The Mantis Shrimp: Supervillain of the Sea

These amazing creatures, also known as stomatopods, dwell in shallow near-shore marine environments. Most are around 15 cm in size, though some can grow up to 40 cm long, which is longer than my forearm. They aren’t true shrimp (which are classified in Order Decapoda, not Stomatopoda), though both are crustaceans within Class Malacostraca. Mantis shrimp possess some unique abilities, which are sharpened by their violent, aggressive temperaments, making them the crustacean supervillains of the oceans.


“Odontodactylus Scyllarus 2” by Roy L. Caldwell, Department of Integrative Biology, University of California, Berkeley, via Wikimedia Commons

So what are their superpowers?

  • Amazing Vision. Animals perceive color by absorbing various wavelengths of light with color photoreceptors, each type absorbing best at specific wavelengths. In humans, the “blue,” “green,” and “red” photoreceptors (which actually have peak absorptions in the blue, green, and yellow ranges, respectively) allow us to see colors ranging from red to violet. Most birds have a fourth type of color photoreceptor that is sensitive to UV wavelengths, allowing birds to see ultraviolet as a color.

    Mantis shrimp? They have up to sixteen types of photoreceptors. Twelve of them are for color.

    Four of these photoreceptors are for ultraviolet alone, and mantis shrimp color vision ranges from red to ultraviolet. But this incredible level of color detail, which allows them to see hues we humans cannot even imagine, is not the only amazing thing about mantis shrimp vision.

    Mantis shrimp can “tune” their vision based on environmental light conditions. Underwater, longer wavelength light tends to fade away faster than shorter wavelength light. This means that in very shallow water, the full spectrum of light is visible, but in deeper water, the environment is composed of shades of blue, violet, and ultraviolet. Mantis shrimp have color filters in their eyes that allow them to adjust to changes in the wavelength composition of their environment.


    “Mantis shrimp near Nusa Kode Island,” by Alexander Vasenin, via Wikimedia Commons

    Each eye is divided into two hemispheres, with an “equator” of six lines of ommatidia. Each line analyzes a different aspect of incoming visual information. The visual fields of the top and bottom sections of each eye overlap to some degree, allowing the mantis shrimp to have stereoscopic vision within a single eye, which is useful, since the two eyes move independently of each other. Mantis shrimp vision is so complex that different sections of the mantis shrimp’s compound eyes are allotted separate tasks. Using one part of the eye, a mantis shrimp might observe the color of an object; using another part, it might analyze the polarization of incoming light. Our own visual system cannot detect the polarization of light, but mantis shrimp can glean useful information about the orientation and texture of objects that reflect and scatter light in various directions.

    If their vision is so complicated, you might ask, how do mantis shrimp process all of the information, given the small size of their brains? As it happens, mantis shrimp employ parallel processing within the retina itself, meaning that much of their visual intake is processed before it even reaches their brain.

  • Mantis shrimp vision enables them to see in ultraviolet, to perceive fantastic color detail, and to detect polarized light – an impressive array of abilities. What other powers do they have in their arsenal?

  • Incredible Punch. As predators, mantis shrimp pursue one of two strategies of attack: spearing and smashing. Many species of mantis shrimp are “spearers,” killing their prey (and, frequently, each other) by stabbing with specialized sharp limbs. As impressive as this is, I find the “smashers” even more amazing.

    For an example of a “smasher,” let’s look at the peacock mantis shrimp, Odontodactylus scyllarus*:


    “Odontodactylus Scyllarus” by Silke Baron, via Wikimedia Commons

    The peacock mantis shrimp has a pair of reinforced, club-shaped limbs specialized for punching, rather like a pair of boxing gloves permanently attached to their bodies. The mantis shrimp prepares for a punch by locking the club in place with a spring-and-latch mechanism, then building up tension against the latch by contracting a muscle, before releasing the latch all at once. The club shoots forward at speeds up to 23 meters per second (that’s about the speed of a running cheetah) and punches the mantis shrimp’s prey with a force of up to 1500 Newtons. The punch only takes a few milliseconds, and the acceleration of the club can equal that of a .22 caliber bullet.

    Such a punch can be devastating, even to strong materials like a clam’s shell, but the attack isn’t over yet. The club punches through the water so fast that surrounding water vaporizes in a process called cavitation, creating a pocket of empty air in the wake of the punch. This space doesn’t stay empty for long. The surrounding water collapses back in on the space and crashes into the mantis shrimp’s prey in a second onslaught, often even more powerful than the first.

    A “smasher” mantis shrimp doesn’t let up the beating until its prey – a crab, a snail, a clam, or even a fellow mantis shrimp – shatters. Their punches are so powerful they can even crack aquarium glass, which make them a fearsome enemy indeed.

  • Body Armor. “Smasher” mantis shrimp can repeatedly pound away at their enemies with enough force to shatter shells, yet they only need to replace their clubs with the occasional molt. How can this be?

    It turns out that the molecular makeup of mantis shrimp exoskeletons – particularly of their clubs – is exceptional. On a micro scale, the structure of the club is extraordinarily resistant to fracture and damage, yet retains its hardness and effectiveness as a weapon. The mantis shrimp’s stiff telson – the hindmost segment of the exoskeleton – also excels at dissipating violent impacts, which mantis shrimp encounter during their frequent bouts of violence against one another.

    Like the resilient, resourceful villains they are, mantis shrimp employ reinforced armor both as protection and as a weapon.

  • Captivating Beauty. On top of these abilities, many mantis shrimp are simply breathtaking to look at – after all, every worthwhile villain needs a good sense of style. A picture will tell more than words alone, so feast your eyes on the glorious mantis shrimp:


    “Odontodactylus scyllarus 1” by Jens Petersen, via Wikimedia Commons

    Not all mantis shrimp are so brightly colored, but some – like the peacock mantis shrimp – are decked out in every color of the rainbow (and perhaps a few that only mantis shrimp can see).

  • Evil Disposition. All these powers packed into one creature might qualify mantis shrimp for membership in the Justice League, but they are no heroes. Behind the rainbow exterior, the exceptional vision, and the intimidating strength, lies the heart of a violent, ruthless predator.

    Mantis shrimp are merciless in their attacks on helpless prey, and they don’t limit their violence to obtaining food. Some species attack each other in ritualized displays of violence, assessing the strength of their rivals by punching them on the telson. This habit of violence toward each other is another reason mantis shrimp need such resilient armor.

The inescapable conclusion? Mantis shrimp possess exceptional vision, proficiency in violence, and strong defense. They are creatures of breathtaking beauty and bloodthirsty temperament. They are true supervillains.


“Mantis shrimp from front” by Jenny, via Wikimedia Commons

What other creatures do you think deserve to be called the supervillains (or superheroes) of the animal kingdom? Leave your suggestions in the comments!

*Odontodactylus scyllarus roughly translates to “tooth-finger Scylla,” an appropriately monstrous name for a mantis shrimp.

References:

Cronin, Thomas W., Roy L. Caldwell, and Justin Marshall. “Sensory adaptation: tunable colour vision in a mantis shrimp.” Nature 411, no. 6837 (2001): 547-548.
Cronin, Thomas W., and Justin Marshall. “Parallel processing and image analysis in the eyes of mantis shrimps.” The Biological Bulletin 200, no. 2 (2001): 177-183.
Cronin, Thomas W., N. Justin Marshall, Carole A. Quinn, and Christina A. King. “Ultraviolet photoreception in mantis shrimp.” Vision research 34, no. 11 (1994): 1443-1452.
Dingle, Hugh, and Roy L. Caldwell. “Ecology and morphology of feeding and agonistic behavior in mudflat stomatopods (Squillidae).” The Biological Bulletin 155, no. 1 (1978): 134-149.
Patek, S. N., and R. L. Caldwell. “Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarus.” Journal of Experimental Biology 208, no. 19 (2005): 3655-3664.
Patek, S. N., W. L. Korff, and R. L. Caldwell. “Biomechanics: deadly strike mechanism of a mantis shrimp.” Nature 428, no. 6985 (2004): 819-820.
Taylor, J. R. A., and S. N. Patek. “Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp’s telson.” The Journal of Experimental Biology 213, no. 20 (2010): 3496-3504.
Weaver, James C., Garrett W. Milliron, Ali Miserez, Kenneth Evans-Lutterodt, Steven Herrera, Isaias Gallana, William J. Mershon et al. “The stomatopod dactyl club: a formidable damage-tolerant biological hammer.” Science 336, no. 6086 (2012): 1275-1280.
Nathans, Jeremy, Darcy Thomas, and David S. Hogness. “Molecular genetics of human color vision: the genes encoding blue, green, and red pigments.” Science 232, no. 4747 (1986): 193-202.

See also:

Webcomic on mantis shrimp, by The Oatmeal.
Radiolab podcast on color, including a discussion on mantis shrimp vision and an full choir illustrating mantis shrimp vision through sound.

What Causes Sore Muscles After Exercise?

Last week (and much of the week before) I’ve been unable to keep up on my posts, due to an ongoing home construction project. I won’t chatter on about the details, but suffice it to say, the project involved a lot of shoveling dirt and digging trenches. So, now that I’m finally able to return to my blog, I’ll talk about something I’ve experienced since the last time I posted – muscle soreness!

Most of you have probably felt it at some point, whether you’re a casual gardener or a professional athlete – you feel great, if a little tired, right after a new workout, but a day or two later, your muscles grow stiff and sore, only to return to normal again within the week. So what’s going on here? Why do you feel fine right after working out, but sore hours later?

This phenomenon is called “delayed-onset muscle soreness” (abbreviated as DOMS), and there are actually a number of proposed explanations. It occurs most frequently after strenuous, unfamiliar exercise, especially if muscles have been undergoing “eccentric” movement, in which muscle fibers are being stretched despite trying to contract at the same time.

One popular – and probably untrue – explanation for DOMS that you might have heard bandied about is lactic acid buildup. While it’s true that lactic acid builds up in muscles during strenuous exercise (it’s a byproduct of anaerobic respiration, which happens when muscles aren’t getting enough oxygen), the body clears that away relatively quickly; therefore, lactic acid buildup doesn’t explain soreness that persists days after exercise. In addition, lactic acid buildup also occurs in non-“eccentric”-type muscle movements, which don’t usually result in DOMS.

The most likely explanation for DOMS is actually a combination of several proposed explanations: muscle damage, movement of enzymes out of muscle cells, and inflammation. According to this hypothesis, muscle tissue is damaged by new or unfamiliar “eccentric” movements, which tear some membranes within and around the cells, allowing enzymes and other molecules such as calcium to leak into the surrounding area. This stimulates an inflammatory response from the body, which leads to the pain, stiffness, and weakness you might feel a few days after an unfamiliar workout.

DOMS doesn’t pose much of a problem in and of itself, since the soreness will go away and damage will heal on its own within a few days, but you should be careful about exercising sore muscles. DOMS can cause temporary muscle weakness, so your usual level of activity becomes proportionately more intense, putting you at greater risk for a more serious muscle strain. That’s not to say that muscle soreness should stop you from exercising – it’s just wisest to be careful.

Several review articles have looked over possible ways to treat or prevent DOMS, but they found no clear standout treatment strategy. There have been a lot of studies looking into various treatments, but on the whole, these studies have been plagued by small sample sizes, shoddy research methods, and conflicting results. Applying ice and compression might help somewhat; the jury is still out on whether NSAIDs (non-steroidal anti-inflammatory drugs) are useful; massage, stretching, and nutritional supplements probably won’t do anything. Interestingly, the most effective treatment by far is light exercise, which will lessen the soreness and stiffness (though only for a little while). Furthermore, repetition of the same “eccentric” muscle movements will decrease DOMS over time, meaning that you might feel less sore after doing the same muscle movements in future workouts.

So if you feel stiff and sore from, say, spending a week shoveling dirt, then you might feel better if you went outside and shoveled a little more.

References:
Cheung, Karoline, Patria A. Hume, Linda Maxwell. “Delayed Onset Muscle Soreness.” Sports Medicine Volume 33, Issue 2, pp. 145-164 (2003).
Connolly, Declan A.J., Stephen P. Sayers, Malachy P. McHugh. “Treatment and Prevention of Delayed Onset Muscle Soreness.” Journal of Strength and Conditioning Research Volume 17, Issue 1, pp. 197-208 (2003).

How Do Bird Lungs Work, Anyway?

Here’s a little more detail on how bird lungs work, compared to human lungs:

As I mentioned in the last post, avian “flow-through” lungs are much more efficient than human lungs. We humans pull in air and push it out of our lungs with every inhale and exhale, and in normal breathing, we only use a portion of our total lung capacity. During gas exchange, the fresh, oxygenated air we breathe in mixes with “old” air that’s been in the lungs for a while. Oxygen makes its way down to the alveoli (the little pockets where gas exchange between actually occurs); here, oxygen from the lungs diffuses out into the bloodstream, and carbon dioxide moves out into the lungs. Finally, carbon dioxide and a good deal of unused oxygen are expelled as we exhale.

This system works well enough, but it’s terribly inefficient. Birds, however, have evolved a much better system, illustrated below:

Bird Lungs Part 1

Bird Lungs Part 2

Step 1. Inhalation: Fresh air is inhaled into the first chamber

Step 2. Exhalation: Air is pushed across the lungs, where oxygen transfers over into the bloodstream (and carbon dioxide transfers into the lungs)

Bird Lungs Part 3

Bird Lungs Part 4

Step 3. Inhalation: Air, now deoxygenated, is pushed into the second chamber. Meanwhile, a new batch of fresh air is inhaled into the first chamber, as in Step 1.

Step 4. Exhalation: Deoxygenated air is exhaled out again. Meanwhile, the new batch of fresh air is pushed across the lungs, as in Step 2.

Bird Lungs Inhale

Bird Lungs Exhale

Step 5: The Cycle Continues…

Each “breath” of air actually remains in the bird’s lungs for two cycles of inhalation and exhalation, and is then fully expelled at the end. Fresh air flows across the lungs with every breath, and “old” air never mixes with new air (as it does in human lungs). Respiration is much more efficient, which allows birds to perform strenuous activities like flying that would be much more difficult with our paltry human lungs.

5 Reasons Birds Are Better Than Humans

I’ll admit, that last post was pretty heavy, so here’s a light little post on 5 ways that birds are more impressive than we humans:

#1: They are living dinosaurs, in the most literal sense.

#2: Their lungs are way, way better than ours. Human lungs are essentially sacks that we partially fill and partially empty with fresh air with every inhalation and exhalation, which isn’t very efficient for gas exchange. Birds, on the other hand, have a “flow-through” lung structure, which creates a constant flow of fresh air across the lungs, and prevents fresh air from mixing with older, deoxygenated air (which is what happens in human lungs). This allows for a much more efficient transfer of oxygen and carbon dioxide between the air and the bloodstream.

#3: Magnetoreception. Birds can detect magnetic fields and frequently use them for navigation. Many birds probably detect magnetic fields using small, magnetite-containing, compass-like structures inside their heads. Not only do birds detect the earth’s magnetic field (useful for migration), but they can even detect local magnetic fields, which allows them to accurately navigate an area even without visual cues about their location.

#4: Most of them can fly. Some can run. Some can swim underwater. Some can hover in midair. Many can sing (and some can do much more). A few can even dance. The point being, they would definitely come out on top in a talent competition, hands down.

#5: They have super-excellent vision. Let me count the ways:

  • They can see in ultraviolet! Some birds have feather patterning in ultraviolet-colored pigments that are invisible to the human eye (and putting UV-blocking sunscreen on their feathers can mask these pigments to the birds).
  • Some birds have two foveae (centers of vision) in each eye. We humans only have one per eye.
  • The American Woodcock has a visual field of 360 degrees.
  • Bird motion perception is so sharp that they could potentially navigate by the movement of the stars. No, not the placement of the stars – they can see the stars moving across the night sky as the earth rotates, where the human eye would only see motionless points of light.

The inescapable conclusion? We humans may think we’re special, but birds are truly amazing.



(As a side note, bird hearts are also pretty impressive. They are generally larger than mammal hearts relative to body size, and have more muscle fibers, with more mitochondria per muscle. As a result, avian hearts have more power in pumping blood. This didn’t make the list, however, because it comes with a major downside: such powerful hearts can lead to very high blood pressure.)

Reference:

Gill, Frank B. Ornithology: Third Edition. New York: W. H. Freeman and Company, 2007. Pp. 31-36, 146-150, 184-191.

How is a Fish Gill Like an HVAC System?

Countercurrent exchange is one of my favorite physiology tidbits. Creatures across the animal kingdom use countercurrent exchange to increase the efficiency of functions ranging from a fish collecting oxygen in its gills to a wolf conserving valuable body heat in cold weather. We humans also employ countercurrent exchange in architecture and engineering – it’s one way to make a building’s heating and air conditioning systems more efficient.

So, what is countercurrent exchange? Imagine that you have any two fluids (either liquid or gas) flowing through two adjacent conduits, and you want to transfer either molecules or heat energy from one fluid to the other as they flow alongside each other. Flow in the same direction is called concurrent exchange; flow in opposite directions is called countercurrent exchange. For reasons that I’ll explain below, the transfer of molecules or heat is much more efficient if the fluids are flowing in opposite directions than in the same direction.

Concurrent Exchange Demo Countercurrent Exchange Demo

But how does it work? Let’s start with a few basic concepts:

First of all, molecules and energy like to spread out, and will tend to move from a more crowded area to a less crowded area. A drop of bright red food coloring will spread out to color a whole glass of water pink; an ice cube dropped in a hot cup of tea will melt and make the whole cup of tea cooler (and weaker); and a dollop of spaghetti sauce on your plate will somehow find a way to stain the tablecloth, the placemat, and the white shirt you’re wearing (or maybe that’s just me.)
molecules
Why does this happen? Keep in mind that molecular movement is very haphazard, and molecules crash into and bounce off each other pretty much at random. Now, say you have two “rooms” with a door connecting them, like in the picture on the right; one room contains a lot of high-energy molecules and the other just a few. In the crowded room, molecules are bouncing around a lot and will likely shove a number of molecules over into the emptier room. Meanwhile, very few of the molecules from the empty room are likely to get pushed back into the already-crowded room. This process continues until eventually, both rooms contain approximately the same number of molecules, although they are still passing from one room to the other. At this point, the two rooms have reached a state of “dynamic equilibrium”.

The tendency of molecules to move from more crowded to less crowded is a useful property when transferring molecules (or energy) from one substance to another. Imagine a fish moving oxygen from oxygen-rich surrounding water to the oxygen-starved blood in its gills: the oxygen already has a tendency to move from the water to the blood, as long as it can pass through whatever barrier lies in between, such as the permeable walls of blood capillaries in a fish’s gills.

The more efficient the transfer of oxygen from water to gills, the better for the fish. Generally speaking, organisms that spend less energy to do the same things will out-compete their less efficient rivals. They will be more likely to survive, produce more offspring, and be overall more successful; this leads to huge evolutionary pressure in favor of efficiency. Thus, the high-efficiency benefits of countercurrent exchange have led to its widespread adoption in biological systems.

So how exactly do concurrent and counter-current systems work? Let’s look at them in the model of a fish gill:

On a basic level, fish gills contain a lot of small, thin blood vessels. Deoxygenated blood is pumped in after circulation through the body to be re-oxygenated by the surrounding water that gets pushed through the gills. The freshly oxygenated blood then continues on to bring oxygen to the rest of the body.

Cocurrent NumbersIn a concurrent exchange setup, the blood vessels are oriented in the same direction as the flow of the water through the gills. This is not a very efficient setup – only half of the concentration of oxygen at most can transfer from the water to the blood. For simplicity’s sake, let’s stay that the fish starts out with an adjacent stream of 100% oxygen-saturated water flowing through the gills containing 0% oxygen-saturated blood, and that both water and blood can hold equal amounts of oxygen. Oxygen will tend to flow from the 100% oxygen-saturated water into the deoxygenated blood when they first come into contact with each other. Further down the line, as the oxygen continues to transfer, you might get 90% saturated water, 10% saturated blood; then 70% saturated water, 30% saturated blood. So far, so good – oxygen is still flowing from water to blood. The transfer of oxygen continues until you finally have 50% oxygen-saturated water and 50% saturated blood – and now, no more oxygen can transfer from water to blood. True, the fish has gained oxygen in its bloodstream, but half of the oxygen remains in the water flowing out of the gills again. All that oxygen, wasted! If only there were a way for gills to harvest almost all of the oxygen from the water passing through…

And that’s where countercurrent exchange comes in to play. Simply by switching the direction of blood flow so that the water and the blood are flowing in opposite directions, efficiency of oxygen transfer increases dramatically. How? Well, may seem counterintuitive, but here’s an image to illustrate the basic difference between countercurrent and concurrent exchange:

Cocurrent DotsCountercurrent Dots

In countercurrent exchange, the water and blood are flowing in opposite directions. At the point where the blood first begins to flow alongside the water (at the right side of the figure above), the water has already traveled the length of the section where water and blood are adjacent. Countercurrent Numbers As you can see in the image, there is a higher concentration of oxygen in the water than in the blood for almost the entire length where they interact, which means that there is a consistent tendency for oxygen to move from the water into the blood. Even at the far right end of the image, when there is hardly any oxygen remaining in the water, this flow from water to blood still continues; although there is very little oxygen remaining in the water, there is none at all in the deoxygenated blood entering the gills.
In short, transfer of oxygen in a countercurrent exchange system can be almost 100% efficient – twice the maximum efficiency of a concurrent exchange system – with a simple reversal of the direction of flow for one of the fluids.

Countercurrent exchange is used for more than just oxygenation of gills. Many endothermic (“warm-blooded”) animals use it to prevent heat from their extremities by conserving heat in their core body.

Imagine a wolf walking in the snow: with body-temperature blood continuously flowing through thin vessels in the wolf’s legs and feet, a lot of body heat will be lost unnecessarily. Fortunately, countercurrent exchange prevents some of this heat loss – in fact, much of the heat carried through the bloodstream will never reach the foot. The artery that carries blood to the foot runs right next to the vein that carries blood back from the foot, allowing countercurrent exchange of heat along the length of the leg. As warm blood travels down the artery, heat continuously leaches over into the cooler blood carried back up by the vein, which is thus warmed up as it travels back to the body. As a result, by the time the blood in the artery reaches the wolf’s foot, it is considerably cooler, and therefore less body heat is lost from the foot in the snow. Interestingly, the foot operates at a lower temperature than the core of the wolf’s body.

So how is a fish gill like an HVAC system? Many air conditioning and heating systems use the same principle of countercurrent exchange when ventilating buildings with outside air. The air intake and output pipes run adjacent to each other, in opposite directions, so as outside air is brought into the building, it is made warmer or cooler by capturing heat from, or dumping heat into, the building-temperature air flowing out the ventilation system. Countercurrent exchange can save us energy, too!

Countercurrent exchange is a simple and elegant solution for a variety of problems in nature and engineering. What other applications can you think of?

What’s That Science? Eurypterids

Orchids? You can buy some of those at the nearest Home Depot. Squids? Most people have probably at least heard of calamari. But what on Earth is a eurypterid?

Well, it isn’t an organism you will find living on Earth any more. Eurypterids, also known as “sea scorpions,” were common underwater predators from about 470 to about 250 million years ago. Long before the rise of the dinosaurs, these chelicerates – relatives of modern-day spiders, scorpions, and horseshoe crabs – lived abundantly in a variety of aquatic habitats, preying upon fish, other arthropods, and each other. Some larger species could reach almost two meters in length – that’s longer than the average American is tall!

Eurypterids were diverse organisms. Some swam, some walked, some crawled and burrowed, and some did all of these things. This variety in lifestyle is clearly visible in the diversity of body plans eurypterids display in the fossil record; some have paddle-like limbs useful for swimming, while others had appendages better suited for walking or gathering food.

Sea scorpions seem to have lived in a wide variety of habitats, ranging from solidly marine ocean floor to freshwater lakes and rivers to brackish estuaries and bays (where freshwater meets and mixes with saltwater). Alas, eurypterids reached their peak during the Silurian Period, around 440 – 420 million years ago, when plants were still in the early stages of moving onto land. If you want to see a eurypterid these days, you’ll have to look into the fossil record. Eurypterids are most commonly found in North American and European fossil assemblages; the rest of the world has a fairly sparse record of these creatures.

Why should you care about eurypterids? Well, who wouldn’t care about a two-meter-long sea scorpion that could have been swimming around a lake or river near you – if you had lived 250 million years ago, that is!

References:

Benton, Michael J., and David A. T. Harper. Introduction to Paleobiology and the Fossil Record. Chichester, West Sussex: Wiley-Blackwell, 2009. Pp 375-8; 482-6; 550. Provided information on eurypterids, and on the colonization of land by plants.

McDowell, M. A., C. D. Fryar, C. L. Ogden, K. M. Flegal. “Anthropometric reference data for children and adults: United States, 2003-2006.” National Health Statistics Reports: No 10. Hyattsville, MD: National Center for Health Statistics (2008). http://www.cdc.gov/nchs/data/nhsr/nhsr010.pdf – see pages 14 and 16. Provided height statistics of men and women in the United States.

Selden, P. A., “Autecology of Silurian Eurypterids.” Special Papers in Palaeoecology Issue 32, 39-54 (1984). Provided information on ecology of eurypterids. http://www.paulselden.net/uploads/7/5/3/2/7532217/autecology.pdf

Tetlie, O. Erik. “Distribution and dispersal history of Eurypterida (Chelicerata).” Palaeogeography, Palaeoclimatology, Palaeoecology Vol. 252, Issue 3-4: pp. 557-574 (2007). Provided information on distribution of eurypterid fossils. http://dx.doi.org/10.1016/j.palaeo.2007.05.011