion abilities.
Image from (Brusca & Brusca, 2003)
1. Jarms, G. & Morandini, A.C. (2019). World Atlas of Jellyfish. Dölling und Galitz Verlag, 816p.
Benefits of Jellyfish
Commonly thought of as pests, jellyfish actually benefit human and ecosystems in numerous ways.
One of the most well-known products from jellyfish is the Green Fluorescent Protein (see video), which allows scientists to see cellular and molecular events inside living cells . Jellyfish toxins are being investigated for anticancer compounds and as antioxidant nutritional supplements . In traditional and modern Chinese medicine, jellyfish are believed to cure arthritis, hypertension, back pain, ulcers, tracheitis, asthma, burns, and relieve fatigue . Jellyfish collagen is a cheaper, biosafe method of cartilage regeneration.
See this video from Fig. 1 by University of California about the scientific use of jellyfish bioluminescence.
Characteristic of marine communities, the benefits jellyfish can give depend upon the size of the jellyfish biomass, up to a certain point, after which there is little benefit. Conversely, the impacts of jellyfish do not scale the same way, so can have an outsized effect of ecosystems .
Jellyfish can be a strong driver of carbon sequestration, through their feces and through the dropping of their dead bodies to the ocean floor . Numerous benthic organisms can gain a large benefit from jellyfish falls through scavenging or predation . Jellyfish also provide an alternate energy pathway for nutrient cycling by being able to eat up and down the food web in ways outside the traditionally conceived food chain (smaller animal eaten by bigger animal) [1,5]. Jellyfish movements up and down the water column may also assist nutrients' movements between different layers of the ocean . The costs of jellyfish impacts are likely greater than the benefits that jellyfish give to ecosystems, and increasing at a faster rate .
The cultural services provided by jellyfish can be worth millions of dollars in tourism revenue in places like Jellyfish Lake, Palau and entrance fees at exhibits and aquariums, like Monteray Bay Aquarium in California, USA . Jellyfish provide a fascinating glimpse into the mysteries of the ocean and inspire art across the world.
Jellyfish consumption by humans by Goldthread.
Jellyfish have been eaten by humans (see video), particularly in Asia, for over 1,700 years, and can be important part of fishing economies . Nineteen countries, mostly in Asia, currently harvest at least ten species of jellyfish considered edible, with Rhopilema esculentum being the most important species [1,7]. Jellyfish production was valued at over US$100 million in 2005, though due to large market fluctuations, the economic viability of jellyfish production is in doubt .
Jellyfish has low nutritional value, so is a low-calorie food, because they are made of mostly water and protein . You can find a jellyfish recipe here, though there is a potential risk from consuming too much alum, which is used in the preparation of jellyfish .
Doyle, T. K., Hays, G. C., Harrod, C., & Houghton, J. D. R. (2014). Ecological and Societal Benefits of Jellyfish. In K. A. Pitt & C. H. Lucas (Eds.), Jellyfish Blooms (pp. 105–127). Springer Netherlands. https://doi.org/10.1007/978-94-007-7015-7_5
Graham, W. M., Gelcich, S., Robinson, K. L., Duarte, C. M., Brotz, L., Purcell, J. E., Madin, L. P., Mianzan, H., Sutherland, K. R., Uye, S., Pitt, K. A., Lucas, C. H., Bøgeberg, M., Brodeur, R. D., & Condon, R. H. (2014). Linking human well-being and jellyfish: Ecosystem services, impacts, and societal responses. Frontiers in Ecology and the Environment, 12(9), 515–523. https://doi.org/10.1890/130298
You, K., Ma, C., Gao, H., Li, F., Zhang, M., Qiu, Y., & Wang, B. (2007). Research on the jellyfish (Rhopilema esculentum Kishinouye) and associated aquaculture techniques in China: Current status. Aquaculture International, 15, 479–488. https://doi.org/10.1007/s10499-007-9114-1
Ates, R. M. L. (2017). Benthic scavengers and predators of jellyfish, material for a review. Plankton & Benthos Research, 12(1), 71.
Gershwin, L. (2013). Stung: On Jellyfish Blooms and the Future of the Ocean. The University of Chicago Press.
Omori, M., & Nakano, E. (2001). Jellyfish fisheries in southeast Asia. Hydrobiologia, 451, 19–26. https://doi.org/10.1023/A:1011879821323
Brotz, L., & Pauly, D. (2017). Studying jellyfish fisheries: Toward accurate national catch reports and appropriate methods for stock assessments. In Jellyfish (pp. 313–329). Nova Science Publishers, Inc.
Dong, Z., Liu, D., & Keesing, J. K. (2010). Jellyfish blooms in China: Dominant species, causes and consequences. Marine Pollution Bulletin, 60(7), 954–963. https://doi.org/10.1016/j.marpolbul.2010.04.022
Wong, W. W. K., Chung, S. W. C., Kwong, K. P., Yin Ho, Y., & Xiao, Y. (2010). Dietary exposure to aluminium of the Hong Kong population. Food Additives and Contaminants, 27(4), 457–463. https://doi.org/10.1080/19440040903490112
Although jellyfish are commonly seen as simple creatures preyed upon by others, they are much more complex than most people think. Species have been shown to be capable of sun compass navigation, daily vertical migration, to avoid predators and respond to environmental cues (salinity, temperature, light, chemicals) . Despite being seen as simple planktonic creatures, some species of jellyfish are capable of active predation and navigation .
The role jellyfish play in the marine ecosystem is complex as they can affect all levels of the food web from microplankton to top predators, feeding on everything they can catch with their stinging tentacles. Previously they were thought to be primarily prey for turtles and fish, though more recent research shows that over 120 species of fish eat jellyfish . This also overlooks their role as strong predators . Jellyfish diets can be as varied as they are, consuming fish, zooplankton, crustaceans and even other jellyfish! Jellyfish can not only eat prey, but they can also eat the larvae of their predators and what they eat, too. This compounding impact on predators and prey helps them take over ecosystems . The newer focus on their wider role has prompted researchers to include jellyfish in modeling of marine ecosystems .
Turtle eating a jellyfish by National Geographic.
Jellyfish can also be part of symbiotic relationships, whether providing protection for older fish from predators or acting as a nursery for younger fish [7,8]. Some species of jellyfish have a symbiotic algae called zooxanthellae that produce food through photosynthesis for the jellyfish while it provides them protection . Even when dead, jellyfish have impacts upon trophic webs. Their decomposing bodies, for example in large quantities after a bloom, can increase nutrient loads, decrease available oxygen and impact macrobenthic communities .
1. Katsuki, T., & Greenspan, R. J. (2013). Jellyfish nervous systems. Current Biology, 23(14), R592–R594. https://doi.org/10.1016/j.cub.2013.03.057
2. Garm A, Oskarsson M, Nilsson D-E (2011) Box jellyfish use terrestrial visual cues for navigation. Current Biology 21:798–803
3. Pauly, D., Graham, W., Libralato, S., Morissette, L., & Palomares, M. L. D. (2009). Jellyfish in ecosystems, online databases, and ecosystem models. Hydrobiologia, 616, 67–85. https://doi.org/10.1007/s10750-008-9583-x
4. Acuna, J. L., Lopez-Urrutia, A., & Colin, S. (2011). Faking Giants: The Evolution of High Prey Clearance Rates in Jellyfishes. Science, 333(6049), 1627–1629. https://doi.org/10.1126/science.1205134
5 Gershwin, L. (2013). Stung: On Jellyfish Blooms and the Future of the Ocean. The University of Chicago Press.
6 Wright, R., Le Quéré, C., Buitenhuis, E., Pitois, S., & Gibbons, M. (2020). Unique role of jellyfish in the plankton ecosystem revealed using a global ocean biogeochemical model. Biogeosciences (preprint). https://doi.org/10.5194/bg-2020-136
7. Graham, W. M., Gelcich, S., Robinson, K. L., Duarte, C. M., Brotz, L., Purcell, J. E., Madin, L. P., Mianzan, H., Sutherland, K. R., Uye, S., Pitt, K. A., Lucas, C. H., Bøgeberg, M., Brodeur, R. D., & Condon, R. H. (2014). Linking human well-being and jellyfish: Ecosystem services, impacts, and societal responses. Frontiers in Ecology and the Environment, 12(9), 515–523. https://doi.org/10.1890/130298
8. Riascos, J. M., Aguirre, W., Hopfe, C., Morales, D., Navarrete, Á., & Tavera, J. (2018). Floating nurseries? Scyphozoan jellyfish, their food and their rich symbiotic fauna in a tropical estuary. PeerJ, 6, e5057. https://doi.org/10.7717/peerj.5057
9. Jarms, G. & Morandini, A.C. (2019). World Atlas of Jellyfish. Dölling und Galitz Verlag, 816p.
10. Guy-Haim, T., Rubin-Blum, M., Rahav, E., Belkin, N., Silverman, J., & Guy, S.-V. (2020). The effects of decomposing invasive jellyfish on biogeochemical fluxes and microbial dynamics in an ultraoligotrophic sea. Biogeosciences, (preprint) 38. https://doi.org/DOI: 10.5194/bg-2020-226