Cephalove

Most species of shallow water octopuses appear to be pretty solitary animals. They live in dens and venture out from them to hunt or find mates; defending these dens and getting busy are the only social interaction that many species of octopuses are observed to have in the wild. I like to think of them as the curmudgeons of the reef environment, keeping to themselves because that’s just the way they like it.

Keep movin', buddy; there's nothing to see here. (Photo by algaedoc)

It might surprise us, then, to learn that Elena Tricarico and her coworkers, working out of the Stazione Zoologica in Naples, just published a paper arguing the octopuses (of the species Octopus vulgaris), can recognize other individual octopuses. While it’s clear that this ability might be important to more social cephalopods (like squids, which form schools), what good could it do for a species with such a hermit-like existence?

It turns out that keeping to one’s self in an area where there are lots of other organisms around requires some social skills – you have to know a little about the folks around you and what behavior to expect from them. For example, if you see the same animal patrolling an adjacent territory each day, it doesn’t do much good to make a huge fuss over it all the time. If you have your territory, and she has hers, it behooves you both to be able to recognize each other so that you don’t waste time and energy chasing off somebody who isn’t actually going to cause you any problems (this is called the “dear enemy” effect.) On the other hand, if a wandering octopus comes through looking for a good nesting site, it would be useful to be able to tell that he’s a stranger so that you could drive him away and keep him from taking over your territory. Thinking about it in these terms, it makes sense that the ability to recognize other octopuses could be a useful ability to have.

To test whether octopuses could do this, Tricarico and coworkers divided up their experimental octopuses into two groups; in one group, pairs of octopuses were housed with a clear divider between them, so that they could see their partner, while in the other group the octopuses had an opaque divider. After letting the octopuses either see or not see each other for 3 days, they watched how these pairs interacted with each other when they were put into the same test tank for 15 minutes. It turns out that pairs that had seen each other before avoided each other more, touched each other less, and spent a longer time ignoring each other when they were placed in the same tank than pairs that had been separated by the opaque divider.

This alone isn’t enough evidence to conclude that octopuses can recognize other individual octopuses – after all, the pairs that could see each other might just be getting used to being around any octopus. To test whether the octopuses had learned to recognize their specific partner or just gotten used to the presence of other octopuses, the researchers did one more test – they put the octopuses back in the test tank, but this time, they put some of them in with the familiar octopus they had been seeing throughout the experiment, and some of them in with an octopus they had never seen before. What they saw was this: when octopuses were placed with another octopus that they were familiar with, they touched each other less, avoided each other more, and their interactions were shorter than when they were placed with unfamiliar octopuses. It looked as if the octopuses had learned to recognize their partner, and responded differently to them than to a strange octopus.

Like all good experiments, this one begs plenty of questions: can octopuses tell who individuals are, or do they just categorize other octopuses as familiar or unfamiliar? Does their ability to discriminate other individuals imply some sort of social cognition, and of what sort (I’d argue that it suggests only very basic social cognitive skills, but opens the door for more investigation,) and, finally, do I need to worry that someday, the octopuses will learn to recognize ME?

That's right, buddy, I'm looking at YOU. (photo by Rowland Cain)

Thanks for reading! I’d like to point out that I took the title of this post directly from the paper it discusses; it was such a good title that I couldn’t think of anything more fitting.

Elena Tricarico, Luciana Borrelli, Francesca Gherardi, Graziano Fiorito (2011). I Know My Neighbour: Individual Recognition in Octopus vulgaris PLOS One : 10.1371/journal.pone.0018710

If you like nature documentaries, you’ve probably seen the following clip (from the BBC’s “Planet Earth“):

Nautiluses are really cool – they’re misfits among cephalopods, having many tentacles and external shells while their fellow squids and octopodes are squishy and eight- or ten-armed. In this clip, at least, they come across as sort of mysterious, spending most of their time floating about in the abyss and only coming up to where it’s easy for us to see them at night. In fact, daily vertical migrations are common in the ocean, with a truly enormous number of organisms moving from shallow to deep waters and back again each day. The nautilus takes part in this migration, probably for its own safety – it’s harder for predators to see you (and eat you) if you stay where it’s dark.

Now I love learning new things about cool animals, but there’s something that I’m more interested in: how do we get that information? Presumably, biological facts are not true just because Sir David Attenborough says they are. How do the producers of that clip (or anybody, for that matter) know that this is what Nautiluses do?

It turns out that the answer is pretty simple: we monitor some of them for a period of time and see what they do. We can’t actually follow them underwater and visually watch them (because it would be difficult and expensive to do this for very long), so we have to use some sort of remote communications device (a radio or ultrasonic transmitter) to see what they do.

This is just what a group of researchers from the University of Queensland, have been doing for some years. Following up on reports about the possibility of daily vertical migrations in Nautilus, they attached ultrasonic transmitters to eleven nautiluses they caught around Osprey Reef in Australia. They kept track of each animal’s vertical movement, and described what they saw.

Nautilus fitted with ultrasonic transmitter - from Dunstan et al. 2011

As far back as 1899, a naturalist by the name of Arthur Wiley made note of the depths at which Nautilus live, both because it made them hard to study, and it meant that they were only be caught in shallow-water traps at night (although to be fair, he provides us no proof of this, only his word.) There are several other reports that support the idea that Nautiluses make vertical migrations (for example, a report published in Nature about the daily migrations of Nautiluses in Palau by Peter Ward et al. in 1984, and a report by Bruce Carlson et al. published in the journal Pacific Science about the same population of Natuiluses.)

Enter Dunstan and co-workers, the group of scientists at the University of Queensland that I mentioned earlier. They followed eleven individuals for up to around 80 days each and also used information from remote-operated vehicle dives to get a clearer picture of what Nautiluses spend their time doing. It’s easy to get an impression of their data from the following graph (click on it to see a bigger version):

Graph of individual recordings of Nautilus depths from Dunstan et al. 2011

Each line is one recording – there are multiple recordings from each animal. The “depth” of the line at the time of day shown at the top of the graph indicates the depth of the recording.

So what do we learn from this? Basically, we learn that it’s not as simple as “shallow at night, deep during the day” – just look at that graph! It doesn’t look like the movement of nautiluses is coordinated very well, either between animals or with times of the day. The authors note that the nautiluses often spend the daytime resting in relatively shallow water, and at night are found in all sorts of depths. In summary, it looks like there are several factors that influence where a nautilus ends up hanging out; purely physically, they can’t go deeper than about 800 meters (2600 feet) due to the high pressure (their shells implode at depths greater than this) or shallower than about 100 meters (320 feet) due to higher temperatures near the surface. During the day, nautilus appear to rest in shallow water or forage in deep water, while at night they are active and move through a whole variety of depths – this is probably influenced by things like food availability, the type and number of predators around, the conditions of the water, and the size and age of the animal. These results don’t contradict earlier results, although the authors note that different populations of Nautilus have been observed to have different patterns of behavior, and chalk this up to differences in their environments.

On a final note, I’d like to mention a study by Kanie and coworkers in which they determined what depth of water would cause a nautilus’s shell to break. To do this, they put a nautilus in a tank of water, and pressurized the tank until the nautilus imploded, all the while measuring its ventilation rate (how quickly it was “breathing”). From this, they came up with a maximum depth for nautilus of 785 meters. At first, this sounded like a cruel experiment to me, but on second thought, it was done very well. They used a single animal (although one might criticize them for that, it means that they only caused harm to a single animal) and in doing so adequately and rather elegantly answered a basic question in cephalopod biology that has implications not only for the ecology of living nautiluses but also for the study of nautiloid fossils and their distribution.

Thanks for reading!

NOTE: This post has been edited from its original form in response to Dr. Peter Ward’s comment (see below). As I did not have access to the research he mentions when I wrote this post (and still don’t), it was inappropriate of me to comment on the state of the field as I did. I’d like to sincerely apologize to Dr. Ward and his coworkers for any disrespect I may have shown to them and their work. I’ll do my best to bring myself up to speed on the research he mentions and include a more detailed discussion of it in a future post, so that I’m accurately representing the field. The text of this post before it was revised is available at this link.

BRUCE A. CARLSON, JAMES N. McKIBBEN, AND MICHAEL V. DEGRuy (1984). Telemetric Investigation of Vertical Migration of Nautilus belauensis
in Palau Pacific Science

Yasumitsu Kanie, Yoshio Fukuda, Hideaki Nakayama, Kunihiro Seki, Mutsuo Hattori (1980). Implosion of Living Nautilus under increased pressure Paleobiology, 6 (1)

Dunstan AJ, Ward PD, & Marshall NJ (2011). Vertical Distribution and Migration Patterns of Nautilus pompilius. PloS one, 6 (2) PMID: 21364981

Arthur Willey (1899). On a Zoological Expedition to the South Seas Proceedings of the general meetings for scientific business of the Zoological Society of London

Ward, P., Carlson, B., Weekly, M., & Brumbaugh, B. (1984). Remote telemetry of daily vertical and horizontal movement of Nautilus in Palau Nature, 309 (5965), 248-250 DOI: 10.1038/309248a0

To recap the last post on the Euprymna/Vibrio symbiosis: Euprymna scopoles (also known as the Bobtail squid) is a tiny species of squid that has two light organs in the underside of its mantle. Vibrio fischeri is a species of bacteria, of which some varieties can live inside of the bobtail squid’s light organs. These bacteria produce light, which the squid uses for camoflauge.

The story of how the two evolved together to make the working symbiosis is long, complex, and as of now incomplete – scientists are still piecing together all of the many adaptations that allow these two species to live together. I’ll try to bring both of us, dear reader, at least one step closer to making sense of it in this post.

A juvenile bobtail squid. Photo by Loh Kok Sheng (click through to see his blog.)

The important thing about this symbiosis is that it is selective. The squid has these little pouches which are just perfect for bacterial growth, but only one species of bacteria is found there. This involves a number of processes of selection – for example, the squid’s immune system sends cells into the crypts of the light organs to eat up invading bacteria, and the lining of the light organs secrete antimicrobial chemicals (like nitric oxide.) Vibrio fischeri has evolved to allow it to prosper under these conditions. The first step of the symbiosis, though, involves the bacteria and the squid finding each other. The ocean is big, and there are lots of bacteria and squid in it; how do these two get together so reliably?

Nyholm and McFall-Ngai address this in a 2003 paper that examines what they call “the first site of symbiont specificity”: the mucus that coats the opening of the light organ crypts in juvenile squid. Let’s start from the beginning:

A baby bobtail squid hatches from its egg. (Awwwww! So cute!) While it developed, its bacterial partner was nowhere to be found. After it hatches, though, a colony of V. fisheri will become established in its light organs within mere hours. Nyholm and McFall-Ngai looked at the surface of the mantle where the crypts open to the seawater and found that specialized cells in this area secrete mucus in response to the seawater that it would normally encounter right after hatching. This mucus helps trap bacteria, which can then colonize the light organ. V. fisheri normally make up about 0.1% of the bacteria found in seawater, though, so in order to beat out the competition, they must have some ability to interact with the light organ in a special way. Nyholm and McFall-Ngai hypothesized that the mucus layer on the outside of the light organ was key to this specificity, and conducted a number of experiments to test this idea.

First of all, they found that if they exposed hatchling bobtail squids to seawater without V. fisheri in it, all sorts of bacteria could be found in the light organ mucus. However, when they used seawater with small amounts of V. fisheri in it (again, on the order of one-tenth of one percent of the total bacteria in the water,) the colonies that formed in the mucus were almost exclusively V. fisheri. This indicates that this mucus excretion has some role in establishing the specificity of this symbiotic relationship, in that it somehow “screened out” all of the other species of bacteria that might have taken hold in the mucus and started to multiply.

Image of a V. fisheri colony in a bobtail squid light organ, labeled with green fluorescent protein. Aa, anterior appendage; pa, posterior appendage. (from Nyholm and McFall-Ngai, 2003)

They also determined that most of the V. fisheri present when they took their measurements had been collected from the water; this is in contrast to a scenario where a few cells were captured and then multiplied. To do this, they used a chemical called nalidixic that prevents cell replication while not affecting cell growth – when exposed to this chemical, bacteria won’t divide, they will simply elongate. By looking at how long V. fisheri cells grew in the light organ mucus, the experimenters determined that the cells were growing at a low rate in the mucus – in fact, they were growing much more slowly than they do in a plain culture! Thus, it’s unlikely that a few cells were captured by the mucus and then dividing into the large colonies they found; rather, there may exist some way for V. fisheri to selectively adhere to the mucus and be efficiently collected from the water (the authors say that this is unlikely, but not completely ruled out – it seems to me a likely explanation, especially taking into account the results of a series of studies that I’ll write on soon.)

The authors than tried using killed V. fisheri, to see if there is something specific to the presence of the bacteria (for example, some component of their outer membrane) that inhibits the growth of other bacteria. They found that, although killed V. fisheri could still adhere to the light organ mucus, they did not prevent the growth of other species of non-symbiotic bacteria. This implies that the bacteria perform some active process that prevents the growth of other bacteria in the light organ and allows V. fisheri to establish its dominance there, even though the mere presence of V. fisheri bacteria doesn’t kill other kinds of bacteria.

This symbiosis, then, which occurs very quickly and very specifically, depends (as most great things do) on mucus. Somehow, V. fisheri interacts with the squid’s secretions to beat out it many competitors. Interestingly, though (and I won’t cover the methods here, for time’s sake) the authors also found that the V. fisheri that colonize the crypts initially are not necessarily able to produce luminescence. It seems that the species of bacteria is selected during the initial stages of colonization, but that later on, specific strains that are better able to produce light are selected for while those that do not produce light are expelled or die off – each stage of selection no doubt involving a complex set of signals between the squid and the bacteria.

Thanks for reading!

Nyholm, S., & McFall-Ngai, M. (2003). Dominance of Vibrio fischeri in Secreted Mucus outside the Light Organ of Euprymna scolopes: the First Site of Symbiont Specificity Applied and Environmental Microbiology, 69 (7), 3932-3937 DOI: 10.1128/AEM.69.7.3932-3937.2003

I’ve recently gotten into microbiology (I got a book on protozoans, and I’m hooked,) so I decided to try to find something microbiological to write about. Lo and behold, after a few Pubmed searches, I came upon some papers about an bioluminescent bacteria called Vibrio fischeri. Of course, not just any bacteria would do for a blog post – this one is special. It lives inside the Hawaiian bobtail squid, Euprymna scolopes, in two special light organs. There, it finds a nice comfy home, and the squid can use the bacteria’s light-producing ability for countershading, so that it is harder to catch and eat. It is easy to see how this is good for both the squid and the bacteria.

(As a brief aside, the genus Euprymna is among the cutest cephalopod taxa. Go ahead and do a Google image search for the string “euprymna”, if you don’t believe me.)

The squid houses its tiny symbionts in a set of paired organs called the light organs (there’s a nice, straightforward anatomical term for ya!) These organs contain many convoluted cavities lined with epithelial cells in which the bacteria live. They connect to the inside of the mantle cavity (which is actually the outside of the body, as the mantle is open to the seawater through the funnel,) through ducts, so that the inside of the light organ is actually continuous with the squid’s mantle epithelia. Vibrio fischeri infect young squids by making their way through these connecting ducts and colonizing the cavities in their light organs. This process has been the subject of a number of very interesting studies; I’ll come back to this later, though.

At the present moment, let’s forget about the bacteria and consider the light organ from the squid’s perspective. The light organ’s structure was described in 1990 by McFall-Ngai and Montgomery. They found that the organ had a structure that was specialized for the projection of light downwards from the squids body. Specifically, the organ is set up so as to maximize the amount of light that leaves the squids ventral surface (its belly, or underside.) In addition to the cavities for the light producing bacteria to live in, the light organ contains a specialized “reflector” that helps to prevent light from escaping from the top of the organ, and a clear lens beneath the light organs to allow light to exit the squid’s body from the bottom. These structures are closely associated with the animal’s ink sac, which it has muscular control of, and so the shape of the reflector and lens can be changed by the action of the squids muscles. This allows the squid to adjust the characteristics of the light that leaves its light organ – specifically, it allows the squid to control how much light leaves the light organ. This is very important, as it’s what makes the light organ useful.

This image shows the light organs of the Hawaian bobtail squid. They are the pair of white and black organs below the eyes. From McFall-Ngai and Montgomery, 1990.

This is a cross section of the light organ. For orientation, imagine that the squid in the above photograph was lying on a table, and we cut it in half horizontally through the light organ, so that one half had the head and tentacles and the other half had the tip of the mantle. From McFall-Ngai and Montgomery, 1990.

How do we know what the light organs are used for, though? They look a lot like they would be used for countershading – that is, the lightening of an animals downward-facing side that prevents predators below them from seeing their silhouette against the light from the surface. Suggestive anatomy is not enough to make a firm conclusion about their function, though – we have to show that the light organs actually function to provide counter-shading.

Let’s fast-forward to 2004, where after stepping out of our Scienterrific Time-Travel Machine (TM) we will crack open the current issue of the journal “Marine Biology” to find an article by Jones and Nishiguchi that purports to show that the bobtail squid really does use its light organs for counter-shading. Successful bioluminescent countershading requires that the animal be able to regulate the amount of light it gives off in response to the amount of light that falls on it from the surface. In this way, it can match its appearance to that of the surface light, and blend in.

To demonstrate that the bobtail squid can do this, Jones and Nishiguchi used the following procedure: A squid was placed in a small container, slightly larger than its body, to prevent movement (the small size of the apparatus may have interfered with the results by stressing the animals, but after removing animals who appeared to be distressed by the experimental procedures, they got consistent results.) Then, a light was turned on at the top of the container. A fiber optic probe placed at the bottom of the container allowed the authors to measure the amount of bioluminescence the squid generated.

They found that the amount of light the squid’s light organ released was proportional to the amount of light they hit its dorsal surface (the upward-facing surface of the squid.) This is pretty good evidence for active counter-shading (also called counter-illumination.) This is the benefit that the squid derives from the symbiosis – it can more effective hide from predators below it. This at first seems a bit puzzling, because the bobtail squid spends much of its time buried in the sand. On those occasions when it leaves its sandy hiding place, however, it is very vulnerable, and these are the times when light organ really, *ahem*, shines.

Now, let’s speculate a bit about the evolutionary history of the light organ (in very general terms, of course.) We have a mechanism for selection: both the squid and the bacteria benefit from the symbiosis. The squid gets a counter-shading mechanism that allows it to escape being eaten, and the bacteria gets a reliable and fertile place to grow and reproduce. But what is the cost of evolving this symbiosis? For one, when they evolve specialized mechanisms for coexisting with their host’s immune system, the bacteria might give up the flexibility to live in other environments (this is not the case in this symbiosis, as there are both free-living and symbiotic forms of V. fischeri, but it is a general problem in the evolution of symbiosis.) The squid’s immune system, following the same tack, had to evolve special mechanisms to allow the bacteria to colonize it, which may have fitness-reducing side effects. Furthermore, being part of a symbiosis makes the animals dependent on each other (in this case, perhaps not entirely dependent upon each other, but at least dependent upon each other in terms of achieving optimal fitness.) As a symbiosis evolves and becomes more completely co-dependent, the organisms involved are increasingly restricted to only those habitats and conditions that their symbionts can also live in. Nevertheless, this symbiosis has evolved, indicating that such problems are not actually detrimental enough in this case to preempt the beneficial effects of the symbiosis.

The ways that the bacteria and the squid have evolved live together on a molecular level are manifold and complex. One particular problem sticks out like a sore thumb: how is colonization of the squid’s light organ so selective? The squid has these nice little crypts that are apparently well-suited for bacteria to live in, and yet they are only colonized by a single species of bacteria. In fact, this problem has been extensively studied. It turns out that, during colonization, the V. fischeri and E. scolopes are engaged in a sort of biochemical dance (if you will excuse a student’s romanticism,) mutually sensing and reacting to each other, and putting out chemical and physical signals in a coordinated fashion to successfully live together. This actually presents many problems: the bacteria must be plentiful, but stay contained; they must survive exposure to the squid’s immune system, but not be allowed to infect the squid as a whole. Other bacteria must be excluded from the crypts. The initial colonization must take place, with the bacteria finding the right place on the squid to enter and moving through the ducts of the light organ to its crypts. This process is of interest to molecular biology because it is a case where molecular signals between an endosymbiont and its host have been identified, leading to its use as a model system for the development of such symbioses in general.

I know, I know, you want to hear about the molecular biology, but it’s already the end of the article! Don’t worry – I’ll get to that in a dedicated post soon enough. It’s a complicated subject, and it’s far enough out of my area of knowledge that I’ll need a little time to learn about it before I can write about it. For now, thanks for reading, and stay curious!

For more info on the Bobtail squid-Vibrio symbiosis, check out this great page by J. Graf. Also, as always, I encourage you to find and read the studies I cited for yourself.

McFall-Ngai, M., & Montgomery, M. (1990). The Anatomy and Morphology of the Adult Bacterial Light Organ of Euprymna scolopes Berry (Cephalopoda:Sepiolidae) Biological Bulletin, 179 (3) DOI: 10.2307/1542325

Jones, B., & Nishiguchi, M. (2004). Counterillumination in the Hawaiian bobtail squid, Euprymna scolopes Berry (Mollusca: Cephalopoda) Marine Biology, 144 (6), 1151-1155 DOI: 10.1007/s00227-003-1285-3

I recently got a request (thanks to arvindpillai at Fins to Feet) to do a post on the shoaling and predatory behavior of Humboldt squid, Dosidicus gigas (also known as the Jumbo squid, and by those who don’t know any better, the giant squid.)  I decided that this would be a good thing to do, because I hadn’t read much about the predatory behavior of D. gigas.  So I spent a week searching the literature for scientific studies on Humboldt squid predatory behavior, and guess what?  I still haven’t read much about it!

It turns out that there is very little known about the behavior of these squid.  The paucity of our ethological knowledge of them is shocking to me, given the disproportionate attention to this species in popular media.  I’ve seen at least one budding cephalopod enthusiast become intrigued by stories about this species to the extent of obsession, and it’s not hard to see why.  Somehow, these squid have gained a reputation as fierce predators that are so bloodthirsty as to be regularly deadly to humans.  As such, popular TV shows and news magazines have run numerous stories about them, usually finding one or two divers who have (presumably) had experience with these squids (or at least heard stories about them) to expound on just how ferocious and aggressive they are.  Invariably, some sensational quip (that is almost always unsupported by scientific literature because, well, that literature does not exist) is used to drive home just how scary these squid are:

“It has probing arms and tooth-lined tentacles, a raptor-like beak and an insatiable craving for flesh — any kind of flesh, even that of humans,” says Pete Thomas in “Warning lights of the sea“. 

Mike Bear, an otherwise anonymous diver from San Diego is quoted in this article as saying “I wouldn’t go into the water with them for the same reason I wouldn’t walk into a pride of lions on the Serengeti.” 

“The Humboldt squid is a voracious predator that will eat anything it can get its tentacles on,” says Kelly Benoit-Bird, an oceanographer, quoted by Mark Floyd in this piece.

With all the hubbub, these guys must be pretty dangerous, right?  The stuff of nightmares, even!  I mean, just look at this bloodthirsty monster!

This is the myth of the Humboldt squid: that they are first and foremost dangerous, indiscriminate killing machines.  This is, frustratingly, the first (and often only) piece of information that is repeated about them in any given article.  But what’s the real story?

Let’s put this in perspective by considering the case of sharks, another predatory ocean-dweller that has been sensationalized as being imminently dangerous to humans (remember “Jaws”?  It was pretty silly, but a lot of people took that era’s shark scares seriously.)  Fatal shark attacks on humans are documented somewhat regularly, and are discussed (albeit infrequently) in the scientific and medical literature (ie. this study on fatal shark attacks.)  I cannot find a single verifiable record of a fatal squid attack on a human in the medical literature (admittedly, I have only searched 3 online databases and Google scholar – I might be missing something.)  The closest thing I can find are fisherman’s accounts in popular media of other fisherman’s stories about hearing about people being killed by Humboldt squid.  Keeping in mind the D. gigas is a rather common animal, is fished for sport by casual fishermen, and is usually encountered in large groups (the commonly cited size is 1000-1200 animals per shoal, but I can’t find anything peer reviewed to support this,) it looks like these squid are all but harmless, given how often it is encountered by lay-people and how few (if any) fatal encounters there have been.

This is not to say that I don’t think it’s possible that a Humboldt might kill a human someday; they are clearly aggressive, as several documented, non-fatal “attacks” on humans show.  I have to say, though, that the media attention that is payed to them (which is probably the reason why so many people are “interested” in them) is really a nuisance.  By making inflated claims about a species that we have little behavioral research about, media outlets encourage people to accept hearsay and horror stories as if they were biological fact.  These stories also draw attention away from other squid species whose behavior is very well characterized (ie. L. Pelalei) which might be better used to teach the public basic information about cephalopods.  Finally, by attempting to catch people’s eye using gorey stories, such articles serve to blind people to really learning about these animals by focusing on how “bloodthirsty” and “horrifying” they are – an effect that can’t be any good for conservation effforts.  I recognize that most people want an entertaining story rather than a dissertation out of their media, but this obvious bias in popular media coverage on this particular variety of cephalopods just bugs me because it is so pervasive and one-sided.

Now that I’m done ranting and raving, and have hopefully convinced you that D. gigas might not be the single-minded killers that they are often portrayed to be, I’ll try to get at the facts (that is, our very limited scientific knowledge) of this species.  Most of the research that has been done on them has been about their interactions with predator and prey species and their movement through their habitat, rather than their ethology.  This is because they are an important species in the study of ocean ecology.  They can be caught regularly in relatively large numbers throughout their range with little risk of damaging populations – this is uncommon among large predators, which tend to be much more rare than those lower on the food chain.  They are also suitable to be tracked using remotely monitored tags (as per Markaida et al. 2005) which are difficult to attach to less robust cephalopod species.  As such, they are convenient and informative to study when trying to learn about how oceanic food chains work.

So, what do we know about their feeding habits?  For one, we know that, as Dr. Benoit-Bird was trying to point out, that Humboldts are active, generalist predators, eating (according to Nigmatullin et al.) all manner of prey including “cepepods, hyperiid amphipods, euphausiids, pelagic shrimps and red crabs… heteropod molluscs, squid, pelagic octopods and various fishes.”  The authors also note that D. gigas is commonly cannibalistic, a facet of their predation that has probably contributed to their mythological status.  They are especially cannibalistic during squid jigging sessions, when they are excited by bright lights and surrounded by their injured conspecifics.  They feed near the surface mostly during the night, especially at dus
k and dawn, and spend their days deeper in the water column (200-400m deep), as was shown by a radio tracking study by Gilly et al. in 2006.  They can vary their diet depending on changes in their environment, showing an adaptability that no doubt contributes to their great abundance (Markaid and Sosa-Nishizaki, 2002).  Interestingly, recent ecological research has shown that their range has recently expanded from its historical locus in the equatorial Pacific ocean off of Central and South America to extend to the Pacific Northwest (as described by Cosgrove and Sendal, 2005, Zeidberg and Robison, 2007, and Field et al., 2007), possibly due to their unique ability to deal with hypoxic conditions that other predatory species cannot.   The squid can retreat into deep water with very little oxygen in between daily trips to feed at the surface, and thus avoid predation by other species such as Mako sharks (Vetter et al, 2008)..  On an unrelated note, if I were a squid researcher named Zeidberg, I’d just go ahead and change it to “Zoidberg”.  It’s too perfect.

There is dissappointingly little to say about the shoaling and predatory behavior of D. gigas.  If there are any glaring omissions in my coverage of the topic, please let me know; however, I think I found most of what’s in the scientific literature.  Basically, we know that they form large shoals, and that they are generalist predators.  More detailed information than that on the behavior of this species will have to wait for a new generation of adventurous ethologists.  Until then, I’ll be turning back to those species of cephalopod about which we have enough information to draw useful conclusions about behavior.  Perhaps someday the sort of experiments that have been done in smaller, more easily handled species will be done in D. gigas, but until that happens, I will probably stay mostly silent about them in favor of covering studies on less glamorous species in detail.

Please excuse me if it seems like I’ve rained on the proverbial parade.  Excuse me also for not getting into the methods of the studies that I’ve cited.  I encourage you to peruse them, but I opted to cover a greater area of research superficially rather than getting in depth about any specific finding in this post, so that I could adequately sum up the state of scientific knowledge of the Humboldt squid.  To lighten the mood, I’ll leave you with one more quote about the Humboldtl, by the realtively famous undersea cameraman, Scott Cassell, who has spent much of his professional career filming these squid (including a documentary titled “Humboldt: the Man-Eating Squid”) and is quoted in this piece by Tim Zimmermann:  “They are one of the most beautiful creatures, and they just happen to be lethal… There is no life form on this planet more alien than a Humboldt squid.”  I guess I didn’t realize that any life form on this planet was “alien”, given that they all evolved here.  Oh well – what do I know?

Thanks for reading!

Gilly, W., Markaida, U., Baxter, C., Block, B., Boustany, A., Zeidberg, L., Reisenbichler, K., Robison, B., Bazzino, G., & Salinas, C. (2006). Vertical and horizontal migrations by the jumbo squid Dosidicus gigas revealed by electronic tagging Marine Ecology Progress Series, 324, 1-17 DOI: 10.3354/meps324001

JOHN C. FIELD, KEN BALTZ, A. JASON PHILLIPS, & WILLIAM A. WALKER (2007). RANGE EXPANSION AND TROPHIC INTERACTIONS OF THE JUMBO SQUID,
DOSIDICUS GIGAS, IN THE CALIFORNIA CURRENT CalCOFI Rep., 48 : http://swfsc.noaa.gov/publications/FED/00859.pdf

James A. Cosgrove, & Kelly A. Sendall (2005). First Records of Dosidicus gigas, the Humboldt Squid
in the Temperate North-eastern Pacific Archives of the British Columbia Royal Museum

Unai Markaida, Joshua J. C. Rosenthal, & William F. Gilly (2005). Tagging studies on the jumbo squid
(Dosidicus gigas) in the Gulf of California, Mexico Fisheriy Bulletin, 103, 219-226

Markaida, U., & Sosa-Nishizaki, O. (2003). Food and feeding habits of jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae) from the Gulf of California, Mexico Journal of the Marine Biological Association of the UK, 83 (3), 507-522 DOI: 10.1017/S0025315403007434h

Nigmatullin, C. (2001). A review of the biology of the jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae) Fisheries Research, 54 (1), 9-19 DOI: 10.1016/S0165-7836(01)00371-X

RUSS VETTER, SUZANNE KOHIN, ANTONELLA PRETI, SAM MCCLATCHIE AND HEIDI DEWAR (2008). PREDATORY INTERACTIONS AND NICHE OVERLAP BETWEEN MAKO SHARK,
ISURUS OXYRINCHUS, AND JUMBO SQUID, DOSIDICUS GIGAS, IN THE CALIFORNIA CURRENT CalCOFI Rep., 49

Zeidberg LD, & Robison BH (2007). Invasive range expansion by the Humboldt squid, Dosidicus gigas, in the eastern North Pacific. Proceedings of the National Academy of Sciences of the United States of America, 104 (31), 12948-50 PMID: 17646649

Byard RW, Gilbert JD, & Brown K (2000). Pathologic features of fatal shark attacks. The American journal of forensic medicine and pathology : official publication of the National Association of Medical Examiners, 21 (3), 225-9 PMID: 10990281

I know these citations are sloppy – for some reason, I’m having some trouble working with the ResearchBlogging citation generator.  I promise I’ll fix it before next time.

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