Science Questions

Jelly fish swim in groups. How do they communicate to stay together?

Sun, 11th Oct 2009

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Question

Steve asked:

Jelly fish swim in groups. How do they communicate to stay together or do they communicate?

Answer

Helen -   I donít think they do actually.  Jellyfish are in some ways extremely simple creatures.  They donít have a brain, so they donít really have the ability to process inputs and sensory inputs like that.  So, I think usually when you see large numbers of jellyfish together, itís probably more likely to be, the fact that the currents and the ocean currents are actually moving them together and keeping them in similar places.  Or also, they can respond to things like the availability of food in the water and chemicals and things like that.  So possibly, they're all following food sources, and thatís why they're all ending up together.  But I donít think we yet have an idea that jellyfish can actually communicate to each other.  Although some of them do have quite complex eyes, which is quite exciting and box jellyfish have eyes.  They're actually quite...

Chris -   What do they do with them?

Helen -   Thatís a very good question.  They have eyes quite a lot like humans and in fact, some of the genes they have are very similar to human genes for creating parts of the eye, but we think that happens in parallel and wasnít from a common ancestor, but we arrived at the same solution to having eyes and what do they see?  We know they certainly respond to daylight, light and dark.  They need to know basically, what time of day it is because they tend to come up the water column at nighttime when they're less easily seen by predators and when itís light, they actually go further down the water column.  So, they respond to light and dark.  And even though they have quite complex eyes, itís actually a very good eye at detecting things like diffuse light to figure out, is it light or dark?  What time is it?  Should I be up or down in the water column?

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Steve asked the Naked Scientists: Jellyfish swim in groups. How do they communicate to stay together? What do you think? Steve, Sun, 6th Sep 2009

Jellyfish can, but don't always swim in groups.

Any degree of communication between jellyfish is likely to be by chemical means: the jellyfish will be producing and releasing various compounds into the water that can probably be sensed by other jellyfish, so it's probable that jellyfish have some idea of whether there are other jellyfish nearby, but I doubt that there's any meaningful communication between them.  Neither do I think that they deliberately group together for mutual defense, as with flocking birds and schooling fish.  With birds and fish it is the speed of the individuals that make the flocking/schooling tactic effective, by making it difficult for a predator to identify a single specific target.  Jellyfish just aren't fast enough for this to work.

I would guess that large groupings of jellyfish occur either because they've all detected the same concentration of food or because they all hatched in common region and, all reacting in the same way to the same common stimulus, haven't drifted apart yet.  If all the jellyfish in a large concentration are similarly sized it suggests that they are all the same age and have grown up together.  Conversely, if they are different sizes and species it would suggest that they have come from different places but have all detected the same source of food. LeeE, Sun, 6th Sep 2009

In addition to Lee's highly probable explanation, there is also the reproduction requirements. Jellyfish are both asexual and sexual reproducers. No mating takes place in sexual reproduction, the egg is fertilised by the sperm released into the sea. Proximity of the parents is therefore essential. Don_1, Mon, 7th Sep 2009

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