Cephalove

Shallow-water octopuses are generalist predators – this means that they can eat a variety of other animals – and good ones too. They have a few different hunting strategies, with the commonest ones involving the octopus groping along the reef, feeling for food with its arms (although octopuses have been reported to hunt by ambushing (pdf link) as well, striking their prey after spotting it.) You can see the groping strategy at work in this video:

It is clear from previous research that octopus arms are capable of movement, even relatively complex movements, on their own. Thus, when an octopus gropes its way around a reef, it might be that it’s central nervous system is doing very little to control its arms; rather, it seems likely that they move mostly “on their own”. Tamar Gutnick and her colleagues at the Hebrew University of Jerusalem recently published a study that investigated if and how octopuses (of the species Octopus vulgaris) can use information from their central nervous systems to control the movement of a single arm. I’ll let them tell you about it:

(By the way, I love video abstracts/experiments. Thanks, guys!)

The researchers took 7 octopuses and trained them to reach into a clear plastic “maze” where they could choose to put their arm into one of three areas. One of the arms of the maze had a piece of food in it. Since they were only given one chance in each session – if they chose the wrong arm in a session, they weren’t allowed to try again – the octopuses learned to find the food by looking at it through the clear walls of the maze and then make the appropriate arm movements to get it. After the researchers covered the clear maze with masking tape, the octopuses, who could no longer see the food, weren’t able to do the task any more – they got about 1 in 3 trials right, exactly what you’d expect if they were choosing randomly.

The results of this study tell us that octopuses can use visual information to direct the movements of their arms, and that they seem to get more accurate with practice. What we don’t know, however, is how an octopus’s brain could pull this off. It’s clear that simple movements are controlled within the arms themselves, as a disembodied octopus arm can make some movements by itself, but it’s unclear how the “higher-up” parts of the brain that receive visual information from the eyes could mix it with tactile information from the arm to direct these sorts of movements.

The skeptic in me says that there might not be much to be excited about. After all, we’ve known that that octopuses use their vision to do things like find their way around, and size up potential food/predators for a long time. The procedure used, even if it’s new, is sort of limited; it’s essentially a simple detour task, where the animal can see its reward but has to take a complicated route to get to it. As Zen Faulkes pointed out in his post on this study, (which is so cleverly titled as to put me to shame), the octopuses weren’t even very good at learning such an apparently simple task. Compare this to the scores of learning tasks that other laboratory animals like rats (and people, for that matter) whiz through, and it seems like a small step. Some experiments using tasks like this fail while others succeed, and there’s no clear consensus as to how and why octopuses learn (or fail to learn) in certain situations, making it even harder to say anything about how octopuses learn.

Nevertheless, there’s some room to be excited; it’s a small step into an mostly unexplored field. Think about just how foreign an experience this was for the octopuses in the experiment – not over the time scale of the experiment, but over evolutionary time. For millions of years, the ancestors of this species have been hunting on the seafloor in shallow waters, where it’s very unlikely that they’d ever encounter a hard, transparent surface that they might have to move around to get food. Even still, when they’re presented with such a situation, they can navigate it, even if they do it with some difficulty. The behavior of these octopuses, then, seems to me to have evolved not only to work well in a specific situation, but to work (at least minimally) in a wide range of situations – their behavior has evolved to be somewhat flexible. In fact, this is a strategy that is used by all animals that can learn (which seems to be most of them) that helps them deal with the fact that there is no such thing as a perfectly stable and predictable environment, and that behavior needs to adapt to deal with this. For example, your ancestors (if you were an octopus) might have fed on a few specific species of crab for the past few hundred years – if something about the environment changes, you need to be able to learn to hunt something else, or you (and your species) are doomed. Looking at it in this light, it’s not very surprising that a laboratory filled with mazes and puzzles built by scientists would push the limits of a cephalopod’s behavioral flexibility – this is a huge change from the environment the animal evolved in. To quote Zen Faulkes, “the point is not that the animals are slow to learn; the point is that they can learn to do this at all.”

This research is also exciting because it begs questions about how the nervous system of the octopus can do this task. In more familiar research animals (that is, mammals), we know that specific parts of the brain (areas of the motor cortex) control the contraction of specific muscles. Besides this, we’ve identified a whole host of brain structures that play various roles in putting together these movements and in using information from the muscles, skin, and eyes to control and refine them. In mammals, both motor and sensory systems are put together in a such a way that their arrangement in the brain corresponds to their arrangement in the body – this is called somatotopy. (Check out this neat little demonstration of the concept by Jaakko Hakulinen.)

According to another study published in 2009 by researchers from the same university, this doesn’t appear to be the case with the octopus. The investigators in that study couldn’t find any clear relationship between activity in different parts of the octopuses’ brain and different movements. While we know where the information from the eyes goes in the octopus brain (to the sensibly named “visual lobes”,) it’s unclear where it goes from there or how it might interact with the neurons that control the arms, or how this information might be put together with sensory information from the arms. How exactly an octopus’s brain uses vision to control ongoing movements, then, is the most exciting kind of scientific problem: an unsolved one.

Thanks for reading!

Zullo, L., Sumbre, G., Agnisola, C., Flash, T., & Hochner, B. (2009). Nonsomatotopic Organization of the Higher Motor Centers in Octopus Current Biology, 19 (19), 1632-1636 DOI: 10.1016/j.cub.2009.07.067

Gutnick T, Byrne RA, Hochner B, & Kuba M (2011). Octopus vulgaris Uses Visual Information to Determine the Location of Its Arm. Current biology : CB, 21 (6), 460-2 PMID: 21396818

A story about squid has been making the rounds in news sources and blogs this weekend. Just two days ago, a paper came out showing that male squid (loligo peleii) react with extreme agression to a certain protein found on the surface of squid eggs. The paper was written by a group of researchers (including the illustrious cephalopod biologists Jean Boal and Roger Hanlon) from all over the world.

The authors of the study noted that, when this species of squid get together in big groups, the males interact with the eggs that the female attach to the sea floor in a curious way – they approach eggs that are already attached to the sea floor, blow water on them, and then touch the eggs with their head and arms. This occurs before and in-between fights that the males have with each other, in which they compete over the opportunity to mate with females.

To confirm that some component of the eggs was causing the squid’s aggression, the experimenters put pairs of male squids into tanks with eggs. Sure enough, touching the eggs made the squids more aggressive.

Squid eggs attached to the seafloor, by Debby Ng

The authors then made an extract of the squid eggs, and used a technique called high-performance liquid chromatography to separate their extract into different parts. HPLC works by forcing a solution through a tube (called an HPLC column) that the various substances in the solution stick to in some way. Depending on the specific conditions, different substances will get stuck in the column for longer or shorter times. Substances that don’t stick to the column very well come out the other side quickly, and substances that stick to the column very well will come out more slowly. The solution coming out of the column at a certain time will contain only some of the substances that were in the original solution, and so it is a useful way to separate a complex mixture up into parts, called fractions.

After the egg extract was divided up into fractions, the various fractions were tested to see if they made males aggressive. After finding which fraction was responsible for this effect, the authors purified it further and identified a specific protein within it and figured out its amino acid sequence.

The culpit, it turns out, is a small protein that is related to a family of proteins called the Beta-microseminoproteins, which have been found in species from humans to scallops to ostriches. The female squid’s reproductive tract secretes this protein as part of the coating of the eggs she lays.

In humans, this group of proteins is found in the male reproductive tract, where it attaches to the outside of sperm cells and may be related to a man’s level of fertility. Nobody has asked whether it also makes males of mammalian species more aggressive, although I suppose it is only a matter of time now.

How could the behavior we’re looking at (that is, the fact that males of the species L. pealeii just flip out when they sense this protein) be good for the squid? Behaviors have to evolve, and so they need to help the individual to reproduce in some way.

It’s easy to see how being aggressive and competitive would help a male mate. The real question is: why use a pheronome? And why on the eggs? In a squid mating frenzy, there is ample stimulation – why don’t male squids simply become intensely aggressive at the site of a large group of other squid? That would seem to involve less energy (and less evolutionary backflips) than first seeing the mating frenzy, then seeing the eggs on the seafloor, then approaching and touching them, and only then being spurred into super-aggression.

For starters, it’s good to remind ourselves that none of the components of that behavioral sequence, odd as it is, evolved specifically for that purpose. Because the type of protein that is used as a pheremone in this case is found in many animals, it very clearly has been around for a long time before squid evolved specific mating habits, filling other necessary (and currently unknown) functions. Similarly, squid were probably spawning in groups and competing for mates before they developed the ability to respond to this protein with aggression.

Of course, this could be wrong – if it turns out that beta-microseminoproteins is used as a pheremone in lots of different species, then it may have evolved this function very long ago. Only more research will tell.

Functionally speaking, though, why would a pheremone evolve as a cue for aggression, even when there are plenty of other cues in the environment for the male squid to respond to (the sight of eggs and other squids being the easiest to think of)? My thought is that, in a system that can change as quickly and as unpredictably as an ecosystem, it always helps to have reduntancy. Having two seperate senses that both cause the right behavior in the right context is better than having just one, because either one might fail at an inopportune time and leave the animal (figuratively) screwed.

The other idea that I came up with is that it might help the squids recognize when a spawning group is made up of the right kind of squid – squids of their own species or sub-species. There are many types of squid that look alike to me, and while I’m sure that squid are better at telling the difference between their own kind than I am, it seems like a relatively easy mistake to make in poor conditions (dark or murky water, or the added confusion of predators being present.) Any squid who couldn’t clearly tell when the squids available for mating were squid they could actually produce offspring with would be at a huge disadvantage in terms of making babies. Again, this question could only be answered with more research.

Thanks for reading!

Scott F. Cummins, Jean G. Boal, Kendra C. Buresch,, Chitraporn Kuanpradit, Prasert Sobhon,, Johanna B. Holm, Bernard M. Degnan, Gregg T. Nagle,, & and Roger T. Hanlon (2011). Extreme Aggression in Male Squid Induced by a b-MSP-like Pheromone Current Biology : 10.1016/j.cub.2011.01.038

Anahí Franchi N, Avendaño C, Molina RI, Tissera AD, Maldonado CA, Oehninger S, & Coronel CE (2008). beta-Microseminoprotein in human spermatozoa and its potential role in male fertility. Reproduction (Cambridge, England), 136 (2), 157-66 PMID: 18469041

Cephalopods have a lot to offer – tentacles, beaks, and big scary (and perhaps cute) eyeballs. Today, though, let’s look at a part of the cephalopod body that doesn’t get paid so much attention to, especially by us neurobiologist types: the ink.

Fossil squid with preserved ink sac. Ink sacs are often easily visable in coleoid cephalopod fossils. (via Maitri on Flickr - click through for original.)

Most coleoid cephalopods (that is, all the living cephalopods excluding nautiluses and a few deep-water octopuses) produce ink. This ink is composed chiefly of melanin, which is a dark brown pigment that is found throughout the animal kingdom. Humans have used cephalopod ink for a variety of purposes, including writing, drawing, dying, and cooking (the fact that both a dark brown color of ink and a genus of cuttlefish are both named Sepia is not coincidence.) In fact, you can buy tubs of cephalopod ink online.

Cephalopods use their ink for a different purpose, though; it helps them get away from sticky situations. When severely startled, cephalopods will release ink from their ink sac, which is pushed out of their funnel with a jet of water (which usually also jets the cephalopod in the opposite direction away from the perceved danger.) The resulting cloud of ink could serve many functions; it could conceal the escaping cephalopod’s location from the predator, serve as a false target for the predator (who would attack the dark ink instead of pursuing its prey,) frighten the predator, or even trick the predator’s sensory systems into thinking it had already caught something (I’ll explain this last one at the end of the post.)

One neat property of cephalopod ink, though, has nothing to do with how predators perceive it, but rather how cephalopods perceive it.

When one squid in a shoal inks (“inking” being the action of expelling ink into the water) the rest of the shoal can certainly see what has happened and be alerted to the immanent danger that way. In addition to this, though, it has been hypothesized that squid can chemically sense the ink in the water, which would give them another way to keep abreast of squid-predator interactions going on around them.

One study that found evidence for this hypothesis (which is actually a part of a series of studies in this line of research) was done by Gilly and Lucero (1991) at Hopkins Marine Station in California. These investigators restrained squid (loligo opalescens) by attaching their dorsal mantle to a platform with cyanoacrylate glue (the same stuff that Super Glue is made of,) and then used a pipette to place small amounts of various substances onto a chemoreceptive organ located behind the squid’s eye.

Photograph and photomicrograph of squid olfactory organ - from Gilly and Lucero (1992)

They recorded the activity of the squid with a video camera, and everything was done remotely, so that the movement of the experimenter’s would not upset the squid and cause extra escape-like behavior. Escape-like behavior was measured in terms of the pressure inside the squid’s mantle, which was recorded via a tiny pressure transducer inserted inside the squid’s mantle. One of their records is shown here – the spikes in pressure reflect jets of water being expelled from the squid’s mantle, as it presumably attempts to escape from the chemical stimuli that signal some sort of danger in the environment.

They found that pipetting ink from an animal of the same species of the test animal onto the olfactory organ caused jetting. Furthermore, they found that a specific component of squid ink, L-DOPA (which is a precursor of melanin, the main pigment in squid ink) caused jetting just as much as did whole ink. On this basis, the authors concluded that L-DOPA is used as a sort of chemical alarm signal between L. opalescens individuals. (I should note that some of the authors cited in this post write that squid ink is actually a cue, not a signal, as a signal results in an action on the part of the receiver that benefits both the receiver and the sender of the signal. Escape responses by squid in response to the ink of conspecifics do not fit this definition.)

A more recent study by Wood, Pennoyer, and Derby (2008) looked at the responses of Caribbean reef squid (Sepioteuthis sepioidea) to squid ink preparations. This species of squid has ink that hangs together in a mucous-ey glob in the water, forming what the authors call a “psuedomorph”, or false animal shape that confuses predators.

Each squid was tested by placing a small amount of an ink preparation into its aquarium and videotaping its behavior during and after this event. The authors used a scoring system to determine how defensively each squid behaved during each test, with points on a scale of defensiveness being awarded for behaviors like jetting, specific postures that are used to hide from predators, and certain color changes that are known to signal alarm. A higher score on this measure of defensiveness indicated that the squid was “alarmed” (or something like that) during the test. Below is a video showing on of their tests, produced by New Scientist:

The authors found that S. sepioidea responsed with alarm to fresh squid ink placed in their aquarium. The ink worked to cause alarm responses even after it had been frozen, albeit not as well – the authors noted that it changed in consistency, and dispersed much more quickly. Ink that was placed into an adjacent aquarium (meaning the squid could see it, but could definitely not chemically sense it) worked very well to stimulate escape behavior. This argues that one of the stimuli that this species of squid uses to respond to ink is its appearance. What about chemoreception, though?

The authors produced “melanin-free ink” by centrifuging fresh ink to remove all of the melanin-containing granules in it. They reasoned that this ink was just link the whole ink chemically, except that it did not contain the specific chemical that made it opaque (melanin). They found that the squids did not respond to this ink that they could not see. These results point to the use of vision exclusively in S. sepioidea in responding to other squid’s ink, apparently conclusively.

Frustratingly, they don’t, really. It would be easy enough to chalk this result up to species differences – one species can chemically sense ink, and the other species cannot. These results, however, don’t say enough to make this claim (although there may exist other research that answers this question.)

In a paper by Lucero, Farrington, and Gilly (1994), squid (L. opalescens) ink was analyzed for the presence of L-DOPA and dopamine (they found it, but that’s not the reason I mention it.) They found that, in seawater, L-DOPA and dopamine are rapidly degraded via oxidation reactions, which would certainly dampen any effects they would have on the behavior of squid swimming in that water. They also found that the L-DOPA and dopamine in squid ink did not degrade this rapidly – these preparations behaved as if they were being protected by some sort of antioxidant contained within squid ink. While the authors used ascorbic acid (a small, soluble molecule) to replicate this effect, it’s possible that any anti-oxidant activity in the squid ink is provided by a protein (or another large, centrifuge-seperable molecule.) When Wood et al. prepared their “melanin-free ink”, they may also have removed some component of the ink that is essential for its activity as a chemical signal (for a hypothetical example, a protein that prevents that oxidation of L-DOPA and dopamine in the vicinity of the ink blob.) They may even have done something that eliminated the L-DOPA and dopamine altogether – they provide not chemical analysis of their ink preparations, and so it’s hard to know. The authors acknowledge that this is a shortcoming of their work in their paper, so there’s been no oversight on their part – it just would have been nice if they’d done a bit more in the way of quantifying the chemistry of the preparations they were using. Oh well – it’s something for the next round of studies, I guess.

I mentioned that squid ink might trick predators into “thinking” they had already caught the squid and were eating it, a trick called phagomimicry. This is because squid ink (and the exudates that other molluscs exude under stress) contains, among other things, a full complement of free amino acids – these are chemicals that predators taste when they eat flesh. If a predator gets a mouthful of ink, if they can sense the amino acids that normally tell them that they’re eating flesh, they may behave as if they have already caught and/or eaten their prey, and give up pursuit.

Thanks for reading!

W. F. Gilly and Mary T. Lucero (1992). Behavioral Responses to Chemical Stimulation of the Olfactory Organ of the Squid, Loligo opalescens Journal of Experimental Biology

WOOD, J., PENNOYER, K., & DERBY, C. (2008). Ink is a conspecific alarm cue in the Caribbean reef squid, Sepioteuthis sepioidea Journal of Experimental Marine Biology and Ecology, 367 (1), 11-16 DOI: 10.1016/j.jembe.2008.08.004

Lucero, M., Farrington, H., & Gilly, W. (1994). Quantification of l-Dopa and Dopamine in Squid Ink: Implications for Chemoreception Biological Bulletin, 187 (1) DOI: 10.2307/1542165

Cephalopods are great subjects for studies on vision, because they are so dependent on their vision that you can get robust behavioral effects by manipulating the visual environment of a test animal. In some new research in the October edition of the Journal of Experimental Biology, CM Talbot and J Marshall (from Queensland) investigate the visual system of the pyjama squid (S. lineolata) and two species of cuttlefish (S. plangon and S. mestrus) – specifically, to find out whether they can respond to polarized light, and in the case of S. lineolata, how photoreceptors are distributed on its retina. I’ve blogged about a study on visual perception in Nautilus before, as well as a study on the retinal topography of squid, so if you would like to see more of the same sort of research, check out those posts.

In these two papers, the authors assessed the ability of their experimental specimens to respond to polarized light by monitoring their optokinetic and optomotor responses to a rotating drum. The optokinetic response is the movement of an animals eyeballs to follow a moving object in the visual environment, while the optomotor response is the movement of the animal’s body to follow movement in the visual environment. The experimenters monitored the optokinetic response in S. lineolata, because it tends to stay motionless on the substrate, partially buried – as such, it will not exhibit an optomotor response under most circumstances. On the other hand, S. plangon and S. mestus both tend to hover in the water, and so show optomotor responses more readily.

A basic scheme of the apparatus used is shown in this figure from the S. lineolata paper:

The animal is in the tank (in this case, prevented from burying itself by being enclosing in a transparent cylinder,) while a drum is rotated around the tank. By varying the pattern on the drum, it is possible to determine the sensory abilities of the animal – assuming that animals generally don’t inhibit optokinetic or optomotor responses, the animal will respond to any pattern it can perceive. If the animal can’t perceive the pattern on the drum (for example, if the drum is visually continuous, as is the case with an all white drum,) it will not perceive any motion and the response will be absent.

The authors used a drum that consisted of alternating stripes of orthogonally oriented polarization filters – that is, the drums were striped, but the difference between adjacent stripes was only in the direction of polarized light that they transmitted. All the stripes transmitted the same total amount of light, and had the same appearance. Thus, the animals would only show an optomotor or optokinetic response to these drums if they could perceive the direction of polarization of the light.

In fact, this is what happened, in all three species. Two control drums were used, one of alternating black and white stripes (to make sure the animals had otherwise normal optokinetic and optomotor responses) and one of a uniform-direction polarization filter (to make sure that the animals weren’t responding to some other part of the drum – the tape used to hold it together, seams resulting from the drum’s construction, etc.,) making it pretty clear that the animals were responding to the alternating directions of polarization and not anything else.

This result is pretty unambiguous, but I’d like to point out a problem that this type of experiment presents in its interpretation: specifically, it’s very difficult to interpret negative results. In this case, it’s very easy to know what it means in terms of the animal’s sensory ability when it responds to a stimulus: it means the animal can detect that stimulus. But what if the cuttlefish didn’t respond (for example, as was found in a very similar study by Darmaillacq and Shashar (2008) in a different species of cuttlefish, Sepia elongata)? It’s hard to know what that means – did the animal fail to perceive the stimulus, or did the stimulus just not mean enough to generate a behavioral response? This is a general problem that crops up in studies on sensation and perception in animals, or any study that relies on an animal perceiving something and emitting a behavioral response. Many things need to happen to get any behavioral response to a stimulus, even one as apparently simple as eye movements. The animal must have a functional sensory apparatus appropriate to perceive the stimulus, it must have the energy and intact musculature to perform whatever behavior it is you’re looking for, it must be expressing no other behaviors that might mask or supress the behavior of interest, it must be motivated to perform the behavior of interest, etc. A negative result in such an experiment means that one of these many things is not the case, but because it’s so difficult to tell the difference between all of these steps between “stimulus” and “behavior”, it’s hard to say what exactly it is that the animal isn’t doing. Is it failing to sense the stimulus, is it failing to respond because the stimulus isn’t relevant, or is it failing to behave because it’s afraid, or stressed, or tired? Darmaillacq and Shashar note that S. elongata has retinal anatomy that looks like it would allow the animal to sense polarized light, and so they are (wisely) wary of claiming that their subjects could not perceive polarized light – but there’s no way to make any claim about S. elongata‘s vision at all from these results (except, of course, the most conservative assertion that S. elongata failed to show an optomotor response to a certain type of polarized-light stimulus under the experimental conditions used in that specific study.)

Fortunately, though, Talbot and Marshall found positive results, and so avoided that quagmire all together. It turns out that all three species they studied can respond to polarized-light stimuli with optokinetic or optomotor responses. They went on to examine the distribution of photoreceptor cells (also called “retinal topography”) in the S. lineolata retina. If you’ll remember from my post on squid visual ecology, it turns out that you can relate the retinal topography of cephalopods to their lifestyle – squids that live near coasts have retinas that are specialized to allow the animal to see below it clearly, whereas oceanic squids have retinas that are specialized for monitoring the water column above them. What might we expect from S. lineolata, an animal who spends much of its time buried in sand? The sensible answer is that is eyes would be specified to look up, since that’s where its predators and prey would be in most cases. Let’s take a look at what Talbot and Marshall found:

The darker the blue is, the higher photoreceptor density is in that area. It turns out that the striped pyjama squid does indeed have a high photoreceptor density in the ventral part of its retina, which probably gives it good visual acuity in the upper part of its visual field (if you don’t know why this is, check out this explanation of image formation in the eye for a primer.) This fits in neatly with what we know about the lifestyle of this squid.

I hope these studies represent the start of a trend towards the study of less “classical” cephalopod species (the “classical” ones being Loligo pealai, Octopus vulgaris, Sepia officinalis.) There’s a lot to learn from the less common species of cephalopods, due in part to the fact they we know very little about most of them.

Thanks for reading!

Talbot CM, & Marshall J (2010). Polarization sensitivity and retinal topography of the striped pyjama squid (Sepioloidea lineolata – Quoy/Gaimard 1832). The Journal of experimental biology, 213 (Pt 19), 3371-7 PMID: 20833931

Talbot CM, & Marshall J (2010). Polarization sensitivity in two species of cuttlefish – Sepia plangon (Gray 1849) and Sepia mestus (Gray 1849) – demonstrated with polarized optomotor stimuli. The Journal of experimental biology, 213 (Pt 19), 3364-70 PMID: 20833930

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 dec
apodiformes (Cephalopoda) Journal of Mulluscan Studies, 76, 180-185 : 10.1093/mollus/eyp055

In this post, I’ll be talking about octopus tactile sensation.  M. J. Wells and J. Z. Young did the classic experimental work on touch discrimination and learning in the octopus, although a bit of recent work has been done on the neurochemical basis of touch learning in the octopus (which I won’t get into here.)

We’ll focus on Tactile Discrimination of Surface Curvature and Shape by the Octopus (1964) by Wells.  This was one of his later papers in a series on tactile learning in the octopus.  Prior to this paper, Wells had already determined that octopus do not use proprioception to discriminate between objects (as a blindfolded person might do when trying to feel what an object is with his hand,) but rather use (almost exclusively) tactile cues about the object’s shape.  Let me explain.

It had been found that a blinded octopus could discriminate, on the basis of touch, between a sphere and a cube.  This could be explained by the presence of some sort of proprioception that monitors the relative position of the octopus’s arms in space – a system like this is known to exist in most vertebrates.  However, Wells carried out a series of experiments that show that this is, if anything, a very subtle factor contributing to the octopus’s ability to perform tactile discriminations.  He found that octopuses learn to recognize the corners of a cube as variations in texture, which are encoded in reference to the extent that the suckers contacting the object are deformed.  For example, a sucker that is on the corner of a cube will wrap around the corner, bending itself along a sharp angle.  This information is encoded as some sort of distinct textural component, and sent along to the brain where it can interact with learning centers (which I’ll discuss in a later post, hopefully) that will allow the octopus to remember what a particular texture means.  Thus, if you teach an octopus to respond to a cube (meaning that you reward it with food when it grabs the cube, and punish it with electric shock when it grabs another object, say, a sphere,) this theory would predict that it would also respond to any object which induces a similar deformation of the suckers that contact it, such as a rectangular prism, or a thin rod.  This is called a transfer experiment, because it tests the extent to which a learned task transfers to situations other than the one it was learned in.  Indeed, Wells found that he could substitute a thin rod for the cube, and the octopus will respond to it as if it is a cube, presumably because the suckers contacting the rod are bent into a relatively sharp angle, as are those contacting the edges of the cube.

This evidence alone didn’t quite clear up the question of how octopus performed touch discriminations, though – specifically, Wells’ experiment with the cube, sphere, and rod did not use enough variations of form and dimension to really probe the mechanism of touch discrimination.  Thus, Wells decided to conduct a number of transfer experiments between differently sized and textured cylinders in order to figure out the characteristics that octopuses use to identify objects by touch.  The stimuli he used are shown here:

The numbers under the cylinder cross-sections indicate their diameter in millimeters.  Wells notes that the octopus he is working with have suckers that are 10mm or less in diameter.  Knowing this, one can gauge the approximate deformation of a sucker that the different cylinders would produce.  For example, a 6mm wide cylider would induce a significant curvature in the sucker, whereas the 38mm cylinder would produce a very slight curvature, and thus would appear essentially “flat” to the octopus, if Wells’ theory is correct.

Wells quantified this difference, and generally found that the greater the difference in curvature between two cylinders, the easier the discriminate was.  This is great, but it doesn’t rule out the proprioception theory.  What if the octopus was actually “feeling” the position of the arm as it bent around the cylinder? 

To solve this problem, Wells used the two cylinders shown at the bottom of Figure 1, those labeled 8* and 6*.  These are “composite cylinders” were made of 7 small cylinders attached together, parallel to each other.  If the sucker-distortion hypothesis is correct, then these objects should be treated as equivalent to small cylinders, because they create equivalent deformation of the suckers contacting them.  If there is some mechanism that determines the shape or position of the grasping arm as a whole, then they should be treated as equivalent to the large cylinders, as they would require the same arm position and curvature to grasp as the 24mm and 18mm “simple” cylinders, respectively.  In fact, this is what Wells found, although the experiments with compound cylinders did not adhere quite as closely to his proposed model regarding differences in curvature as did those with the simple cylinders.  This might be expected, as the actual curvature experienced by the suckers is more variable with a more complex object.

Wells tested his idea further, by offering already trained octopuses P1 (which was grooved) and P4 (which was smooth.)  Other than their texture, these objects did not differ at all.  If sucker deformation is the basis of discrimination, we would predict that P1 feels most like a small-diameter rod to an octopus, as it would deform the suckers touching it greatly.  P4, on the other hand, would feel like a large-diameter cylinder, because, well, it is.  In fact, this is what Wells found – octopuses who were trained to take the larger diameter cylinder transfered this learning to the P1/P4 discrimination, and tended to take the smooth one.  Animals who were trained to take the smaller diameter cylinder tended to take the grooved one.

Wells goes on to consider discrimination using a cube with rounded corners (which proves difficult for an octopus) and a cube/rectangular prism discrimination (which is also difficult,) but I’ll let him tell you about those, as the point is amply made already.

What about the neuroanatomy of this system?  Wells provides us with a figure showing the cross-sectional structure of a single sucker, including the receptors that putatively monitor mechanical distortion of the sucker (in the area labeled “2″ at the rim of the sucker, towards the bottom of the diagram.

 

These receptors detect the mechanical forces from the object deforming the rim of the sucker, and then send this information to the ganglia of the arm.  It seems likely (although I don’t know that it has been tested) that these mechanoreceptors don’t send their information the whole way to the central nervous system, but rather input into some processing system in the nervous system of the arms first.  It would be interesting to know the minimum number of steps that information from the suckers might go through before it gets to the brain, because this would give a rough idea of how ”
processed” the sensation is before it gets to brain areas involved in learning.  J. Z. Young’s “Anatomy of the nervous system of Octopus vulgaris” didn’t seem to have a clear answer for this question, so for the time being, I’ll assume that it’s unanswered (though, if I’m wrong, please point me to the literature.)

All in all, this might seem like a poor way to distinguish two things from each other.  When you keep in mind the fact that octopus can’t discriminate objects based on weight, either, even though it can adjust its posture and muscle tone to hold a heavy object, it would seem that the octopus has a sort of crappy tactile sensory system.  We should ask, then: what does the octopus use this for?

When octopuses hunt, they often use a “blind” foraging strategy.  They will pounce on an area where prey is likely to be with their arms and web spread open and then feel for prey.  Alternatively, in rocky areas, an octopus might feel around in cracks for prey items.  If the octopus feels a prey item, she grabs it, moves it towards her mouth, and eats it.  It seems likely to me that the sort of touch discrimination that Wells trained octopuses with is not anything like what is demanded of them under ecological conditions.  For one, it is likely that octopuses are sensitive to movement as well, as they must be able to discriminate between rocks and prey, both of which might be similarly textured.  While hunting, an octopus also has other sensory systems to rely on.  They’re not primarily visual predators, but they can be, spotting prey and then attacking it (as they do when shown a live crab in an aquarium.)  They also probably have chemoreceptors on their arms which could help them identify objects under their web.  It doesn’t seem to me as if lacking proprioceptive input to the central nervous system is at all a deficit to the octopus in its natural habitat.

Thanks for reading!

M. J. Wells (1964). Tactile Discrimination of Surface Curvature and Shape by the Octopus Journal of Experimental Biology, 41, 433-445

I’ve been reading M. J. Wells’ book “Octopus”, one of the “classic” works on the octopus.  It was published in 1978, and is an essentially complete review of research on octopuses (primarily O. vulgaris, the common octopus) up until that time.  It must have been fun to write the book, because he had done much of the research he cites (he cites at least 45 of his own papers in the book.)

Wells has published a large number of experimental studies on octopuses, and I’ve become very interested in his series of experiments dealing with the sensory capabilities of the octopus.  In this series of posts (there will be 3 or 4, I anticipate, over the next week or two) I’ll relate his findings, as well as others, about the sensory systems of the octopus.  Forgive me if I write brief posts, as I’m rather busy writing other things at the moment.

I’ll jump right into the octopus visual system.  It’s a good place to start, I think, because it’s relatively easy for us to imagine what it’s like to see, and so to think about octopus visual perception by analogy.  It’s relatively harder to imagine what it’s like to control eight boneless arms (just like it’s hard to think about what lateral line perception is like in fish – we simply do not have the anatomy to do it.)  We’ll move on to other senses in the next few posts.  For now, though: vision!  (For a brief look at the anatomy of the octopus visual system, check out my earlier post on that topic.)

Vision in octopus has been studies mostly through the technique of visual discrimination tasks.  Simply put, an octopus is taught (via food rewards for correct answers and/or electric shocks for incorrect answers) to attack one visual stimulus and not another.  The visual stimuli are then varied, and it can be discovered what stimuli the octopus can discriminate between.  If two shapes are not discriminated between, they can be said to be perceptually identical (or at least perceptually similar) to the octopus.  These experiments were widely done (well, as widely done as anything in octopus neurobehavioral research) throughout the 1950′s, 60′s and 70′s, with some research still going (albeit at a slower pace and with shifting foci) today. 

Octopuses, it turns out, can see pretty well.  They can reliable learn to discriminate a variety of shapes, and can even tell the difference between rectangles that are identical except that one is rotated through a small angle relative to the other.  An interesting deficit showed up after a variety of experiments were carried out, however – Octopuses are unable to recognize the difference between certain mirror images!  For example, an octopus does very poorly when discriminating between two diagonally oriented rectangles, even though they are perpendicular to each other.  This result is not too surprising, as this is the case to a greater or lesser extent with many vertebrates (to whom the octopus is often compared in its sensory abilities.)  It is, however, remarkable in its extent – it seems like a big deal not to be able to make such a discrimination at all.

In any case, these experiments gave theorists of visual perception a lot of things to think about (I’ll write about their theories later in the series.)  What I find interesting about the whole series of experiments, however, is that they laid the groundwork for the investigation of the role of statocysts in octopus visual perception.  Statocysts in cephalopods are like the vestibular systems in our ears: they are tiny, fluid filled chambers that contain hair cells which detect the movement of this fluid (see The Fine Structure of the Octopus Statocyst (1965) by V. C. Barber for a more complete description.)  It appears that the main function of the statocysts is to help the octopus maintain its equilibrium by detecting changes in velocity as well as the direction of gravity.  For example, an octopus whose statocysts have been surgically removed will move unsteadily and fail to make normal eye movements in response to the movement of its environment.

Under normal conditions, an octopus will keep its pupil slit horizontal, relative to the pull of gravity.  However, when the statocysts are removed, the octopus will no longer be able to do this; her pupils will be fixed relative to her body position, and will not adjust to remain horizontal.  This is a figure from Proprioception and Visual Discrimination of Orientation in the Octopus (1960) by Wells, showing this effect:

 So what happens when you test these octopuses on orientation discrimination tasks?  It turns out that they can no longer discriminate between differently-oriented bars.  This is a big deal, because it can tell us something about the way that the visual system of the octopus processes shapes.  Specifically, it reveals that there is something important about the orientation of the retina that makes it possible for the octopus to discriminate shapes.  When the octopus cannot keep its retina level, it cannot discriminate between long shapes and tall shapes.  Thus, we know that the circuitry processing visual information in the octopus is relying on the input from the eye always being in the same orientation.

This can be contrasted with the way that people (and probably other vertebrates, although I don’t know this for sure) determine orientation in their visual field, which is determined partly by non-visual cues but also largely by the presence of some sort of horizon line in the visual field (see A Horizontal Bias in Human Visual Processing of Orientation and its Correspondence to the Structural Components of Natural Scenes (2004) by Hanson and Essock for a neat demonstration of this.)  If there is no available horizon line, orientation is gathered from other parts of the context, such as common or known objects.  This means that in humans, even when the orientation of retina is perturbed, the visual processing system can use what it knows about the visual environment to adapt to the new orientation.  As long as the input is otherwise normal, it doesn’t matter which way the retina is turned – that is, one can still make orientation discriminations when one’s eyes aren’t on the level.

This might seem puzzling or maladaptive on the part of the octopus, but a little thinking about the visual ecology of humans and octopuses reveals that it’s perfectly sensible.  Octopuses, living in the ocean, don’t often have very clear horizon lines or reliably-oriented objects around them.  Their strongest clue about orientation is the pull of gravity.  People, living on land, have reliable horizon lines almost all the time, so they can take advantage of this to orient themselves.  In addition, octopus eyes have muscles that can rotate the eyeball around any which way relative to the body – human eyes do not.  Octopuses never needed to evolve the complex orientation-correcting systems that vertebrates did, because they can just hold their eyes level.  They can, in a sense, afford this apparent deficiency in their visual processing system, because they compensate for it easily.  If this was the ca
se with people, every time we laid down, bent over, or leaned to one side, the world would be full of novel objects that we couldn’t make sense of!

I hope this was informative.  I can’t wait to write the next one, about octopus touch and proprioception.  See you next time!