MICHELLEHAYWOOD
A model of efficiency
Michelle Haywood considers the movement techniques employed by jellyfish * as they navigate in search of prey
JELLYFISH ARE UBIQUITOUS IN OUR seas , ranging in size from a few millimetres up to the dustbin sized barrel jellyfish . The strange thing is that they are passive feeders . They don ’ t actively hunt their food , just stumble across it as opportunistic predators . The tentacles will catch anything that happens to drift or swim into them . Those tentacles have venom that kills the prey . Jellyfish are carnivores eating zooplankton ( the larval stages of many marine creatures ), fish eggs and small fish .
Once their prey is snagged on the tentacles , there is the challenge of ingesting and digesting it . After all , the purpose for eating anything is to take the biological molecules from your prey and use them for your own ends . Shorter oral arms pull the snared food towards the mouth . The mouth sits underneath the bell and is directly connected to the gastrovascular cavity or stomach . The stomach contains digestive enzymes which break down the structure of the prey and the molecules are absorbed into the cells of the stomach . The indigestible food is expelled via the mouth again .
There ’ s a bit of a problem with this approach . A jellyfish also uses its mouth for spitting out water to give it some propulsion .
Digesting food interrupts this ability . So , when a jellyfish is eating it can ’ t swim . That means digestion has to be a fairly fast process because a jellyfish can ’ t move on a full stomach .
Larger jellyfish may also have a poisonous sting that they shoot out . This is triggered by something touching the tentacles . Paralysing prey is a sensible approach because there is a real possibility that the prey tries to escape and causes damage . Its much better to have your prey immobilised as you digest it ( its how I like my food too – not wriggling !).
Jellyfish use some of the energy for propelling themselves along . In fact , they are the ocean ’ s most efficient swimmers . They are not as fast and manoeuvrable as other swimming creatures such as salmon , tuna or sharks , which rely on muscle mass , but they are incredibly efficient . The jellyfish body is slightly elastic and it uses this attribute to recapture energy as it moves . This means it uses less energy to travel a greater distance than any other swimming animal .
To swim , the jellyfish opens its bell and then creates jet propulsion by squeezing its bell to a smaller volume . When a jellyfish contracts its body , two vortex rings ( doughnuts of water rolling into themselves ) are created . The first ring is shed in the wake as the jellyfish moves . As the bell of the jellyfish relaxes , the second vortex ring rolls under the edge of the bell and spins faster . This sucks water in and pushes the centre of the jellyfish , giving it a speed boost .
Such a technique is very energy efficient , but only works for low speeds and fairly small body sizes . Jellyfish use approximately half the energy of any other marine creature to travel , so their cost of transport ( as measured by how much oxygen they use to move ) is really low .
Efficient movement means that there is more of the consumed energy available for growth and reproduction . So , there you have it , jellyfish may have no brain , only very small sensory cells on their tentacles and use their mouth as an anus , but they are the ocean ’ s most efficient swimming creature . Happily , their top speed is only a couple of centimetres per second ! �
* We know … they still aren ’ t fish !
A compass jelly drifting past Lundy Island in the Bristol Channel
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