Two part question really,
is it true that Jellyfish have eyes but no brain. I have no particular knowledge in the area, just what i have been told by a friend.
if the above is true are there any theories out there that someone could explain here or link to as to how this set up works. having done a quick google search i’m aware there is no definate answer, but still i am interested to hear the theories if there are any.
thanks
Cubozoans, which are probably jellyfish depending how you define the term, do indeed have eyes, complete with lense, retina, iris etc. But no brain.
You mean how they manage to co-ordinate the eyes with muscle movements? Nobody has any freakin’ idea. And oddly this is probably the only case in biology where not having clue hasn’t led to inifnite theories. Ive never even seen anyone speculate on it.
In humans the eyes are not seperate from the brain, but extentions of the brain from how I understand it. If so, and the eyes are basically simular, then by default the jellyfish must have a brain if it has eyes.
In chordates (such as humans) the eyes are developed from light sensitive nerves off the major nerve bundle. The major nerve bundle is the brain.
In mollusks (such as scallops or cephalopodes), however, the eyes are developed from light sensitive patches on the skin. So there is at least one way to make functional eyes without needing a brain.
It has been a very long time since I thought about Cnidarian anatomy, but I seem to recall that most of them have a neural net rather than a centralized ganglion system. As their only “muscular” action is to contract, and they seem to contract uniformly over their entire body, the eyes are probably useful to keep them upright and pulsing in the direction of the surface.
Note: the foregoing contains large proportions of Wild Assed Guess.
aha, thanks for the replies so far. to my non technical mind a neural net is just a brain by a different name, is that a very incorrect statement?
also am I understanding it right that essentially eyes are just light sensors with varying degrees of complexity depending on species? I dont want to get too sidetracked from the jellyfish question but does that mean assuming you could wire them up different species could use other species eyes?
I’m not an expert in biology, but the brain is part of the central nervous system, and chordates also have a peripheral nervous system. The CNS collects data from the PNS, then sends out instructions to it. (E.g., your brain receives data telling it where a ball is, then tells the muscles in your arms and hands where to move to catch the ball).
Some work goes on the the PNS, e.g., your finger touches something painful, and your arm moves the finger away before the brain has received the data. But apparently jellyfish just have a neural net which corresponds with the PNS, and no CNS.
here is a good discussion on jellyfish eyes and brains. The main problem with studying jellyfish is keeping them alive
http://pharyngula.org/index/weblog/comments/jellyfish_eyes/
ps while jellyfish do not have a centralised brain, they do have a loose nervous system that allows some coordination
Mordib
The planarian has eyespots. These are two bundles of light-sensitive nerves. There are cephalopods with these eyespots in two pits. OTTOMH, I can’t think of an example with pits covered by a clear membrane. Put an iris on that membrane and you’ve got eyes just like ours. Then, there are cat and dog type animals with a reflective membrane on the back of the eye to reflect more light to the retina. The best eye belongs to a type of shrimp (or was it a crayfish?). Better vision than ours, one segment is covered for seeing below water, and one sees in the ultraviolet.
Semi-Hijack
While we’re on the subject of jellyfish, could somebody clear things up for me regarding jellyfish being colony creatures, and the floats, tendrils etc being seperate organisms?
You mean a Porteguese Man O’ War ? They are indeed colony creatures. Here’s a mention.
So the eyes just trigger reflex reactions?
While that would be perfectly accurate for most purposes, unfortunately in this case it’s too oversimplified to draw any such conclusion.
To expand on what Dr. Fidelius said…
In chordates the optic nerve and retina develop from outgrowths of the neural tube, which at that stage has nothing remotely like a brain attached to it. But the lens and various other parts of the eye are derived from the ‘skin’.
In molluscs the eye is derived from the ‘skin’, but the nerve chord still grows out meet it to produce the required neural components.
To add to the confusion the eyes of insects are formed from ‘skin’ in a completely different manner, and don’t even include any neural tissue.
What makes all this so astounding is that precisely the same genes regulate eye development in all groups. A mouse breed that is genetically eyeless can be ‘cured’ by splicing in the one or more eye development genes from an insect, and vice versa.
What that suggests of course is that there is just one basic way to make a triploblast eye, and it evolved once. A lot of tinkering has gone into the basic design since then but insect and mollusc and chordate eyes are all homologous structures at least in their fundamentals. IOW we can conclude that you never need a brain to make an eye, but if you do have a brain then you can tack that on to eye development as kind of front end.
So no, if you have eyes you definitely don’t need a brain, not even if you are a triploblast. Jellyfish aren’t event hat advanced. So they definitely don’t; need a brain for eye development, which is just as well because they don’t have one.
Jellyfish don’t simply contract over their whole body. They can control their muscles just as any other animal can and as such are capable of limited directional swimming. This is taken to the extreme by cubozoans which are active predators. I know that seem like a bizarre concept when you fist see it applied to a jellyfish, but if they see a fish they will actively swim towards it, which is presumably why they evolved such complex eyes in the first place. At least, they certainly wouldn’t need such complex eyes just to detect which way the surface is and to remain upright. They have statocysts which can do the perfectly well, far better than the eyes can mange it.
Not in any way. The neural net of the jellyfish is just what it sounds like: a literal net of nerves that crisscrosses the bell. It could never justifiably be called a brain. A brain may justifiably be called a neural net, but it is highly compact, not dispersed as in the jellyfish. IOW brain = neural net. Neural net =/= brain.
Not a chance. You could probably manage this for closely related animals, so a human could probably use a mouse eye or even a lizard eye. But once you get away from that there’s no hope at all. The eye itself is only a tiny part of vision. Most of it occurs within the brain and is concerned with interpreting the input. The human brain simply has no way of interpreting the signals from an insect eye. We’d probably know the eye was there but all we’d get would be a varying tingling sensation, or a sensation of cold or something equally meaningless.
To put it another way, a radio telescope and radio antenna are both just light sensors with varying degrees of complexity. But we can’t wire a microwave antenna to your TV no matter how hard we try. Your TV simply lacks the hardware to make any sense of the signals form a radio telescope. You might get static but there is no way of rendering the output into any form that makes sense to the rest of the system.
Jellyfish are not, no. Jellyfish are individual animals just like you and me.
However there are related creatures called colonial hydrozoans, and they are pretty much what you describe. The most common example of these are the portugese man’ o’ war and similar creatures. The float is one animal, the tentacles are several more individuals, the ‘stomach’ is made up of several more and then there are several gonad individuals.
When I say ‘related” to jellyfish, the relationship is very distant, about as distant as between a human and a sea squirt. Colonial hydrozoans are recently descended from an animal that looked more like a sea anemone than jellyfish.
No. Reflex reaction is meaningless when talking about an animal with no brain and no central nervous system.
Sing it!
Tunicate girl won’t you come out tonight,
Come out tonight,
Come out tonight!
Tunicate girl won’t you come out tonight,
And spawn by the light of the Moon!
(Sorry. It has been a very long week at work…)
Please clarify.
The only additional information you gave is that a brain is “highly compact.”
Functionally, a network of “neurons” with input/interconnections/output could be called a “brain” whether compact or not, right?
Is the problem with the jellyfish that the network of cells are nerves but not neurons?
More importantly, jellyfish have eyes without a face.
/How’s about a date?
//Got no human grace.
///:: Ducks hurled slimeballs. ::
Then how can they be considered members of a colony instead of organs in an organism?
That about sums it up. I could also have added “situated anterioriorly” and "non-sensory” but that’s only 99% accurate.
Not really. The term brain as applied to non-vertebrates is kind of hazy around the edges because of course brains didn’t evolve spontaneously from nothing. But one of the defining features of a brain is that it is it be a compact mass of neurons. Compact itself is of course a relative term, but we all know it when we see it. If you look at this picture of the human nervous system and compare it to this picture of a cnidarian neural net it should be quite clear. Although there are neurons throughout the human body they are quite clearly far more densely concentrated in the head.
A network of neurons with input/interconnections/output doesn’t qualify as a brain. If it did then you I could say there was a brain in your finger, and your eye, and your arsehole… All parts of the body have neurons with input, interconnections and output. That alone does not a brain make.
A brain needs to be a clearly concentrated collection of neurons. It also needs to be dedicated exclusively or primarily to processing rather than sensation or direct innervation of muscles.
IOW a brain needs to be what you think of when someone says brain. It needs to be a dedicated organ for dealing with information. A jellyfish has nothing even approaching that. Even flatworms have more of a brain than jellyfish do, and all they have is a slight concentration of neurons in the ‘head’.
No, in fact the opposite. In common speech nerve and neuron are interchangable. If you want to make a distinction then technically jellyfish have only neurons and no nerves. Technically a nerve is collection of neurons all bundled together. Jellyfish have all their neurons dispersed as individual cells.
This is based on a very rusty memory, but I think the Portugese Man-O-War has a weird multi-stage lifecycle that ends up with one solitary polyp (the result of sexual repoduction) going into a frenzy of asexual reproduction to produce a huge number of other polyps which then differentiate their function (based on magic, for all I know). So conceptually a Man-O-War is more like an ant colony than an individual organism.
The likelihood of me getting all this right is fairly small - I read it when I was about 10.
Slaphead got it pretty much right.
Imagine if a human baby was born with just a body and a head. Then at 12 months it grew another complete body and head out of the left shoulder. Is it one organsim or two? Since we consider conjoined twins as two individuals it is two individuals. Now imagine the second body transfomed into an arm and lost most of the features of the head and body. We still have two organisms. Then imagine that another body and head grows out of the groin, and transforms into a leg. And so one and so forth.
We know we have multiple different organisms because they exist breifly as juvenile organisms before transforming into what appear to be organs. And they always retain traces of that juvenile form.
The concept of individual vs. colony gets pretty vague at this level, but we can safely say that these are colonies simply because of the way they develop. Now if they were to simply develop directly from a single cell we would have a much harder time deciding. But that isn’t the case.
In reality I guess it’s true to say that humans and all other animals are colonial organisms. It’s just that in our case the last solitray form was unicellular, so we produce new colony units from single cells just like the ancestor. Colonial hydrozoans have amore recent solitary ancestor and reproduce new units via that intermediate form as well.
