Through The Sandglass

Georgia Tech Crab Lab: guest post on the art of sidewinding

Over the life of this blog, I have written a number of times on the remarkable and innovative work carried out at Georgia Tech’s Crab Lab, investigations into how critters get around in loose sand and how this can be applied to robotics (see, for example, here, here, and here). Crab Lab is headed up by Dan Goldman, and I was recently delighted (and flattered) that he contacted me about the “Sand” book. I asked if he and his colleagues would be willing to contribute a guest post – and here it is. Courtesy of Henry Astley, a postdoc “who loves all things snakes” and collaborator Joe Mendelson: the state of the art on the physics of sidewinding.

 

In spite of their barren reputation, deserts around the world teem with life, including a wide array of animals from beetles to camels. While much has been written about their adaptations to deal with two of the most notable characteristics of deserts, extreme temperatures and scarce water, far less attention has been paid to their interactions with the other distinguishing characteristic of many deserts: sand.

The ability to move from place to place is crucial for animals to find mates and resources, regulate their body processes, avoid predators, and colonize new environments. But sand makes locomotion difficult, whether moving on it or through it. Particularly problematic is that sand will behave as a solid under certain loads, but will yield and flow like a fluid under others, and very small differences in foot placement and movement can be the difference between moving and becoming hopelessly stuck. Animals deal with this challenging substrate in a variety of ways, whether by anatomical changes or selecting the best movement patterns.

Perhaps the strangest movement pattern of desert animals is the famous “sidewinding” locomotion, seen in certain snakes of sandy deserts around the world. While prominent herpetologist Clifford H. Pope wrote in 1955 “A study of [sidewinding] is recommended to anyone who likes to be confused,”, the underlying motion is quite clear when examined in detail. The snake lifts its head and moves it forward, placing it on the ground, then repeats this motion in a propagating wave down the body – see Science paper supplementary movies.

This produces a characteristic trackway consisting of a series of parallel lines, with the imprint of each scale on the belly clearly visible, showing that the snake does not slip (which would smear the tracks and erase the fine scale imprints). Ultimately, this seemingly complex motion can be reduced to a pair of waves producing vertical and horizontal body undulation, +- 90 degrees out of phase, which propagate together down the body. This simple model may be a “neuromechanical template”, a simple model of a motion which captures all the essential features, potentially serving a simple “target” for the animals to control their locomotion (see Sidewinding with minimal slip: Snake and robot ascent of sandy slopes). This two-wave template of sidewinding also produces sidewinding locomotion when applied to a snake robot, allowing the robot to move on sand effectively.

 

 Screen-grab from Science supplementary movies

This provides a great experimental tool, because the physics of sand have yet to be reduced to simple systems of equations (as has been the case in fluids for almost 200 years), making computer modeling of results difficult and time-consuming. However, the snake robot provides a “physical model” for movement in sand, allowing us to test hypothesized biological mechanisms. Further observations of biological snakes have revealed the modifications of the two-wave template responsible for effectively sidewinding up inclines and turning (see the recent PNAS paper), which in turn have further improved the effectiveness of the robot.

In spite of these insights, the serpents of the sand still hold many mysteries. Why do some snakes sidewind, while others don’t? Why can some species move effectively on sand, while others fail? How do sidewinders deal with obstacles? Can we reconstruct the evolution of this remarkable mode of locomotion from the tracks it has left, or has this history been lost in the sands of time? 

[My sincere thanks to all at Crab Lab for their work and this post. Photographs by Henry Astley]

On the Edge of the Sahara: a guest post by Ann Lingard

At the edge of the Sahara, I stood for a while and watched a dune move. It pushed out its toes, testing the flat ground in front of it; a small trickle of sand cascaded down the lee side and staked a claim, a first footstep of a few millimetres, across the stony flat.

We were in south-east Morocco, and had spent much of that morning crossing the reg, a gravel plain, and were now into the erg, a broad sea of dune-waves. It was a windy day, the sky hazy with particles and the ground beneath our feet blurred by a spray of sand. The particles whipped round our calves as we tramped up each windward slope. From each crest – the sharply-carved edges so familiar from tv documentaries and photos – there would be a view of more dunes; and then we would stride down the lee side where the negative pressure, created as the wind skimmed above our heads, sucked the disturbed sand upwards into our faces.

Having long ago read Michael’s book Sand, and having skimmed the relevant sections in The Desert (his new book arrived two days before our flight), I was delighted to experience for myself the physics and lexicon of dune-formation (saltation, reptation – lovely words): to watch the bounding sand-grains knocking into each other, leaping in the air, and co-operating to form small avalanches; to see the different patterns of aeolian ripples - and to draw comparisons with water-formed ripples on my home patch, the Solway shore.

Less delightful was the determined ingress of sand grains into clothes, socks, boots, eyes, hair and later, food (but no ‘sand in the sandwiches’ here – much tastier ‘sand in the salad’ and tagine instead).

And then, after about four hours’ walking, we came to an area of flat ground on which the dunes were scattered, not contiguous, encroaching individually or in small groups. Formerly this had been the bed of a lake, on the flood-plain of the River Draa, but after the dam was built upstream near the Atlas mountains and the river’s path moved for irrigation purposes, the lake inevitably dried out, leaving a stone-scattered and gently-contoured plain. Now, the sand was taking advantage of this empty space and was piling up and moving in, millimetre by millimetre.

However, it wasn’t only the dunes that were intriguing, for whenever we came to areas of reg, the flat and stony desert, there were scatterings of red or grey clay potsherds – handles, fragments of bowls, even a couple of large, almost-whole pots. Some pieces were engraved with simple patterns, and a few had remnants of green glaze. Had these fragments been washed down from dwellings in the Atlas or the Draa Valley? How old were they? Sadly, other than a comment that they were ‘perhaps a hundred years old’ it wasn’t possible to get a definitive answer from our Berber guide.

The camels, carrying three days’ water, the food, cooker and utensils, and our tents and baggage, moved faster than we did, and one of the camel-drivers had staked a sack on top of a ridge to show exactly where that night’s camp was being pitched. The camp-site, on flat ground between the tall dunes of Zahar, felt remote, yet without the use of GPS or compass, our guide had led us to this spot.

The next morning, an hour after we had set off, a lightly-laden camel and his driver suddenly appeared from between the dunes and crossed our path, following an unseen (to us) route. Elsewhere we saw camel footsteps imprinted and dried into what had once been mud. Humans and their animals were moving purposefully through this landscape, clearly using natural signs and age-old knowledge. Lacking this knowledge, and with only the footprints of other members of our party to follow, we would soon have been lost.

But not all the footprints were caused by human-related passage: parallel scrapes of the clawed feet of laboriously-moving beetles; a lizard’s skittering footsteps straddling the straight line of its tail; small mammalian footprints – gerbils, scampering and churning the sand around their burrows; and the focussed, straight-line padding of a fox: apart from these signs, no-one would have known that animals were living in this landscape.

After four days trekking through erg and reg, wadis and hamada, and feeling the chill of February nights that were deeply silent except for the occasional camel grumbling, and seeing into the depths of the Milky Way, I came away with a sense of wonder at what I had seen – but also knowing that, apart from saltation and reptation, I really didn’t understand a thing.

  

Sunrise at the well of Tilhatine

 

[Ann is an old friend of mine and of the Solway Firth, a sheep-raiser and weaver – and a wonderful writer. Enjoy Solway Shore Stories, her blog, Solway Shore-walker, and her extraordinary and compelling novel, The Embalmer’s Book of Recipes, now out in its second edition. See also Ann’s website. Since she has recently been trekking in the Moroccan desert, I requested a guest post – and here it is, so many thanks, Ann.]

Guest post: Baltic gold: the amber rush

Danuta Ziętala writes:

When you visit the Polish coast you have to take a souvenir: a necklace made of amber, a bracelet made of amber, or other jewellery. Prices depend on size, working, the precious items inside (mosquito!).

I want to encourage you to experience the "amber rush". Last spring I experienced it while assisting my friend Tomasz in taking some dunes measurements. Spring and autumn are best to look for amber.

I was lucky twice: (a) we reached Jantar village (jantar in old-polish language means amber), (b) last night there was a storm. Early morning Tomasz and I got to the beach and saw local people wandering in the water. They know that storm = new supply of amber. They sell it to tourists and jewellers. Of course you can see amber all over the beach. Small pieces are glittering between sand grains – an incredible and pleasant feeling.

You start collecting one after another and cannot move a meter because there are so many of them. The most interesting for me was that if you want to find a bigger piece, you should check the wooden sticks washed ashore. Why? It turns out that amber is as light as wood and they flow together in the water. 

Where is my amber????

There were so many sticks. They even built the shore.

Everybody could find some amber. I found one, and another and another.... this was my "amber rush." I forgot about the world. I forgot my duties. Tomasz was looking at me with understanding and did his work, patiently waiting till I gathered all the amber I could see. 

I keep my amber on the window sill. It glitters in the sunny days. Some really small pieces I used to prepare a tincture which is a treatment. I am proud of my amber collection and after Schwarzenegger I can say: "I'll be back."

Jantar is a small tourist village located in Pomerania District in Poland. It is so special because of mouth of Wisla River, which brought amber from central Poland. Visit Poland so you can see the beautiful sandy beach and experience the amber rush on your own!

Thanks for this, Danuta - keep up the hunt and Tomasz, keep up the dune-measuring!

The Great Sand Dunes – as seen through the lens of ‘Written in Stone’

 

Earlier this year, I discovered a superb geological and photographic blog that I had not encountered before. ‘Written In Stone…seen through my lens’ is by Jack Share, who describes himself as follows:

I was the kid in the Jackson Browne song “in ’65 I was seventeen”, when my educational efforts were directed toward a degree in zoology and a career in the health professions. Years later, the fossils I found as a kid in Central New York led me to paleontology, their evolutionary relationships and to developmental biology. It wasn’t long before I came to appreciate the importance of an education grounded in geology. Former life and former landscapes are indeed inseparable. In this blog I offer an interpretive and photographic perspective of our world, both past and present, and both within my New England home and well beyond.

Jack wrote to me about his two posts on the Great Sand Dunes of Colorado, the first a comprehensive geological review (the ‘the juicy geological details’), the second a photographic essay, ‘Climbing the geology of the dunes.’ I thoroughly recommend a visit to these posts – and to the rest of the blog. Jack has a natural ability to write clearly and informatively but also to make geology fun, and his photographs are great. As a lure, here is a selection (reproduced with his permission).

Guest post: The 'sandwave' - a homage to Elvis?

John Lackie writes:

Last year I had the pleasure of going on a four-day residential field course as part of an Open University geology module (SXG288); one of the sites we visited had a spectacular feature that might be of interest to those who appreciate sand in all its forms.

The site was Bowden Doors in Northumbria (NU 072323), a long, west-facing crag of Fell Sandstone, 3-8m high, a favourite of both climbers and geologists. The sandstone is derived from the erosion of rock above what is now the Southern Uplands of Scotland and the sand was carried south-eastwards by a braided river system during the early Carboniferous period. There are several unusual geological features that I won't reveal (to avoid spoiling future visits by OU students), but the thing that impressed me most was the 'Sandwave'. I was sure Michael would know of this - but he didn't and he invited me to write a guest blog about it.

Inevitably the pictures I had weren't suitable so I revisited the crag some months later: once home I discovered that none of the pictures had come out - or had disappeared into the digital ether. A third visit was a bit more successful.

 

Looking south-east at the northern end of the crag: it is capped by an impressive quiff, not of hair but of sandstone - the 'sandwave'. (Note person on extreme right of the picture for scale).

Here's another view - with climbers in the distance:

Although it resembles a 1950's quiff, it is really the eroded remnants of the last remaining layer of cross-stratified sand, now compressed into stone, that has resisted the Northumbrian weather more successfully than the lower layers.

Looking up at some of the lower layers:

So, an impressive location and an interesting contrast to some of the coastal sites we visited where there are some fantastic fossils and lots of other geologically interesting features. The scenery is good too!

 

[John and Ann Lackie are friends who live on the borders of the Lake District – Ann, under her author’s alter ego of Ann Lingard, has written for this blog in the past. The topic of the ‘sandwave’ arose during a rambling conversation over the course of a delightful pub lunch in the heart of the hills earlier this year. Thanks for this, John - I'll have to make a pilgrimage myself! For more on cross-bedding see this old post.]

 

The sands of Breezy Point

As the residents of Breezy Point continue on their path back to normality after the ravages of superstorm Sandy, part of that normality is that Larry Deemer has been out and about on the beach again, superbly documenting the shape-shifting art of the sands. Here, many thanks to Larry, is a selection of his latest photographs – enjoy!

Guest post: One Grain of Sand, by Susanne Rieth

 

“One grain of sand, it is all that remains of my vast empire." This line is from the classic film The Never Ending Story. When the mythical land of Fantasia is destroyed, the protagonists are able to wish it back into existence using that one grain of sand, which had escaped destruction. If I could have wished Rockaway back to normal using sand I would have been in luck. Because of our sand? Instead of disappearing it climbed into our houses, streets, cars, beds, bathtubs and just about every other imaginable place.

As Memorial Day approaches one can feel the return to normalcy knocking at the door. Some of the signs of destruction haven't quite left us yet, although more and more our hometown is starting to feel like itself again.

One of the ways that I kept my head through the storm and the winter months that followed was by remembering what I love most about living here. The beach. Every morning, I walk the shoreline with my pooch. Today I invite you to join me, to see what we find in the sands of Rockaway.

Our first find, a paper plate with a child’s drawing on it.

Next, Nina and I encountered this guy, in quite a compromising position.

It’s a bit of a hobby of mine to rescue horseshoe crabs that end up flipped over and helpless. After he was vigorously sniffed, he was re-released into the wild

Next up, this purple broom head.

Lines in the sand that remind me of tree branches.

A series of seashells and sea glass.

A little patterned feather.

And a shred of yellow caution tape.

So there you have it, the best of one morning’s exploration in the sand along the shore. And should the day ever come when I can wish upon a sand grain, I’ll be in luck. Right now, there are about a thousand of them in my shoes.

..................................................................................................... 

[As I wrote in my previous post on the rebuilding of Breezy Point, Susanne, a resident of neighbouring Rockaway, contacted me about using Larry Deemer’s photos on her Rockaway Rises blog (which she started because she “was tired of looking at all the negative news stories and photography about Rockaway.” Larry was, of course, delighted, and Susanne used some of his Wave to Wave photos from his book of the same title. It also seemed like a great opportunity for a guest post, and Susanne kindly agreed – this is the very enjoyable result. Susanne grew up in Breezy Point and is a freelance photographer and photo editor who “spent several weeks after Hurricane Sandy digging out my parents’ flooded basement before driving to Florida where I had to stay for 6 weeks because my office had also been flooded and my job temporarily moved down there.” Thanks for this post, Susanne, and best wishes for a great Memorial Day weekend to you and everyone in Rockaway and Breezy Point.]

The waves roll on - and Breezy Point tries to rebuild

Back in the very early days of this blog, one of the great and rewarding pieces of serendipity was getting to know Larry Deemer, sand aficionado and photographer extraordinaire. I first published a selection of his stunning images back in April 2009, and there have been subsequent collaborations on ice, horseshoe crabs, and scale. And in the meantime, he has published a glorious book of his sand pattern images. Most importantly, also in the meantime, we have become good friends and it’s a great pleasure to post a selection of his latest photos of the creative conspiracy between waves, foam, and sand.

But there’s a backstory, and a dramatic one. In that first post I wrote that Larry is “lucky enough to live in Breezy Point, a coastal neighbourhood in Queens, the borough of New York City,” and indeed my wife and I were lucky enough to visit Larry, Lou, and Buck and enjoy their company, their home, and their beach. But then, in October of last year, Hurricane Sandy rolled the dice and the luck of the residents of Breezy Point changed – catastrophically. They were hit not only by damage from the storm (which would have been relatively manageable), but by a devastating fire that raged out of control and destroyed over a hundred homes. Over 200 more were terminally damaged and have been bulldozed. Larry was, in this sense, lucky: the water damage to their house was reparable and they have been back home since the middle of January, feeling, as Larry has commented, “like pioneers.” But not home as it used to be – houses around them are no longer there, and the process of rebuilding is stalled by the absurdities of bureaucracy. We use the term “community” somewhat loosely these days, but Breezy Point was – and is – a real community; just read some of the stories on the web to appreciate what the word really means.

But nature continues her activities, oblivious to human hardship, and the beach, although “the 6’ dunes are now 2’ dunes',” continues to be an inspiration. Thanks, Larry.

[Soon after writing this post, I heard from Susanne Rieth who runs a blog called Rockaway Rises - Rockaway is right next to Breezy Point. The blog is well worth a look, described thus: "Rockaway Rises is a community for sharing and enjoying the positive aspects of life on our fair peninsula.  We welcome submissions of photography, art work and uplifting stories that remind us why we love it here, and why we will continue to thrive here in spite of Hurricane Sandy.  No devastation and destruction.  Only gratitude for what we still have, the ocean and our community."] 

Guest post: a gallery of sand photographers' styles

 
Kurt Meyer writes:

SHOOTING SAND

For the last 15 years, on and off, I have been collecting sand. The “off” times resulted from not having an answer to the question, ‘what do I do with all of this sand?’ I have tried various methods of displaying my sands as you can see below. Two-ounce bottles with overhead lighting being the best. 

But the only people who could regularly view the display were my wife, the neighbors, some family and my dog, Zena, and she could care less. In fact, anything that draws attention away from her is not well tolerated.

Things got interesting again, however, when I decided to try my hand at getting pictures of my sands. At first, I tried a digital microscope and although it turned out to be an interesting exercise, the quality just wasn’t there. The edges, for example, are usually blurry, the definition is often lacking and the color is rarely true. These problems would even be too much for a Photoshop package. You can, however, get a good idea of what is lurking in your sand sample at 20x, as in this image of sand from Fintragh beach, but it is not suitable for framing.

Today I still use the digital microscope but only for screening sand samples. It is pretty good at predicting which sands will be “photogenic.”

My first foray into serious sand macrophotography began with my purchase of a used Nikon D70. The following picture was taken with that camera body and a 60mm 1:1 lens. Even though the sensor is only a 6.1MP it still provides a pretty good image. This photo is of one of the famous green sand beaches in Hawaii, Pocket Beach to be exact. The olivine, lava and coral make a striking image [at the head of this post - MW]. 

At any rate, this new photographic endeavor answered the big question, ‘what do I do with all of this sand?’ Answer, SHOOT IT.

There are a number of photographers who do outstanding sand photography. I would like to showcase several of them. This is, by no means, an exhaustive list. This only reflects the photographers that have posted images on my website, Tropics of Sand. I will attempt to compare and contrast their styles. As you will see, the top photographers all have their own style. Over and above the camera equipment used, the thing that defines a photographer is their approach to getting the shot. All of the images that you will see below are unique to the photographer. I can tell a Heider photo from a Couette and a Sepp from a Kenney in the blink of an eye. I am sure that you will, too.

SAND PHOTOGRAPHERS

(click images to enlarge)

Leo Kenney: Large field, great clarity

Leo is a veteran of creating outstanding sand calendars and posters. He has a prodigious collection thanks to the fact that he has a legion of former students who dutifully send sands to him from their travels (cheater). Leo tends to zoom out a bit which produces a larger field than most. He uses a Canon 1-5x lens that allows him to get that clarity. Leo’s Picasa Web albums are here.

 
Clockwise from top left: Miller’s Point, Cape Town, South Africa; Monticello, Corsica; Jeffrey’s Bay, South Africa; Rye, New Hampshire. Center: Cap Lardier, France.

Siim Sepp: Great definition of full field shots and individual grains.

Siim also uses the Canon 1-5x lens although Siim does more close up work. He uses a full field for some of his photos but he also shows exploded views of individual grains of sand elements. Siim has a “how-to” on the mechanics of getting a sand photo on his website, Sand Atlas. On that site, he also provides geological information on sand composition that is very informative.

 
Clockwise from top left: Diamond Head, Hawaii; Sapphire crystals, Songe, Southern Tanzania; Zakynthos, Greece; Waddell Sea floor (-3500m), Antarctica (Globigerina forams, yellow circles);  Center: Glass Beach, Kaui, Hawaii.

 

Alexander Heider: Tight shots, great color.

Herr Heider gets excellent close ups of his sands without sacrificing detail. As you can see in this series of sands from Ireland he is able to keep detail at close range. He also agonizes over white balance which gives very exacting color to his images. His tight shots and true color make his photos stand out.

 
Clockwise from top left, an Irish selection: Ardmore, Waterford; Ventry Strand, Dingle Bay; Dooagh, Achill Island, Fanore, Galway Bay; Center: Fintragh Bay, Killybegs.

 

Alain Couette: Soft shots, great detail.

Alain will not reveal what he uses as a lighting source but whatever it is, it gives his photos a “soft” look and it is also rich in detail. Alain has numerous images on his website, Arénophile ou Psammophile? He has sand pix plus images of forams that are very interesting. Alain has a distinctive style that I would recognize anywhere.

 Clockwise from top left: Boracay Island, Angol, Philippines; Nusa Lembongan, Ceningan Island, Indonesia; Rio Grande do Norte, Fernando de Noronha Island, Brazil; Luzon island, Nasugbu, Philippines; Center: Sanur, Bali, Indonesia

 

Carla Lagendijk: Daylight shots, great variety and picture perfect color.

Carla is one of the pre-eminent sand photographers today. As you can see her shots are very rich in detail. This is not due to the lens but the fact she shoots her photos in daylight, the sun illuminating every nook and cranny which in turn provides outstanding definition. The other thing that separates Carla from the pack is her prodigious collection. She has sand from anywhere you can imagine which greatly increases the chances of having outstanding material to work with. Below are 4 shots that typify her style, and of course, no examination of Carla’s work would be complete without a sample of one of her “selections”. These are selected bits of coral, forams, shells, etc., from various sand samples. I hesitate to use more than one for the simple reason that they are too awesome. If you can look at these all day, why would you look at anything else?

 Clockwise from top left: Bahia de Samana, Dominican Republic; Myrdal, Vik, Iceland (olivine and obsidian); Al Bustan, Oman;  Brétignolles, France; Center: Enubuj Island, Kwajalein Atoll, Marshall Islands.

 

Kurt Meyer: Exploded views, contrasting backgrounds.

I now use a Nikon d1500 with the same 60mm lens. What I like to do with my photos is create an edge where heavy minerals or carbonate elements can be isolated from the main body of sand allowing for a better view of the individual grains. This has been dubbed the “black corner” technique. The 5100 has a 16.2 MP sensor that will enable you to enlarge photos without losing definition. Also, I use black or other colored glass as a background to provide contrast.

 
Clockwise from top left: Cape Point, South Africa; Dondra Beach, India; Mardakyan, Azerbaijan; Grand Ile, France; Center: Point Reef, Vanua Levu, Fiji.

 

You can see more photos by all of the above photographers on tropicsofsand.com. I would love to add more images this weekend but Zena has other ideas.

 

[All images reproduced with the photographer’s permission, composite formats by MW.

Kurt is based in Pennsylvania, and writes that while he has been photographing sand for the last three years, he has collected it for “15 odd years.” Nothing odd about that as far as I’m concerned, but perhaps those years were odd in other ways, and sand collecting represented an opportunity for normality. Interestingly, he enjoys diving for samples. Anyway, thanks Kurt for all the effort in putting this together – and thanks to all the arenophile photographers who contributed to this piece.]

Sand critter guest post: the South Texas sand crab

By Zen Faulkes

South Padre Island in Texas has a world-class beach, with lovely fine-grained sand that is wonderful to walk on. The sand grains close up seem to be mostly grains of light or transparent quartz, with just the occasional bit of shell grit. At least, that’s what it looks like to my completely untrained eye.

 
I personally am not so much interested in the sand, however, as some of the creatures living in it. I have been spending the last few years digging around in the beaches there, studying a small local sand crab, Lepidopa benedicti. Because they are so little known, the species doesn’t have a common name; the entire superfamily is generally called “sand crabs” or “mole crabs.” I got started working with sand crabs for my doctoral research because I was interested in how they are able to dig so quickly and efficiently into the sand, and why they couldn’t walk like other crustaceans (Faulkes & Paul, 1997a, b).

  

For the last couple of years, I have been studying these crabs more from an ecological viewpoint. During my regular collections, I eventually made a conscious mental note of something that is incredibly obvious when you say it out loud. Some of these crabs are white, while others are almost a battleship gray.

 
When I looked in the scientific descriptions of these species, the colour of this particular species was never mentioned, and other Lepidopa species were usually described as white. Why are they different colors? We often think of colours as being important signals for animals; think of warning colours, for instance. Sand crabs spend almost all of their time buried in sand. In ten years of digging on this beach, I have seen these sand crabs above the sand twice (weirdly, on the same day!), and even then only for a couple of seconds. The crabs have small, poorly developed eyes. It seemed unlikely that they are using these different colors to signal each other.

Although the eyes of some sand crabs are small, they sometimes take note of their surroundings. Some very nice experiments with sand crabs showed how environmental factors can affect colour. For instance, mole crabs (Hippa) are found on either white coral beaches or black volcanic beaches, and they tend to match their background. What’s even cooler is that you take an animal from a white beach, and put it into black sand, and wait, it will tend to darken up after it molts (picture from Bauchau & Passelecq-Gérìn 1987; see also Wenner 1972).

This doesn't seem to explain the color differences I'm seeing in the local sand crabs, however, because they’re all being collected from one beach, which is a light tan as far as the eye can see.

Another idea that came to mind was that this was related to whether an animal was a male or female. That also turned out not to be the case, as each colour had an even split of sexes.

But two pieces of evidence suggest that these colours are not just incidental. There are more gray sand crabs than white ones. And the gray ones tend to be larger than the white ones, although there is overlap (data from Nasir & Faulkes 2010; carapace drawing by Boyko 2002).

Something that might help sort out this puzzling little question would be to look at other populations. The animals I have collected over the last few years are nowhere near the largest on record for the species. The largest individuals in this species have been found on the Atlantic coast of Florida (Boyko 2002). The average size of sand crabs there is about the size of the biggest individuals I’ve found on South Padre Island. And it makes me wonder, if the Atlantic L. benedicti are that much bigger, are they more likely to be that dark battleship gray?

References

Bauchau AG, Passelecq-Gérìn E. 1987. Morphological color changes in anomuran decapods of the genus Hippa. Indo-Malayan Zoology 4(1): 135-144.

Boyko CB. 2002. A worldwide revision of the recent and fossil sand crabs of the Albuneidae Stimpson and Blepharipodidae, new family (Crustacea, Decapoda, Anomura, Hippoidea). Bulletin of the American Museum of Natural History 272(1): 1-396. http://dx.doi.org/10.1206/0003-0090(2002)272<0001:AWROTR>2.0.CO;2

Faulkes Z, Paul DH. 1997a. Coordination between the legs and tail during digging and swimming in sand crabs. Journal of Comparative Physiology A 180(2): 161-169. http://dx.doi.org/10.1007/s003590050037

Faulkes Z, Paul DH. 1997b. Digging in sand crabs (Decapoda, Anomura, Hippoidea): interleg coordination. The Journal of Experimental Biology 200(4): 793-805. http://jeb.biologists.org/cgi/content/abstract/200/4/793

Nasir U, Faulkes Z. 2011. Color polymorphism of sand crabs, Lepidopa benedicti (Decapoda, Albuneidae). The Journal of Crustacean Biology 32(2): 240-245. http://dx.doi.org/10.1651/10-3356.1

Wenner AM. 1972. Incremental color change in an anomuran decapod Hippa pacifica Dana. Pacific Science 26: 346-353.

 

[“Doctor Zen” first got in touch with me in the spring of last year to kindly point me to some research on bubbles behaving like granular materials. I had a look at his personal web page at the University of Texas-Pan American, and discovered his research interest in decapod crustacean behaviour, in particular the unusual crayfish, Marmorkrebs; I was particularly interested in his work on crustaceans digging in sand – the slipper lobster video on his site is strangely compelling. I asked if he would contribute a guest post and he kindly agreed. I hope that readers find this as fascinating as I do – I’m reminded of some of the intriguing evolutionary questions raised by beach mice and the bleached earless lizard. Thanks, Zen, for this – well worth waiting for!].