“Jellyfish? See colours? That’s crazy talk! They’d need eyes to do that! They don’t even have brains, do they?”
Some jellyfish do have eyes to go along with their well-developed central nervous system. These are box jellies, which are generally better known because some of their number contains rather deadly toxins. If that wasn’t scary enough, they have eyes. In fact, they have lots of eyes.
Rhophalia are sensory structures that you can see as black dots about two thirds of the way down the bell (pictured right). Close up, they look like part of an old-school movie monster (pictured below). Each rhophalium has six, count ‘em, six, eyes. Four of the eyes are quite simple, but two have lens eyes, just like vertebrates and cephalopods. These eyes are fairly crummy by human standards – they’re blurry and probably pretty low resolution – but they do the job well enough.
To see colour, you need to tell different wavelengths of light apart, and to do that, you need at least two different visual pigments. (Humans have three.) O’Connor and colleagues went looking for different visual pigments in Chiropsella bronzie (pictured above right). This involves firing light of all different colours at the eye, and looking to see how much is absorbed at each wavelength. They found only one absorbance peak – i.e., lights of one particular wavelength alone were being absorbed.
They also tried using labels for visual pigments taken from zebrafish. Zebrafish have at least six different visual pigments, and these molecules are often very similar across species. Again, only one of the five different immunolabels that they tried reacted with the cells in the largest lower camera eye.
All of which indicates that these animals are only seeing black and white – at least through their largest eye. I suppose it’s possible that they might have different pigments in different eyes, but if they don’t have multiple pigments in their largest eye, it seems unlikely they’d have them in their smaller eyes.
A side finding was that the jellyfish visual pigments “bleach,” meaning some chemical bonds are broken in the molecules. Most invertebrate visual pigments do not do this, but vertebrate pigments do. Thus, it’s another way that jellyfish eyes are resemble vertebrate eyes (camera eyes with bleaching pigments) more than most other invertebrate eyes (compound eyes with non-bleaching pigments).
A previous study had suggested that a box jellyfish might see colour, but that was a different species (Carybdea marsupialis). Maybe there’s some real diversity in the visual abilities of these different jellies, and some can see colour. But it seems that C. bronzie lives in a world of gray.
O'Connor, M., Garm, A., Marshall, J., Hart, N., Ekstrom, P., Skogh, C., & Nilsson, D. (2010). Visual pigment in the lens eyes of the box jellyfish Chiropsella bronzie Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2248
Rhophalia picture from here.