Keeping with the theme of sensory systems, I thought I'd review some newer research on squids.
While searching for recent cephalopod neurobehavioral research (which is pretty scant) to blog about, I came upon Makino and Miyazaki's study on the distribution of retinal cells in the retina of squids. I have a soft spot for visual neuroscience that I picked up from working with my first research advisor, who works on the visual system of frogs. In any case, this is a good paper (although it was a bit hard to get my hand on,) and I'll review it here.
The study aims to look at the distribution of retinal cells in the retinas of a variety of squid species. This has been done in several vertebrates, with the general finding that animals have retinas that perform well for their lifestyle. Seems pretty simple, right? For example, fish who live in "closed" environments have dense retinal ganglion cells (RGCs) in the area of the retina that sees light from directly ahead, while oceanic fish have a strip of high-density RGCs that stretch laterally across the whole visual field. Thus (to make a horribly crude generalization,) cave and reef dwelling fish have focused binocular vision, while oceanic fish largely lack this but have a greater ability to monitor their whole visual field, ie. for predators or food items.
In vertebrates, retinal ganglion cells are often mapped in this sort of study. By the time RGCs exit the retina, they are carrying visual information that is already processed into the very basic components of visual perception (namely, hue and tone contrast.) As vertebrates have complex retinas, it is also possible to map photoreceptors in vertebrate retina, or a variety of other types of cells (which might be more or less informative.) Cephalopods, however, only have one type of visual cell in their retina - the retinal cell (or rhabdomere.) So, the authors chose to map this. It is useful to keep in mind that this is not directly comparable to the mapping of retinal ganglion cells in vertebrates - it could be the case that the density of visual cells in an animal's retina is not always correlated with the importance of that piece of the visual field in further levels of visual processing. This problem is partially solved in studies on vertebrates by the use of RGCs, in which the processing of information from photoreceptors is already underway. With cephalopods, however, there is currently no way to probe this any deeper, and so for now it remains an assumption - albeit a pretty noncontroversial one - that rhabdomere density is correlated well with the relative importance (behaviorally and neurophysiologically) of portions of the visual field. (For more on cephalopod visual anatomy, check out my earlier post on cephalopod eyes.)
The image to the left shows cell counts (in retinal cells per mm) across the retinas of the 5 species of squid. I added color to this image to make it easier to see the distribution of cells. It's important to not that the colors are relative within each figure, and do not represent absolute cell density, which is shown as (difficult to read) numbers on the boundaries of regions. Also note the scale bars, which are 10mm in every image.
In terms of orientation, keeping things straight gets a little tricky (as it does with all cephalopods.) Dorsal-ventral orientation is pretty easy - remember that the lens of the eye inverts the light coming through it, so that the ventral part of the retina forms the top part of the visual field and the dorsal part of the retina forms the bottom part of the visual field. Anterior is the direction the squids' arms point in, so the anterior retina forms the posterior part of the visual field. The posterior retina is the part that forms the anterior part of the visual field. This is the part that is used when squids look forward to form a binocular image.
Using this data, the authors estimated the visual axes of the squids, based on the location of the highest density of photoreceptors. The visual axis is the general point of focus, which is known to be of utmost behavioral importance in vertebrates. When you follow a moving object with your eyes, you are keeping it in your visual axis. The location of an animal's visual axis is key to its visual ecology - many predators have forward facing visual axes so that they can see their prey accurately, while prey species often have very laterally oriented visual axes (think of rabbits and deer) so that they can monitor more of their environment at any given time. Thus, we'd expect that squids with different lifestyles have different visual axes, because they will be looking for food and predators in different places.
In coastal squid (E. morsei and S. lessoniana), the visual axis is directed downwards, presumably reflecting the importance of monitoring activity on the substrate that these species live on. In oceanic squid (T. pacificus, E. luminosa, and T. rhombus,) the visual axis is directed upwards, and the eyes have a much greater density of photoreceptors overall. I think the retinal cell density map of E. luminosa is especially interesting, because the concentration of cells on the extreme posterior edge of the retina suggests that binocular vision is disproportionately important to this species. The authors conjecture that this eye may be specialized to detect and track bioluminescence in the open ocean, but this is purely speculation.
These findings are important because they expand our knowledge of cephalopod eyes, which are a model evolutionary system. If we can begin understand the impact of ecology on the organization of visual systems (which is part of the emerging field of visual ecology,) we can generate a wealth of testable hypotheses about the ecological conditions that occurred during the evolution of differnt species eyes, as well as the other sorts of adaptations we might see in sensory systems as they diverge (or converge) during evolution. It's also a nice piece of evidence that our rather basic theories about visual ecology and the structure-function relationship of the visual system are largely correct. This is good to know, as we base an incredible amount of more complicated neuroscience research on these theories.
Thanks for reading!
Akihiko Makino, & Taeko Miyazaki (2010). Topographical distribution of visual cell nuclei in the retina in relation to the habitat of five species of decapodiformes (Cephalopoda) Journal of Mulluscan Studies, 76, 180-185 : 10.1093/mollus/eyp055