Jellyfish are amazing creatures. Gently floating along in the oceans and some fresh-water lakes, these gelatinous umbrella shaped blobs with their trailing stingy tentacles have outlived many other animals for almost 700 million years.
Perhaps their simplicity has helped in that feat. They do not have eyes, but some possess light-sensitive ocelli and sensory statocysts that help the jelly maintain its balance. They do not have a specialized digestive system, excretory system, respiratory system or circulatory system, nor do they have a central nervous system or skeletal system. Jellies ‘breathe’ through their skin by diffusion. Their food is taken up into a gastrovascular cavity, digested and the nutrients absorbed into the body. They are just blobs using a hydrostatic skeleton that controls the water pouch in their body to manipulate pulsating movements.
Yet being blobs has not prevented them from being vicious hunters. They hunt passively using their tentacles as drift nets for fish, crustaceans, plankton, fish eggs and other jellies. Jellyfish sting their prey using cnidocysts which, upon release, pierce the victim’s skin and inject venom. Sting effects can range from no effect to extreme pain and death depending on the species of jelly and victim.
Humans and jellies have an interesting relationship. In some Asian countries, jellyfish belonging to the order Rhizostomeae are being fished. Dried and served in soy sauce and sesame oil, they are a delicacy. Studying jellies for research can be very difficult, yet in 1961 a green fluorescent protein was extracted from a jelly and is now being used as a fluorescent marker in biomedical research all around the world. Blooms of jellies have devastated fish farms globally and caused power plants to be shut down out of fear of blockages in the water-intake pipes.
All of these jelly babies have quite a complex life cycle. Fertilized eggs develop into larval planulae covered with tiny hair-like cilia. It settles onto a firm surface and develops into a polyp. This small stalk has a mouth and some upward-facing tentacles feeding it continuously with whatever tiny creatures float past. This stage can last for years. The polyp reproduces asexually by budding, which produces free-swimming ephyrae. These swim away into the oceans and grow into adult medusae. The medusa can be as tiny as a few millimetres or as a ginormous 2 metres in bell diameter. They can live up to several months as fully grown adults, typically until the medusa has spawned a new generation. However, there is one pretty much immortal jellyfish, Turritopsis dohrnii, which can transform from a medusa back to the polyp stage when life gets tough as an adult.
The name jellyfish, little mortal, is quite misleading; they are neither vertebrates nor fish. Not even all jellyfish are jellyfish. The Portugese man-of-war is not actually a jellyfish, nor is it even an animal at all, but an entity formed of a colony of tiny animals called zooids. It is what is known as a siphonophore. The zooids are closely integrated, so that they are only able to survive together. There are at least three specialised types of polyps. Polyps dedicated to digestion and reproduction, polyps dedicated to the 50 metre long tentacles and the venom in them; and lastly, the polyps that make up the gas-filled bladder above the water. This bladder is said to look like an 18th century Portugese warship and is filled with carbon monoxide generated in a gas-gland. It can be deflated to allow the colony to sink out of harm’s way from its main predators tuna and sea turtles.
True jelly or just pretending, they are some fascinating creatures, ever so peacefully bobbing along in the oceans’ currents. Imagine being a jellyfish that could turn into a jelly baby whenever you get exam stress! Jellyfish are clever little floaty animals!