An Introduction to Jellyfish




Sunday 3rd November 2019 marks World Jellyfish Day; where everyone should appreciate and celebrate one of nature’s most beautiful and mysterious creatures! With over 500 million years of fossilised history, jellyfish are one of the most ancient animals still in existence and come in an array of colours, shapes, and sizes.




Moreover, the name ‘jellyfish’ is actually a misnomer as, by definition, jellyfish are a free-swimming relative of corals and anemones, belonging to the phylum cnidarians. In terms of common characteristics, cnidarians are identifiable by their jelly-like bodies (which are over 90% water) and radial symmetry.






Jellyfish are complex creatures, with their lifecycles following several stages. Female jellyfish produce eggs which are fertilised by the males, developing initially into planula larva before transitioning into the polyp stage of development where they anchor to a rock or substrate area. Then, the polyp undergoes larval development, growing into an ephyra (essentially, a juvenile jellyfish) before reaching its adult medusa form.



The main features of a ‘true’ jellyfish include:

· A lack of a central nervous system

· A bell-shaped body

· Long, trailing tentacles which contain stinging cells known as cnidocytes that allow them to both entrap prey and protect themselves from larger predators such as sharks.


Other highly recognisable characteristics include their slow and rhythmic movements within water: whilst they are mostly carried by oceanic currents, they can also autonomously move by pulsating the simple tissue of their bodies to push ocean water behind them.


In terms of their diet, jellyfish are carnivorous, with their main food source being plankton (a diverse collection of smaller organisms including the larva of other fish and marine life); however, some species also feed on small fish.


With regards to keeping jellyfish in captivity, kriesel or cylindrical tanks tend to be the most effective solution. These tanks suspend the jellyfish within the water column, which is necessary to enable their locomotive abilities.





Jellyfish require a bit more care than your average saltwater aquarium inhabitant. A jellyfish aquarium requires a very stable eco-system with extra care taken with maintenance schedules, as they are very sensitive to sudden changes in water quality, e.g. pH levels, salinity and temperature and will not tolerate low levels of dissolved oxygen. Their anatomy also creates challenges, as they benefit from being fed more frequently than other aquarium fish, due to their simple stomachs being unable to store large volumes of food. Being planktivores they prefer to eat live foods such as mysids and newly hatched brine shrimp, they but will accept frozen marine fish food and dried powder food.


For Jellyfish systems we recommend the following pieces of equipment and method

- Heater and chiller to maintain a stable temperature and levels of dissolved oxygen.

- Refugium with bright LED light to export nitrates and phosphates as well create an area for plankton like food sources to breed and populate.

- A pair of good reliable return pumps for flow and water movement through the system.

- UV steriliser to reduce pathogens and algae spores.

- Dosing pump to frequently dose liquid food and minerals.

- 20% weekly water changes

- Feed at least twice a day



Jellyfish are commonly seen in many public aquariums and commercial venues like restaurants and bars, due to their unique appearances and aesthetic beauty. Here are five of the most popular species kept in captivity :


· Moon Jellyfish



· Cassiopea (upside-down jellyfish)




· Sea nettle jellyfish




· Spotted jellyfish




· Cannonball jellyfish



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