With strangely shaped bodies that can, when necessary, be effective weapons, jellyfish are the beauties of the marine world, as mysterious as they are transparent.

It was pale blue with occasional glints of white. Mute and diaphanous. Pale, transparent and utterly without sound, appearing and disappearing in a ghostly ballet in the blue waters, dark at the surface, turning a darker shade in the depths. Scattering its magical rays in every direction in the water’s darkling blue, it danced Prokofiev’s Cinderella, or Alexander Gorsky’s Paris is Burning. Nearing the surface, it briefly assumed a cloudy golden hue under the sun’s rays, before again plunging silently into the depths, leaving behind tiny bubbles in the water, and in their wake its tentacles waving in every direction—like hair rippling in the water. Long, fine hair, continuously undulating. Like the thousand-fold snaky locks of the much feared Medusa of antiquity. And for that reason, in other words, because its tentacles have been compared to the locks of that dread figure of Greek mythology, these fascinating sea creatures are also known to zoologists, perhaps not undeservedly, as medusae.
I speak of jellyfish. Those mute members of the marine kingdom, as mysterious as they are transparent, with their amazing body shapes which they can, when necessary, employ as effective weapons, and their myriad other characteristics. Jellyfish may strike fear in those unfamiliar with them; and sometimes they are perceived as only a furtive and enigmatic shadow. But we share the same ecology and therefore we need to know them, these creatures that accompany as we glide downwards in the blue-green waters.

There are two body types among jellyfish in general. One is the medusa, which takes its name from Greek mythology, the other the polyp-shaped variety. The former are bell-shaped and swim freely in the water. The latter, being attached to a rock or to the sea bottom, have more limited mobility. Frogmen occasionally witness the ballet of these large jellyfish in the sea’s depths. But jellyfish lack the ‘rigid spine’ of other mobile creatures. How then are they able to move? The jellyfish body consists of two layers, an ectoderm and an endoderm, the latter containing the digestive and respiratory systems. These two layers are separated by a jelly-like layer called the mesoglea, which comes under pressure when the other two rub against each other, causing water to spurt out the jellyfish’s mouth, a bit like a jet engine. Afterwards the animal resumes its original shape. In other words, the rhythmic movements of jellyfish underwater are the result of this alternating contraction and relaxation.

Intake of nourishment and expulsion of waste are both performed through a single aperture in the jellyfish’s body. So how does the jellyfish capture its prey? Most jellyfish, a species that has been around for hundreds of thousands of years, possess a perfect weapon unique to members of only one of nature’s phylla, the Cnidaria, to which the jellyfish also happens to belong.
This venomous weapon, known as a nematocyst, is found inside a specialized cell structure known as a cnidocyte in the jellyfish’s tentacles. Thanks to its toxin, a jellyfish can capture even rather large prey. Waiting poised inside the cnidocyte, it is triggered by the prey, which it then zaps like an arrow, causing instant paralysis. The jellyfish’s sting can pose a threat to humans as well. A species of jellyfish called the Physalia, for example, which lives in the tropical Pacific, has nematocysts so potent as to be able to paralyze a human within seconds with fatal outcome. Some species of jellyfish are observed to live in colonies. Polyps and medusae of varied characteristics come together to form a single individual; alternatively, whole colonies of only polyps may be encountered. The individuals that make up the colony function like the organs of a single creature, in a division of labor with different groups responsible for mobility, nourishment and reproduction.

There is another species of jellyfish which resembles the others in shape but is actually quite different. These are the so-called ‘comb jellies’. Known scientifically as ‘ctenophora’, this variety exhibits eight rows of comb-like plates; hence its name. Comb jellies lack the nematocysts of their fellow jellyfish, but don’t be deceived! If you decide to touch one some day thinking, so what, it doesn’t have nematocysts, don’t blame me if you end up with a serious skin rash! For they are clever little beasts that ingest the other species, concealing their toxic tentacles to use to their own advantage when the time comes.
And before I forget: the phenomenon known as bioluminescence can be observed in most jellyfish and ctenophores. But the only way to experience this spectacular display of light and color is to dive with them to the depths of the sea.

Are up you for it?