MARINE MEDUSAS

Did you know that jellyfish are made up of 95% water and some species boast 24 eyes of varying ability? Naturalist Georgina Jones reveals more about these fascinating and beautiful creatures.

Beach walkers coming across amorphous blobs of jellyfish washed up out of the ocean aren’t likely to think of imageforming eyes, symbiosis or energy efficient movement. Unless it’s associated with the plough snails enthusiastically scooting towards their prey. As it turns out though, some jellyfish have better eyesight than snails, and, apart from the inevitable exception to the group, the stalked jellyfish, have the most energy efficient movement in the animal kingdom.

Rather impressive for brainless animals that are almost entirely composed of water.

Jellyfish are made up of about 95% water and are related to corals, anemones and bluebottles. The standard body shape is a bell with tentacles, in various arrangements; and jellyfish mysteriously manage their lives without many of the physiological systems vertebrates depend on. Like blood, a heart or brains.

Jellyfish have a radial nervous system or nerve net, dispersed throughout their bodies. This can smell, respond to stimuli such as food or danger, and detects light. They use heavy crystals on stalks to ensure these eye spots are always oriented towards the sky. Some specialist species, though, have no less than 24 eyes of varied abilities. While most jellyfish have only simple eyes capable of distinguishing between light and dark, box jellies that live in mangrove swamps have eyes that are capable

of distinguishing colour and size. Some of their eyes always face upward toward the sky, others always face downwards, while still others function to avoid obstacles. All these different eyes work together to ensure the box jelly stays in the food-rich mangroves. Just how a creature without a brain manages this is still not well understood, but it may be that having differing eyes that perform only a few specific functions each may simplify information processing.

Most jellyfish, like anemones, and bluebottles, get most of their food by trapping and stinging prey in their tentacles. Some jellyfish, like the reef-building corals, have symbiotic algae called zooxanthellae living in their tissues, which provide them with nutrition that the algae generate through photosynthesis. The jellyfish in turn, provides the zoox, as they are known, with minerals that it absorbs from the sea water. These spotted jellies use their light and dark sensing eyes to stay in the sunlight during the day, ensuring that their zoox can maximise their time photosynthesising. That staying in the sun also minimises their likelihood of coming within reach of predatory anemones may be simple coincidence. Or possibly the jellies can also detect this danger.

Open ocean jellyfish have less mutually beneficial associations: pelagic goose barnacles may take up residence on their tentacles, benefiting from the plankton swept towards them. There are also several species of fishes that use the tentacles as a refuge, occasionally also nibbling on the jellyfish’s flesh. This refuge, however, depends on the little fishes constantly avoiding being stung by the tentacles, either by physical avoidance or else by fooling the jellyfish’s recognition systems by coating themselves with concealing slime. Should these strategies fail, the jellyfish will feast on its uninvited guests.

Long trailing tentacles trap and kill floating prey.

Comb jellies resemble jellyfish but have no stinging cells, a defining characteristic of the group.

Though they may seem fragile, jellyfish are in fact capable of living in both oxygen-depleted and relatively saline environments, unlike most other marine animals. Their stinging tentacles and accommodating digestive systems are capable of trapping and consuming a wide range of prey animals, from zooplankton to crustaceans and small fishes; some jellyfish will also consume planktonic microalgae.

Since they have only one body opening, their mouths function tidally, taking in food and ejecting wastes. Their mouths also function in their reproductive cycle.

Jellyfish have a rather complex life cycle. They are usually either male or female, and each sex will produce either sperm or eggs that are released according to light cues, usually at dawn or dusk. In most species, the sperm and eggs are released into the water, but some species take sperm into their mouths and so into the body cavity where it can fertilise the eggs. Jellyfish develop through several larval forms before changing into the classic adult jellyfish medusa structure.

Almost all jellyfish move using rhythmic pulsations of their bells. This form of movement is extremely energy efficient: muscular contraction of the bell generates a vortex, which propels the animal, and as the muscles relax, a second vortex is generated, further propelling the jellyfish with no extra expenditure of energy.

This form of locomotion is used by all jellyfish except the stalked jellyfish. They don’t use it because, unlike all other species of jellyfish, after a brief career as juveniles in the plankton, stalked jellyfish settle on substrates such as seaweed and kelp and live their lives, from a jellyfish perspective, upside down. Their bells are extended to a stalk which attaches to the seaweed and their mouths face upwards, surrounded by modified tentacles, resembling flowers, if carnivorous ones.

Box jellies typically have only four tentacles and cuboid bells.