We don't call them jellyfish anymore, because they're really not fish at all. Jellies are weird, gelatinous animals. They are found in all of the world’s great seas and some are even found in fresh water.
How jellies reproduce
We can compare the jelly life cycle to that of the butterfly’s. Butterflies are caterpillars, which turn into pupa, which then later emerge as butterflies. Similarly, the jelly has polyps that transform into strobula, which bud off ephyrae (baby jellies), which then mature into jellies.
Generally, jellies swim around as male and female. The adult animals then release gametes (sex cells, like eggs and sperm) and these form planula – they look like furry Tic Tacs – that swim around for a few days until they find a place to settle and call home. Once a planula settles, it transforms into a polyp that stays attached to the sea floor for its entire life (unless it becomes dislodged by some external force). This polyp’s sole purpose is to eat and clone itself as much as possible. The clones can then later – when conditions become favourable (seasonal changes) or when conditions become unfavourable (red tides) – transform into strobulae that bud off dozens and dozens of baby jellies.
Jellies are record-holders
Jellies are some of the top animals in the oceans. Contrary to popular belief, the blue whale is not the longest animal in the ocean: it’s actually the lion’s mane jelly. Its tentacles can grow to 44m long, eclipsing the 33m length of the blue whale.
One of the most dangerous animals in the ocean is a jelly, too. The irukandji jelly is literally as small as a pinky nail but it causes one of the most pain-induced deaths. Yes, pain-induced: the venom itself does not tend to kill its victims; instead the side effects of the sting are what do the trick. Victims tend to die from severe pain, cardiac arrest and organ failure.
Jelly venom is also unique in the natural world. It is very different from most of the venoms that we are familiar with. It actually works exactly like the lethal injection. The venom floods the prey’s cells with potassium, causing the cells to short circuit. It's one of the only known naturally occurring venoms that stop the heart of its prey in a contracted state. So saying that “a jelly shocks you” when you are stung is technically correct.
How jellies eat
Every time a jelly pulses, it eats. It wisps food around its bell and through its curtain of tentacles. With the next pulse, the food that gets stung is flicked onto its oral arms or lips. The lips then carry the food into the central mouth. Jellies like upside downs and blue blubbers have thousands of tiny mouths on their oral arms that lead to a central gut.
Because jellies are constantly eating, they are constantly growing. When jellies don’t eat, they simply shrink, but can eat again to regain their lost body weight or regrow damaged body parts.
Jellies at the Aquarium
The Benguela compass jelly (Chrysaora fulgida)
This compass jelly is one of three that are endemic to our coast. It gets quite large. Its umbrella’s circumference can be 80cm across and from the top of its head to the end of its frilly oral arms, it can grow to just over 2m long. Its sting is only as bad as a bee sting.
The compass jelly goes through many colour changes as it matures, starting as maroon-coloured ephyrae, becoming transparent as it starts looking like a jelly, then becoming a light pink and developing maroon-pink compass markings once mature.
Swarms of these jellies can shut down power plants and mines, and cause great damage to fisheries and aquaculture all along the west coast of South Africa. They mainly eat other jellies (they are what’s known as “jellyvivorous”), as well as plankton and fish likes anchovy, horse mackerel and pilchard.
Upside down jelly (Cassiopea xamachana)
This jelly is commonly found in mangrove ecosystems and can tolerate a wide range of salinities. This species of jelly hosts photosynthetic algae, which provide most of the energy for the jelly’s day-to-day activities. These jellies mostly lie around and “sun tan”, and only ever move if disturbed or if they need to find a better spot to “tan”. These jellies can also eat food for energy, like other jellies.
Comb jelly (Beroe cucumis)
Comb jellies are not jellies at all, but their bodies are made up of a similar jelly substance. Instead of tentacles, these animals have teeth made of cilia, which they use to eat other comb jellies. Their mouths can stretch wide enough to swallow an entire comb jelly of the same size as them.
The pretty light show that these jellies give off, however, is not emitted from them. They have plates of cilia all along the supporting “spines” of their bodies that reflect and refract light, much like fast-moving prisms.
Amakusa or Malaysian jelly (Sanderia malayensis)
Amkusa jelly are jellyvivorous like the compass jelly. Because of their long and delicate tentacles and oral arms, they often become entangled up in each another. This is one of the few jellies that cannot be exhibited in a swarm. The Amakusa jelly has a sharp lingering sting and is restricted to warmer waters.
Box jelly (Carybdea branchi)
This species is quite interesting as it's restricted to colder water, whereas its cousins are mainly found in warmer waters. Box jellies are a group of animals known to have very well developed eyes and other sensory organs that help them distinguish between shapes, obstacles and movements. Because of their box shape, they are able to propel themselves in the direction they wish, allowing them to actively hunt through schools of fish and shrimp.
Their sting is very painful and can leave swelling and lesions comparable to that of a wasp. Reports of cardiac arrest have been noted but to date, no one along the South African coast has died from the sting of this jelly.
The blue blubber jelly (Catostylus mosaicus)
Because of its smooth texture, the blue blubber jelly is a popular ingredient in many Far Eastern dishes. This jelly group’s members come in many colours, including brown, maroon, cyan and white. They are very energetic swimmers, so they require lots of food to keep them going. Unlike many other jellies that have long tentacles around their bells, blue blubbers have thousands of tiny little mouths on their eight arms that help it catch and eat food. They are commonly found in river systems along our east coast but live happily in our warmer coastal waters as well.
Moon jellies (Aurelia sp.)
Moon jellies are one of the most widespread jelly species in the world. There are many variations of this species and new ones are still being added. They are one of the easiest jelly species to grow and keep, thus they are popular in aquariums. They have a mild sting.