According to studies. It isn't for homework so don't bash me with complaints.

For this kind of questions it would be advantageous to provide the source.

To my knowledge there is only one group of jellyfishes that actually have the ability to sense colour (or possess anything but the most rudimentary eyes). So not all jellyfish are able to react to colour at all.

If those indeed react adversely to a colour, then it is likely that something that something reddish in nature might harm them and that a negative taxis to is likely an adaptation to harm. Out of my head I do not know any example as I actually forgot which jellyfish is actually able to sense colour.

Jellyfish have a lot of visual sensors, iirc, and red may be a bit overwhelming to them. I don't know for sure.

 

Perhaps blood in the water is a visual signal for a jellyfish to vacate the area before predators like sharks begin arriving.

 

Red and yellow can sometimes be a signal that an animal is poisonous. I'm not sure if jellyfish need to worry about other sea creature's poison but this could account for the aversion.

Turtles eat jellyfish. Most other creatures avoid them. Turtles are green or brown.

 

Some jellyfish are red. It must be hell for them.

Thanks.

I just thought of something else. If jellyfish are being scared by the color red, it would have to be at surface depths. Water absorbs red light so at a certain depth red isn't even visible.

 

Maybe the jellies are only affected when they get closer to the surface. Where exactly did you find that jellyfish are scared of red, CrazCo?

Most jellyfishes are only able to distinguish between light and dark. AFAIK only box (or something similiarly named) Jellyfishes actually have colour sensors.

Also they are only useful near the surface anyway.

http://starbulletin.com/2005/07/10/features/memminger.html