Here at Cephalove, I tend to focus on octopuses. The reason for this is pretty simple – out of all of the cephalopods, the most behavioral research has been done on octopuses, so they are the easiest cephalopod to write about. To keep things lively, though, and to do something special to commemorate my first new post as part of the SFS Network, I want to dedicate this post to the oft-forgotten member of the cephalopod family: the nautilus. Specifically, I want to talk about the eye of the nautilus. First, to cover the basics: nautiluses are the only extant cephalopods who still have an external shell. There are only six species of nautilus still around, 4 in the genus Nautilus and 2 in the genus Allonautilus, with the most well-know being N. pompilus, the chambered nautilus (although this name accurately describes the shell of this species, all species of nautilus have chambered shells that help them achieve neutral bouyancy.) A nautilus has about 80-100 small tentacles protruding from its shell, with which it catches food – mostly small crustaceans and fish. They are found in the Pacific ocean, where they live in the deeper parts of coral reefs. Here’s a picture (in the public domain – thanks, J. Baecker!) showing a chambered nautilus: You can’t see it too well here, but the eye protrudes on a stalk (the optic stalk, to be precise) from the head. If you are used to looking at octopuses, cuttlefish, and squid (and aren’t we all, by now?) you’ll notice that the nautilus eye looks very different from theirs. In fact, it is lensless, with the pupil being open to the water (sort of – there may actually be a sort of covering over it, but I’ll get to that later.) This sort of eye is called a pinhole eye, and is also seen on giant clams. It works very much like a pinhole camera, where an image is formed (albeit a relatively crude one) by restricted the angles at which light can enter the eye. As those familiar with photography know, smaller pinholes create a sharper image; however, they are less sensitive, and require a much greater intensity of light to form a good image. The nautilus’s eye works similarly, and so its pupil size is ultimately the result by a compromise between resolution and sensitivity. A cross-sectional diagram of the eye (from Muntz and Raj, 1984) is shown below. The dashed line shows the area where the eye connects to the rest of the body, and the thick line shows the location of the retina. The gap at the right of the image is the pupil.
How good is the nautilus’s eyesight? To find out, we’ll take a look at Muntz and Raj (1984), a paper that examines the spatial resolution of the nautilus eye through two different methods. First, the authors used the nautilus’s natural tendency to maintain its orientation and direction in the water to determine its approximate visual acuity. Then, they constructed model nautilus eyes and took pictures through them. These are both elegant (although incomplete) ways to get an idea of what the nautilus sees, which is a very basic question in the problem of understanding the behavior of a particular organism. I’ll skip over any more discussion of the anatomy of nautilus in this post – that’s another story for another day. Muntz and Raj succinctly address the anatomy of the eye in the paper, so look there if you’d like more information on the subject.
To test the functional acuity of the nautilus’s vision, the authors exploited the animals optomoter response. This is a reflexive motor behavior that keeps the animal in a stable orientation relative to landmarks in its environment. This is very useful for a neutrally buoyant creature living in the water, who might otherwise easily be flipped over or turned around by water movement. The authors did this by putting a nautilus into a tank in the middle of a drum. The inside of the drum was striped, and the drum was slowly rotated around the tank. If the nautilus could perceive the stripes, it would presumably rotate with the drum, trying to maintain its orientation to its surroundings. If it could not perceive the stripes, it would let the drum rotate around it without attempting to correct for this movement. A diagram of the apparatus, taken from the paper, is shown below.
By varying the width of the stripes, the authors were able to determine just how well resolved nautilus vision is. After some calculation, the authors concluded that the nautilus has a minimum visual resolution between (roughly) 5 and 11 degrees of the visual field. In comparison, a human has a minimum visual resolution resolution of about 1 minute of arc (1/60 of a degree.) This means that as long as two objects are more than 1/60 of a degree apart in the image formed on a human’s retina, she will be able to tell that there are two objects there. The nautilus, on the other hand, requires that the objects be at the very least 5 degrees apart in order to resolve them separately. Thus, if stripes smaller than the equivalent of 5 degrees of visual field were used, the nautilus could not perceive them, and so showed no orienting response to this stimulus.
There’s a problem with this interpretation, though. It might be that the 5-11 degree limit is not a limit of the eye, but rather just a differential behavioral response to different types of stimuli. What if nautiluses tend to orient themselves to large objects (like the seafloor, or the light of the surface) but not to smaller objects (like animals swimming around them, or floating debris?) This certainly seems adaptive. To help make the argument that the 5-11 degree limit was a limit of the eye itself, the authors constructed a model eye and took pictures through it. In this way, they got a sense of the resolution of the image formed on the retina, which sets (albeit not directly) a lower limit on the resolution of the image that the brain can form. Seen below is a set of such images. The top left image shows the chart that they used. The remaining three were taken through model eyes with increasing pupil diameters, corresponding to the range of pupil diameters found in wild nautiluses. Under the experimental conditions, the top line is distinguishable at a minimum resolution of 4 degrees, and the bottom is distinguishable at a resolution of 2 degrees. As you can see, some of the model nautilus eyes could not resolve either line, suggesting that the minimum resolution found in the behavioral test (5-11 degrees) agrees with the optics of the eye. Now that we know a bit about the optics of the Nautilus eye, what about that open pupil? (I told you I’d get back to this.) Having a gaping hole in one’s eye seems like it presents a problem to the animal, as it leaves the delicate interior of the eye open to the water – and any tiny particle or organism that might be floating around. A simple experiment by W. R. A. Muntz (1987) discovered for one mechanism that protects the interior of the nautilus’s eye. First of all, notice the groove that runs from the pupil to the bottom of the eyeball:
That groove is lined with cilia that sweep mucous downwards across the surface of the eye. Muntz place drops of india ink on the surface of detached nautilus eyes, and found that the the india ink traveled downwards across the eye and into the groove. Drops of ink were even seen moving over the opening of the pupil, indicating that this sheet of mucus is viscous enough to form a semi-solid covering over the pupil. Without a lens to cover this opening, the nautilus seems to have evolved another way to keep particles out. Well, I hope you enjoyed this as much as I did. Thanks for reading! W. R. A. Muntz (1987). A Possible Function of the Iris Groove of Nautilus Nautilus: the Biology and Paleobiology of a Living Fossil, 245-247 DOI: 10.1007/978-90-481-3299-7_16
W.R.A. Muntz, & U. Raj (1984). On the visual system of Nautilus Pompilus Journal of Experimental Biology, 109, 253-263