Tag Archives: behavior

Slow Worms at Wormsloe

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Every time I visit the Georgia coast, traces that have been there all along make themselves apparent to me for the first time. One would think these personal discoveries would stop happening after more than fifteen years (on and off) of going to that coast and studying its traces, especially after writing a 700-page book about them (Life Traces of the Georgia Coast). Nevertheless, they happen, and when they do, these insights underscore the importance of doing regular field work in the same places. However familiar it might seem, there’s always something different you missed previously while there. So before each trip to the Georgia coast, I make sure to become wide-eyed and expectant, rather than jaded and bored.

Intersecting-Worm-Trails-Wormsloe-1Mysterious trails in a sandy road, crossing and re-crossing paths. What could have made them? And why so many? If curious, read on. If not, I’ve heard there are some Web sites with pictures of cats that require your viewing. (Photo by Anthony Martin, taken at Wormsloe Historic Site, Georgia.)

The latest example of this exercise in place-based humility happened just a little more than a week ago during a short time at Wormsloe Historic Site, south of Savannah, Georgia. Sarah Ross, the President and Director of the Wormsloe Institute for Environmental History, invited me there to give a nature walk and talk to guests at a private event on the evening of Saturday, October 11. After the walk and talk (which was a big success), we all watched lovely and enlightening story-telling by a local Gullah/Saltwater Geechee performance group (The Saltwata Players), had a delicious dinner, partook in great conversations fueled by nice wine, and I got to sell and sign copies of the book I mentioned earlier. In short, the proverbial good time was had by all.

My wife Ruth and I stayed in a guest cabin on the grounds of the former plantation that night; in morning, I got up just before dawn to start tracking and otherwise looking for traces. In the dimness, only a few raccoon and deer tracks stood out on the sandy roads, as well as a pile of scat that had been inside a feral hog only a few hours before. A nearby salt marsh beckoned, and because the low tide had exposed its banks, I walked out onto a nearby dock for better views of its exposed surfaces. The dark mud was pockmarked by thousands of holes, most belonging to mud-fiddler crabs and other burrowing invertebrates that call this place home.

Salt-Marsh-WormsloeA small part of the salt marsh at Wormsloe Historic site where it abuts the maritime forest, and during low tide. See all of those holes in the foreground? I wonder what those might be?

Salt-Marsh-Wormsloe-BurrowsEach and every one of these holes is the burrow of a small marine-adapted animal – fiddler crabs, polychaete worms, and more. In other words, an ichnologist’s dream come true. (Photos by Anthony Martin.)

Less than a hundred meters from this dock is the home of Craig and Diana Barrow. Mr. Barrow is the ninth-generation heir of Wormsloe, but donated its grounds to the state of Georgia so that it could become a natural laboratory for researchers studying its environmental history. Ruth and I were acquainted with the Barrows from two previous visits to Wormsloe, and Craig – a great outdoors enthusiast – had eagerly given us personal tours of the woods, fields, and marshes on the property.

Having hunted for most of his life, Craig is a good tracker, and we’ve had in-depth discussions on animal-track forms, trackway patterns, aging of tracks, scat, and related topics. I find these conversations refreshing. Academic hierarchies, journal articles, impact factors, grant amounts, and other dull concerns become meaningless when you’re in the field with experienced naturalists. Here are some traces. Let’s learn.

Thus as the dawn light started to illuminate the maritime forest, I was not surprised to see Craig already outside his home, and to have him enthusiastically invite me to hop onto a golf cart with him to go look for tracks. He had already been out earlier on one of the sandy roads near his house and spotted three red foxes, so he wanted to check on whether their tracks were there, too. Within minutes, we arrived at the spot where he saw the foxes, and we quickly confirmed his sighting by identifying their fresh tracks in the loose sand on the road.

That was also about when Craig asked me a question that I answered wrong at first, then corrected once I gathered more data. You know, like any good scientist should. His question was “What’s this?”, and he was referring to a thin, shallow, and meandering groove in the sand. “Beetle trackway,” I answered instantly, without looking too closely. Then I squatted to show him the tiny tracks that would be on each side of the groove, where I imagined the beetle had dragged its abdomen.

Worm-Trail-Wormsloe-4Oh look, a beetle trackway, and right next to the tracks of a red fox (Vulpes vulpes)! How exciting! Gee whiz, I gosh-darn love science! Isn’t it neat? Wait a minute: what’s that earthworm doing at the end of a beetle trackway? (Photo by Anthony Martin, taken at Wormsloe Historic Site.)

That’s when I realized there were no tracks on either side of the groove. This was a trail made by a legless animal. “Wait a minute, this isn’t from a beetle,” I said. “Maybe a worm?” And by “worm,” I meant earthworm, but my small amount of experience with identifying earthworm traces made me a little uncomfortable with elaborating further on that idea. After all, I didn’t want to appear too ignorant about such common animals, and ones I had written about in both my book and on this blog (Of Darwin, Earthworms, and Backyard Science and Darwin, Worm Grunters, and Menacing Moles).

Fortunately, an earthworm saved me from further embarrassment by having the decency to be at the end of one of these trails, moving and otherwise actively demonstrating how these traces had been made. With our eyes and brains properly (and instantly) trained by this association between trace and tracemaker, Craig and I glanced around us. We were rewarded for looking, and promptly became astonished. The road was criss-crossed with hundreds of earthworm trails for as far as we could see, and most of them had living worms at their ends.

Even better, a few of these trails connected to open, small-diameter vertical burrows. My second insect-biased mistake of the morning was to initially identify these burrows as the shafts of halictid bee burrows. However, too many earthworm trails connected directly to these holes. Again, like any good scientist should in the face of contradictory evidence, I changed my mind. These traces were also from earthworms, and showed where the earthworms exited their subterranean homes.

Vertical-Burrow-Worm-WormsloeEarthworm burrow marking exactly where it left its home for the surface world, and intersecting a trail. (Photo by Anthony Martin, taken at Wormsloe Historic Site.)

What really surprised me, though, was the length and complexity of the trails. These were not simple meandering paths, but complicated records of earthworm decision making. These worms may have been slow, but their traces certainly weren’t dull.

Worm-Trail-Wormsloe-2This trail was made by one earthworm that moved from right to left. The pointed grooves on either side of the main trail are from where its “head” and “tail” ends probed the sand.

Worm-Turning-Wormsloe-1Here’s an earthworm in action, moving from right to left. Compare this to the next photo to see how movement of both its forward and rear ends changes the trail, putting newer traces on top of the previously made ones.

Worm-Turning-Wormsloe-2See what I mean? Small but multiple movements from both ends of a worm – as well as its middle – make this much, much more than just a “worm trail.” So don’t be calling it that. (Photos by Anthony Martin, taken at Wormsloe Historic Site.)

Yes, I know, there’s a bigger question that looms over all of this ichnological minutiae: Why were so many worms on top of the ground, instead of in it? What could have caused hundreds of them to leave their homes and risk the perils of dehydration and predation at the surface?

I speculated aloud that their mass stranding might have been related to vibrations imparted to the road. After all, Charles Darwin had noted how earthworms reacted like this to subsurface vibrations, associating these with their mortal enemies, burrowing moles. This was independently verified by “worm grunters” of the Appalachians, who took advantage of earthworm-mole co-evolution to get bait for their fishing. Craig backed up my idea by saying that he had grated the road the previous day. So perhaps the vibrations from his vehicle and activities had persuaded the earthworms to come up and out of the ground.

Later, though, I wondered whether another much larger stimulus had invoked such aversive reactions in the earthworms, one that persisted for more than a day after the road had been grated. What else could have done this, impelling these earthworms to flee, much like urban hipsters sensing the first few notes of a nearby Justin Bieber concert, and leaving spilled PBR’s in their wake?

Then it came to me. A full moon that weekend had caused higher tides than normal in the area, ranging from 2.6 to 2.9 m (8.5-9.6 ft). As a result, saltwater probably crept high enough in the soil profile to trigger a collective reaction in the earthworms, which do not fare well once salty water starts filling their homes. Yes, that would do it.

Salt-Marsh-WormsloeHi, terrestrial earthworms. Remember me? I’m a salt marsh with 2.5-3 m high tides, right next to where you live.

Assuming this hypothesis is correct, what we saw there on that sandy road of Wormsloe Historic site was a great example of a marine ecosystem forcing animals living in a terrestrial ecosystem to drastically change their behaviors. Best of all, these animals made a suite of traces that reflected this sudden change in their behaviors. If preserved in the fossil record, such trails and burrows might even be recognizable to geologists and paleontologists, some of whom are quite fond of calling every invertebrate trace fossil a “worm burrow” anyway.

All in all, this field experience at Wormsloe taught me a lesson about keeping my senses open to noticing and wondering about traces wherever I go, as should you, gentle readers. Look for those moments when the worm has turned: they will teach you something new.

Worm-Trail-Wormsloe-1

On the 8th Day of Ichnology, My Island Gave to Me: 8 Crab Legs Walking

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For today’s photo and discussion, let’s take a look at one of my favorite tracemakers of the Georgia barrier islands – the ghost crab (Ocypode quadrata) – and its most commonly encountered traces on the dunes and beaches of those islands, tracks. Because I like these animals and their traces so much, I will restrain myself from prattling on too long about these semi-terrestrial crustaceans and their traces. Instead I’ll focus on this one example of ghost crab tracks and what they tell us about its anatomy and behavior.

Ghost-Crab-Tracks-SapeloTracks made by a ghost crab (Ocypode quadrata) while walking along the side of a coastal dune on Sapelo Island. Ghost crabs use eight of their ten appendages for walking (legs, or pereopods), and the other two (claws, or chelipeds) are used for grasping and pinching. Because most of their walking is done sideways, one set of four legs often precede the other set. Look for the repeating pattern of four clustered imprints toward the lower left of the photo, and notice how these are clearly defined, whereas the upper sets are elongated, suggesting that this was the trailing set of legs. But wait: what’s with the two drag marks in the center of the trackway? Please read on. (Photograph by Anthony Martin; scale in centimeters.)

What caught my eye about this trackway is that it shows it was made by a real decapod (= “ten legged” crustacean), and not, say, some uncomfortably large spider that just happened to go for a walk along a Georgia-coast beach. The two central drag marks are from its claws, which the crab held low enough while it walked so that these left impressions on the sand. Like many decapods, one claw on a ghost crab is larger than the other. Appropriately, this is called its superior cheliped and the smaller one is its inferior cheliped.

Normally when ghost crabs walk, their claws point down, so if they hold them low enough, the claw tips register on the sand and leave such drag marks. In this photo, the offset you see between those claw-tip impressions is because of their size differences, where the big claw is in front of the small one. For this crab and its trackway pattern, I think it was moving from right to left, so the upper drag mark would be from its big claw, and the lower one from its small claw. Unless if I’m wrong on that, in which case, just reverse what I said.

Have ghost crab tracks been interpreted from the fossil record? No, not yet, although trace fossils of their burrows are very common. So if any potential candidates of such trackways are discovered, one of the first questions I would ask is whether they also preserve claw impressions, showing these really were from a ten-appendaged critter, and not some arachnid imposter.

Further Reading

Ocypode quadrata: Atlantic ghost crab. Animal Diversity Web (University of Michigan).

Atlantic Ghost Crab: Ocypode quadrata. David Knott, Department of Natural Resources, South Carolina. (PDF)

Links to Previous Posts in This Theme

On the 12th Day of Ichnology, My Island Gave to Me: 12 Snails Grazing

On the 11th Day of Ichnology, My Island Gave to Me: 11 Plovers Probing

On the 10th Day of Ichnology, My Island Gave to Me: 10 Beetles Boring

On the 9th Day of Ichnology, My Island Gave to Me: 9 Molluscans Hiding

Traces of Toad Toiletry and Naming Trace Fossils

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Sometimes I envy those people on the Georgia barrier islands who, through sheer number of hours in the field, come upon animal traces that I’ve never seen there. But this was one of those instances where the find was so extraordinary that I will suppress my jealous urges, celebrate the person who found it, marvel at it, and share its specialness with others.

Gale Bishop, a fellow ichnologist who is currently on St. Catherines Island, found an intriguing sequence of traces during a morning foray on its dunes and beaches there last week. In his second life – his first was as a geology professor at Georgia Southern University – he has transformed into an indefatigable sea-turtle-nesting monitor on St. Catherines and coordinator of a teacher-training program. Part of his daily routine there, among many other duties, includes looking for mother-turtle traces – trackways and nests – during the nesting season, which in Georgia is from May through September.

Along the way, with his eyes well trained for spotting jots and tittles in the sand, Gale often notices oddities that likely would be missed by most people, including me. The following photograph, which he shared on the St. Catherines Island Sea Turtle Program page on Facebook, is from a find he made about 7:15 a.m. on Saturday, July 7. Take a look, and please, if you haven’t already, forget the title of this post as you ponder its clues.

A mystery in the dune sands of St. Catherines Island on the Georgia coast, begging to be interpreted. No, not the shovel: those are never mysterious. Look at the traces to the left and above the shovel. What made these, what was it doing, and who else was in the neighborhood afterwards? Oh, and again, stop staring at the shovel. (Photograph by Gale Bishop.)

Gale called me out specifically when he posted this and several other related photos on Facebook, and asked me to tell a story about it. I gave him my abbreviated take in the comments, kind of like an abstract for the research article:

Looks like southern toad (Bufo terrestris) to me. What’s cool is the changes of behavior: hopping, stopping, pooping, and alternate walking (which people don’t expect toads to do – but they do).

That was my knee-jerk analysis, which took a grand total of about a minute to discern and respond. (After all, this was Facebook, a forum in which prolonged and deep thinking is strongly discouraged.) But I also kept in mind that quick, intuitive interpretations later need introspection and self-skepticism, especially when I’m making them. (See my previous post for an example of how wrong I could be about some Georgia-coast traces.) So rather than fulfill some Malcolm Gladwell-inspired cliché through my intuition, I sat down to study the photo with this series of questions in mind:

  • Why did I say “Southern toad” as the tracemaker for the sequence of traces that start from the lower left and extend across the photo?
  • What indicates the behaviors listed and in that order: hopping, stopping, pooping, and alternate walking?
  • What signified the changes in behavior, and where did these decisions happen?
  • Why did I assume that most people don’t expect toads to walk (implying that they think they just hop)?

The first leap in logic – how did I know a Southern toad (Bufo (Anaxyrus) terrestris) was the tracemaker – was the easiest to make, as I’ve often seen their tracks in sandy patches of maritime forests and coastal dunes. These hardy amphibians leave a distinctive bounding pattern, with the front-foot impressions together and just preceding the rear-foot ones; the toes of their front feet also point inward. In the best-expressed tracks, you will see four toes on the front feet and five toes on the rear.

Close-up of bounding pattern (from lower left of previous photo), showing front-foot impressions just in front of and more central than the rear feet impressions. Direction of movement is from bottom to top of photo. (Photograph enhanced to bring out details, but originally taken by Gale Bishop.)

The only other possible animal that could make a trackway pattern confusable with a toad in this environment is a southeastern beach mouse (Peromyscus polionotus). Still, mice mostly gallop, in which their rear feet exceed their front feet as they move. Mouse feet are also very different from those of a toad, with toes on both feet all pointing forward (remember, toad toes point inward). So although dune mice live in the same environment as these tracks, these weren’t mouse tracks. The only alternative tracemakers would be spadefoot toads (Scaphiopus holbrookii) or a same-sized species of frog, such as the Southern leopard frog (Rana sphenocephala). But neither of these species is as common in coastal dunes as the Southern toad, so I’ll stick with my identification for now.

Mouse tracks – probably made by the Southeastern beach mouse (Peromyscus polionotus) – on costal dunes of Little St. Simons Island, Georgia. The two trackways on the left are moving away from you, whereas the one on the trackway on the right is heading toward you. All three show a gallop pattern, in which the larger rear feet exceeded the front feet. Scale = 10 cm (4 in). (Photograph by Anthony Martin)

The second conclusion – the types of behaviors and their order – came from first figuring out the direction of travel by the tracemaker, which from the lower left of the photo toward its middle. This shows straight-forward hopping up to the point where it stops.

From there, it gets really interesting. The wide groove extends to the left past the line of travel and had to be made by the posterior-ventral part of the toad’s body (colloquially speaking, its butt). This, along with the disturbed sand on either side of the groove, shows that the toad turned to its right (clockwise) and backed up with shuffling movement. That’s when it deposited its scat, which I’ve also seen in connection with toad tracks (and on St. Catherines, no less). This really helped me to nail down the identity of the tracemaker, almost being able to declare, “Hey, I know that turd!”

Southern toad bounding pattern that abruptly stops, followed by clockwise turning, backing up, and, well, making a deposit. (Photograph by Gale Bishop, taken on St. Catherines Island.)

How about the alternate walking? Turns out that toads don’t just hop, but also walk: right side, left side, right side, and so on. This pattern – also called diagonal walking by trackers – is in the remainder of the photo (with the direction of movement left to right). When toads do this, the details of their front and rear feet are better defined, and you can more clearly see the front foot registers in front of the rear and more toward the center line of the body.

This side-by-side movement is also what imparted a slight sinuosity to the central body dragmark, which was from the lower (ventral) part of its body, or what some people would call “belly.” In my experience, most people are very surprised to find out that toads can walk like this, which I can only attribute to sample bias. In other words, they’ve only seen frogs and toads hop away from them when startled by the approach of large, upright bipeds.

Close-up of alternate walking pattern and body dragmark made by Southern toad. Direction of movement is from upper left to lower right. (Photograph enhanced to bring out its details, but original taken by Gale Bishop on St. Catherines Island.)

But wait, what are those two dark-colored depressions in the center of the alternate-walking trackway? Well, it doesn’t take much imagination to figure those out, especially if you’ve already had a couple of cups of coffee. Yes, these are urination marks, and even more remarkable, there are two of them in the same trackway. So not only did this toad do #2, but also #1 twice.

Southern toad urination mark #1, not too long after doing #2. (Photograph by Gale Bishop.)

Urination mark #2 , which you might say was #2 of #1, but with both #1’s after #2, or, oh, never mind.

Notice that the second mark seems to have had less of a stream to it, which makes sense in a way that I hope doesn’t require any more explanation or demonstration.

So to answer to one of the questions above – what signified the changes in behavior – you have to look for the interruptions in the patterns, much like punctuation marks in a sentence. The commas, semi-colons, colons, dashes are all part of a story too, not just the words.

The summary interpretation of what happened. Let’s just say that this frog (or toad, whatever) didn’t come a courtin’.

Through the series of photos Gale shared in an album on Facebook, he also showed that he was following a protocol all good trackers do: he changed his perspective while observing the traces. Likewise, I teach my students to use this same technique when presented with tracks and other traces, that it’s a good idea to walk around them. While doing this, they see changes in contrast and realize how the direction and angle of light on the traces alters their perceptions of it. At some points, a track or other trace may even “disappear,” then “reappear” with maximum clarity with just a few more steps.

A different perspective of the same traces, viewed from another angle. Do you notice something new you didn’t see in the previous photo and its close-ups? (Photograph by Gale Bishop, taken on St. Catherines Island.)

Now, because I’m also a paleontologist, this interesting series of traces also prompts me to ask: what if you found this sequence of traces in the fossil record? Well, it’d be a fantastic find, worthy of a cover story in Nature. (That is, if the tracks somehow went across the body of a feathered dinosaur.) Right now, I can’t think of any trace fossils like this coming from vertebrates – let alone toads or frogs – so let’s go to invertebrate trace fossils for a few examples of similarly interconnected behaviors preserved in stone.

In 2001, a sequence of trace fossils was reported from Pennsylvanian Period rocks (>300 million years old), in which a clam stopped, fed, and burrowed along a definite path, with all of its behaviors clearly represented and connected. The ichnologists who studied this series of trace fossils – Tony Ekdale and Richard Bromley – reckoned these behaviors all happened in less than 24 hours; hence the title of their paper reflected this conclusion.

Ichnologists have a sometimes-annoying and always-confusing practice of naming distinctive trace fossils, giving them ichnogenus and ichnospecies names. (For a detailed discussion of this naming method, I talked about it in another blog from the dim, dark, distant past of 2011 here.) For instance, Ekdale and Bromley stated in their study that three names could be applied to the distinctive trace fossils made by a single clam, with each a different form made by a different behavior: Protovirgularia (burrowing), Lockeia (stopping), and Lophoctenium (feeding).

Along those lines, another ichnologist (Andy Rindsberg) and I also suggested that an assemblage of trace fossils in Early Silurian rocks (>400 million years old) of Alabama, with many different ichnogenera, were all made by the same species of trilobite. The take-home message of that study, as well as Ekdale and Bromley’s, is that a single species or individual animal can make a large number of traces. This also means that ichnodiversity (variety of traces) almost never equals biodiversity (variety of tracemakers).

So let’s go back to the toad traces, put on our paleontologist hats, and think about a “what if.” What if you found this series of traces disconnected from one another: the hopping trackway pattern, the diagonal walking pattern, the urination marks, the groove, and the turd, all found in disparate pieces of rock? Taken separately, such trace fossils likely would be assigned different names, such as “Bufoichnus parallelis,” “B. alternata,” “Groovyichnus,” “Tinklichnus,” and “Poopichnus.” (Please do not use these names beyond an informal, jovial, and understandably alcohol-fueled setting.)

Color, present-day version of the variety of traces made by a Southern toad (above), and a grayscale imagining of it fossilizing perfectly (below). Key for whimsically named ichnogenera in fossilized version: Bp = “Bufoichnus parallelis,” Ba = “Buofichnus alternata,” G = “Groovyichnus,” P = “Poopichnus,” and T = “Tinklichnus.” Please don’t cite this.

Granted, the environment in which Gale noted these traces – coastal dune sands – are not all that good for preserving what is pictured here, but other environments might be conducive to fossilization. To quote comedian Judy Tenuta, “It could happen!”

So if someone does find a fossil analogue to Gale’s evocative find on St. Catherines Island, I will understand their giving a name to each separate part, even if I won’t like it. The most important matter, though, is not what you call it, but what it is. And in this case, the intriguing story of toiletry habits left in the sand one July morning by a Southern toad is worth much more than any number of names.

Further Reading

Ekdale, A.A., and Bromley, R.G. 2001. A day and a night in the life of a cleft-foot clam: Protovirgularia-Lockeia-Lophoctenium. Lethaia, 34: 119–124.

Halfpenny, J.C., and Bruchac, J. 2002. Scats and Tracks of the Southeast. Globe Pequot Press, Guilford, Connecticut: 149 p.

Jensen, J.B. 2008. Southern toad. In Jensen, J.B., Camp, C.D., Gibbons, W., and Elliott, M.J. (editors), Amphibians and Reptiles of Georgia. University of Georgia Press, Athens, Georgia: 44-46.

Rindsberg, A.K., and Martin, A.J. 2003. Arthrophycus and the problem of compound trace fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 192: 187-219.

Darwin, Worm Grunters, and Menacing Moles

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In my most recent previous post, I teased readers with the promise of revealing how Charles Darwin used a piano as a scientific tool for studying the behavior of earthworms. Regardless of whether or not you already looked up the answer through The Google, by reading Darwin’s last book (The Formation of Vegetable Mould through the Action of Worms with Observations on Their Habits), or other means, I will now gladly make connections between the seemingly disparate subjects of Darwin’s musically inclined experimentation, earthworm behavior, and fishermen of the southeastern U.S. catching earthworms as bait.

What makes this earthworm (Diplocardia) run away as fast as its little chetae, mucus, and peristalic movement can carry it through the soil? Let’s just say it’s not picking up good vibrations. Photograph by Bruce A. Snyder, from here, from www.discoverlife.org.

In writing about earthworms and their traces in my upcoming book, I devoted several pages to Mr. Darwin’s fascination with earthworms. In this exploration, I tell how Darwin was on to something when he tried applying sound – which included those made by playing musical instruments – to earthworms he had gathered from the English countryside. These musical performances were not an instance of Darwin trying to entertain these worms, boost their self esteem, or otherwise help them get in touch with their emotions. Rather, he was simply testing whether worms reacted to sound. What happened? Well, instead of me describing his results, I’ll let Darwin’s words inform you directly:

Worms do not possess any sense of hearing. They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest tones of a bassoon. They were indifferent to shouts, if care was taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet.

– Charles Darwin, The Formation of Vegetable Mould through the Action of Worms with Observations on Their Habits (1881), p. 27.

Hence it was with deep appreciation last month when I gazed at the piano in the drawing room of Down House, the former Darwin family home, and thought about these experiments. Smiling, I imagined Darwin carefully watching a container of worms while he or someone else in his family forcefully banged on the keys of this piano. Of course, you also can’t help but wonder what was played “as loudly as possible.” Were these single, random notes, chords, or actual musical compositions? If the last of these, what pieces were played? Ideally, I like to think Mr. Darwin or one of his family members played a sea shanty learned during his days on The Beagle (or perhaps even songs learned from pirates), rather than just pounded random notes up or down a scale.

As conclusive as Darwin’s paragraph might seem about the lack of earthworm reactions to sound, he, like any good storyteller, then injected a dramatic twist when reporting his results. He followed up the preceding paragraph with one describing how earthworms, although deaf, are extremely sensitive to vibrations transmitted through solid media. Here he revealed exactly which notes were played (C on the bass clef, G in the treble clef, C in the treble clef) while two worms were in pots placed on top of the piano.

The vibrations transmitted through solid media – not air – caused the worms to withdraw from the soil surface, presumably hiding from the source of the vibrations. As an extension of this experiment, Darwin also used a fork to agitate the soil underneath other worms, which then provoked them to move up to the surface. Darwin correctly surmised that this stirring activity, like sound, also sent vibrations through the soil, which likewise produced aversive reactions in the earthworms.

These responses made sense in an evolutionary way, and show how Mr. Darwin was applying his principle of natural selection to the predator-prey relationships that had evolved between earthworms and moles. The behaviors he observed would have favored the survival of earthworms that associated vibrations with their most feared predators, and reacting accordingly, which is to say, fleeing in terror. And just what were their aversion-inducing predators? They were not robins or other species of birds – early, punctual, or otherwise timed – but the earthworm version of graboids: burrowing moles.

Eastern mole (Scalopus aquaticus) emerging from its burrow, seeking earthworms and other fresh food. Photograph by Kenneth Catania, from Fairfax County Schools.

Graboid emerging from its burrow, seeking humans and other prey. Note the eerie resemblance of its behavior to that of an eastern mole, albeit orders of magnitude larger and accompanied by a keen interest in large, surface-dwelling, bipedal prey. Photo from Wikipedia, but originally taken from the greatest ichnologically inspired horror film of all time, Tremors.

So you didn’t know about graboids, those burrowing predators of the underworld? Fortunately, this educational video provides all of the details you need to know. But if you’re interested in studying their neoichnology, be careful, and stay on the pavement.

As yet another example of ‘backyard science,” Darwin observed many traces of the European mole (Talpa europaea) in the fields just outside Down House, most of which were their mounds, or “molehills.” Indeed, last month as I admired one of Darwin’s original wormstones in the pasture behind Down House, I also noticed a good number of molehills on the grounds. Rather stupidly, I neglected to take a photo of one of these. (I mean, how cool would it have been to share images of the traces of moles that descended from those whose traces Darwin noticed?) Nonetheless, some of my photos of the grassy area near the wormstone show 20-30 cm wide bare patches in this otherwise meticulously maintained lawn. These spots, I suspect, are traces of the Down House groundskeepers, who probably level molehills as quickly as they appear, an ichnological version of “whack a mole.”

The pasture just behind Down House (Charles Darwin’s former home), with a “wormstone” in the lower right, and a few bare patches of ground just to the left. Could the latter mark recent sites of mole tunnels and molehills leveled by Down House groundskeepers, or are these just places where grass did not grow, and hence the products of an ichnologist’s overactive imagination? Anyway, I did see molehills out there, but don’t blame y’all for being a bunch of skeptical scientists and wanting more evidence than my just saying so.

OK, now how does all of this wonderfully elucidated Victorian-era science relate to the ecosystems and biota of the southeastern United States? Enter the “worm grunters.” Worm grunters are people who, independently of Darwin, figured out the same adaptive responses of earthworms to underground vibrations. Through their own experiments, worm grunters, who were interested in efficiently gathering many worms in a short time for putting on fishhooks (or making money selling earthworms to people who put them on hooks), rubbed steel slabs across the top of wooden posts stuck in the ground. Much later, researchers interested in finding out how this technique worked calculated frequencies of the seismic vibrations that caused earthworms to flee upward away from perceived predators.

The southeastern U.S., including the Georgia barrier islands, not only has its own species of earthworms (Diplocardia mississippiensis), but also has its own species of moles: the eastern mole (Scalopus aquaticus) and the less common star-nosed mole (Condylura cristata). Both types of moles no doubt strike fear in the multiple hearts of earthworms, and natural selection being how it is, the fastest burrowing moles (who are most likely to catch worms) also cause considerable vibrations from their digging. This accordingly means the earthworms that detect and escape these vibrations live long enough to reproduce and pass on whatever genes that aided in such perceptions.

In getting caught by this mole, this earthworm may have just won the worm equivalent of a Darwin Award, depending on whether it had reproduced or not. (Which it probably did, considering earthworm hermaphroditism means they are at least twice as likely to get lucky.) Photo from University of Illinois Extension; Home, Yard, and Garden Pests Newsletter, here.

Thus a visit to Down House in southern England and consideration of Darwin’s contributions to ichnology and behavioral ecology are not so far removed conceptually from the practical knowledge gained by some people in parts of the southeastern U.S. Moreover, many of these same people are of English, Irish, or Scottish descent, and effectively applied the same knowledge surmised by Darwin about worms and moles, which is kind of neat in a heritage sort of way.

Would all of these findings count as applied science, despite its historical lack of Ph.D.-bearing investigators, grant funding, publications, and press conferences announcing the results? Yup. After all, science is about its methods.

So next week, we’ll take a closer look at the traces moles make on the Georgia barrier islands. Do these moles just go after earthworms in the forests and meadows of those islands? Nope. After all, science is not just about its methods, but also surprises.

Further Reading

Darwin, C. 1881. The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits. John Murray, London, U.K.: 326 p.

Edwards, C.A., and Bohlen, P.J. 1996. Biology and Ecology of Earthworms (3rd Edition). Springer, Berlin: 426 p.

Gorman, M.L., and Stone, R.D. 1990. The Natural History of Moles. University of Chicago Press, Chicago, Illinois: 138 p.

Hendrix, P.F. 1995. Earthworm Ecology and Biogeography in North America. CRC Press, Boca Raton, Florida: 244 p.

Mitra, O., Callaham, M.A., Jr., and Yack, J.E. 2009. Grunting for worms: seismic vibrations cause Diplocardia earthworms to emerge from the soil. Biology Letters, 2009: 16-19.

Coquina Clams, Listening to and Riding the Waves

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A little more than a week ago, I co-led a class field trip to Cumberland Island, Georgia and the nearby Okefenokee Swamp for a course titled Ecosystems of the Southeastern U.S. Although I had been to both places more than a few times, none of the students – and a few of my colleagues – had never visited either, potentially casting these already special places in a more exciting light for them.

Nonetheless, as is typical with any field trip to a Georgia barrier island, I also noticed new phenomena while on Cumberland, once again demonstrating how field trips with students ideally also cause the instructors to be filled with wide-eyed wonder. Even better, a few seemingly lowly small bivalves – coquina clams (Donax variabilis) – provided the intellectual highlight for me while we were on Cumberland Island. This is saying something for an island bearing charismatic livestock as a touristic draw.

Resting traces (or are they escape traces?) of coquina clams (Donax variabilis) in the upper intertidal zone of a beach on Cumberland Island, Georgia. These clams are buried just underneath each bump of sand, but some others are much deeper and safer. How do I know that? You’ll find out. (Photograph by Anthony Martin.)

We first noticed the clams as a death assemblage just above the uppermost part of the surf zone on the beach. Their shells, some evident as single valves and others as pairs still hinged together, had been deposited by waves following a high tide, then moved slightly by the wind. These finely ribbed and polished shells readily showed why the specific name of coquina clams (D. variabilis) is applied to them, as they display a gorgeous variety of colors: yellow, orange, beige, blue, pink, and other schemes that surely would inspire interior decorators seeking paisley themes.

Coquina clams with both valves intact or apart, some partially covered by windblown sand, and variably colored. (Photo by Anthony Martin, taken on Cumberland Island, Georgia.)

Just a little bit lower on the beach and in the freshly scoured intertidal zone, we then noticed many small bumps of sand. Underneath these bumps were living clams that buried themselves, which helps to avoid drying out between tides or predation by ravenous shorebirds. With regard to the latter, these bivalves still would have been easy targets for shorebirds intent on acquiring some fresh clam snacks: think of a person ducking under a blanket to avoid being eaten by a lion and how well that might work as a tactic. (Please, just think about it and don’t actually test this idea.)

However, instead of simply writing off all coquina clams as inept burrowers who deserve to die at the beaks of their avian overlords – a similar fate experienced by dwarf surf clams (Mulinia lateralis) – we should look well below the surface, and I mean the sand surface. Look again at the photo first shown above. See all of those tiny, paired holes in between the bumps? Those are the traces of siphons from more deeply buried coquina clams, which are much more likely to escape from bivalve-munching birds while also keeping moist until the next high tide.

Here’s the same photo as above, but zoomed in so you can see the details. Did you notice all of the little holes in between shallowly buried clams? If so, bravo. If not, oh well. (Despite the cropping, turning, and otherwise shuffling electrons, this photograph is still by Anthony Martin, and was still taken on Cumberland Island.)

Coquina clams are actually accomplished burrowers, a necessary adaptation for nearly any small animal living in the high-energy surf of a Georgia beach. In the event of a wave breaking on a beach and washing away the top layer of sand, thus exposing a coquina clam, it will open its valves only enough to stick out its foot, which it then vibrates rapidly. This movement loosens the wet sand underneath, and the clam’s smooth, streamlined shell does the rest of the job, allowing it to glide into its self-made local pit of quicksand and vanish from the surface.

Once under the sand, this clam remains in a vertical position and projects its siphons upwards, making paired holes visible at the sand surface. The resulting burrow is Y-shaped, with the clam body making the lower part of the “Y” and the two siphons leaving thinner traces above. On the Cumberland Island beach the given day we observed their traces, I suspect the more deeply buried clams had the benefit of wetter sand early on as the tide dropped, then the ones hiding under mere caps of sand did the best they could with less wet sand later.

Vertical sections of the Y-shaped burrows made by a coquina clam, dwarf surf clam, or similar small, burrowing bivalves on the Georgia coast, in which the clam body is removed and sediment filled in the empty spaces from above. Both views, taken at right angles from one another, assume the clam is not moving up or down in the sand, which actually isn’t very realistic. (Illustration by Anthony Martin.)

How do we apply this knowledge to the fossil record? Paleontologists have found similar small, Y-shaped burrows in fine-grained sedimentary rocks, trace fossils named Polykladichnus. Some of these burrows are interpreted as the works of suspension-feeding bivalves, although polychaete worms or small arthropods are possible tracemakers, too. Where the lower parts of bivalves rested in sediment and left impressions of their lower halves, which were later filled by overlying sediments, are trace fossils called Lockeia. As a result, paleontologists can reliably identify a former presence of bivalves in rocks that might not have any of their shells. (By the way, please remember that trace fossil names are not the names of the bivalves that made the traces, but of the trace fossil itself. Yeah, I know, that’s confusing. But if you need more of an explanation, I have one for you here.)

So what else is significant about coquina clams? Well, for one, their shells were abundant enough to have formed the framework for a loosely cemented limestone common in Florida, coquina limestone. This is a rock encountered by nearly anyone who has enjoyed (or suffered through) an introductory geology lab exercise on sedimentary rock identification. Students of mine have often compared these rock samples to popcorn balls or similar sugar-cemented treats, although I’ve noticed that no one has been tempted to eat them, no matter how long I kept them in lab that day.

Coquina limestone from Florida. Notice that some of the pieces are from other species of bivalves with rougher, corrugated shells, so these rocks are not made entirely of coquina clams. (Photograph by Mark Wilson of Wooster College, and taken from Wikipedia Commons here.)

But I saved the best tidbits of information for last, and something I’ll bet most people don’t know about these clams that makes them, like, totally cool. These little bivalves respond to sound and migrate seasonally. Yes, that’s right: these clams have their own form of “listening” and they can move en masse once the seasons change on the Georgia coast. Here’s how they do both:

Stop, Look, Listen – Of course, clams lack ears (although rumors still persist that they have legs). Thus they do not hear in the sense we do, but instead respond to low-frequency vibrations caused by waves striking the shore. Once these vibrations are detected, they react by: popping out of the sand; jumping up into the water; body surfing on the wave; and quickly reburying themselves in the sand once dumped by that same wave. Like some aficionados of heavy metal or punk rock, louder is better, as higher-decibel waves cause more clams to jump up out of the sand and into the water, an aquatic version of a molluscan mosh pit.

Shelled Migration – Coquina clams, like caribou, wildebeest, and arctic terns, migrate. Unlike the vast distances covered by those animals, though, coquina clams simply move up and down the slope of a beach with the changes of the seasons. Using their wave-surfing and burrowing abilities, they move from the lower intertidal zone – which is where they live during the spring, summer, and fall – to the upper part of the beach, which becomes their winter homes.

So I hope that all of this pondering over a few shells, bumps, and holes on a Cumberland Island beach has helped lend an appreciation for the small wonders on any given Georgia barrier island. Who knows what little discovery the next field trip will bring, or whether some facsimile of what is seen might also be preserved in the fossil record? As the old saying goes, time will tell, whether that time is in the present or the geologic past.

Further Reading

Ellers, O. 1995a. Discrimination among wave-generated sounds by a swash-riding clam. Biological Bulletin, 189: 128-137.

Ellers, O. 1995b. Behavioral control of swash-riding in the clam Donax variabilis. Biological Bulletin, 189: 120-127.

Pemberton, S.G., and Jones, B. 1988. Ichnology of the Pleistocene Ironshore Formation, Grand Cayman Island, British West Indies. Journal of Paleontology, 62: 495-505.

Ruppert, E.E., and Fox, R.S. 1988. Seashore Animals of the Southeast. University of South Carolina Press, Columbia, South Carolina: 429 p.

Turner, H.J., Jr., and Belding, D.L. 1957. The tidal migrations of Donax variabilis Say. Limnology and Oceanography, 2: 120-124.