Tag Archives: ghost shrimp

Ghost Shrimp Whisperer

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When you hear the word “shrimp,” you probably picture those that show up in grocery stores and restaurants throughout the world, which are then consumed voraciously by their terrestrial admirers. Also, some recent attention has been given to mantis shrimp, and deservedly so, because they are among the most gorgeous and terrifying of marine invertebrates today. But there are other marine crustaceans bearing the name “shrimp” that are neither gracing seafood buffets nor awesome predators, yet are worthy of our adoration, documentary films, and epic songs, the latter of which will be no doubt performed on Eurovision 2014. Yes, you guessed it: I’m talking about ghost shrimp.

Ghost-Shrimp-Burrow-Tracks-JekyllWhat’s this? We’re looking down on the surface of a Georgia beach at low tide. The collapsed top of a ghost shrimp burrow is in the lower left, but it’s connected to a trackway, which ends in a shallow horizontal burrow, which holds the maker of all three types of traces. Lots of other ghost-shrimp burrow tops are in the upper part of the photo, too. Life doesn’t get much better than this for an ichnologist. You may now envy me. (Photo by Anthony Martin, taken on Jekyll Island, Georgia; scale in centimeters. )

Why ghost shrimp? Because they can burrow like nobody’s business. Take a typical ghost shrimp in the Bahamas or the Caribbean, such as Glypterus acanthochirus. This crustacean is only about 10-cm (4-in) long, but if it lives for eight years and burrows continuously through that time, it will have processed a cubic meter of sediment. Individual ghost-shrimp burrows can go as deep as 5 m (16 ft). These would be like a human shoveling more than a cubic kilometer of dirt, or a vertical shaft about 100 m (330 ft) deep, but without a shovel, backhoes, augers, drilling rigs, or other tools. These vertical shafts then connect with extensive branching tunnels, making complicated networks in the sand and mud below the level of the low tide. Now multiply that industriousness by millions, and we’re talking about enormous volumes of sediment processed by ghost shrimp in their respective shallow-water environments. Ghost shrimp are like the ants of the ocean, only not as organized: no queens, workers, soldiers, or other divisions of labor, just lots of individual shrimp burrowing, eating, mating, and defecating.

Ghost-Shrimp-Burrow-TopsEvery one of these holes is the top of an occupied ghost-shrimp burrow. Now imagine meters-long vertical shafts from each of these going down into the beach sand, then turning into branching horizontal networks of such grandeur, they would further embarrass naked moles rats, which are already apologizing for how they look. (Photo by Anthony Martin, taken on Sapelo Island, Georgia. Human foot (upper right), still attached to human, for scale.)

Ghost shrimp share a common ancestor with crabs, lobsters, crayfish, and shrimp, all of these having four pairs of walking legs and one pair of claws. (Mantis shrimp are actually not true shrimp – or even decapods – but stomatopods.) Ghost shrimp are also known by marine biologists and ichnologists as callianassid shrimp, belonging to an evolutionarily linked group (clade), Callianassidae.They burrow through sand and mud using their front two claws, but also carry sediment on their other legs. Ghost shrimp are also well-known for depositing much of the mud on Georgia beaches as elegantly packaged little cylindrical fecal pellets. These bear enough of a resemblance to “chocolate sprinkles” on cupcakes that they become tempting to sample, until you remember that they’re, like, you know, fecal.

Ghost-Shrimp-Fecal-PelletsGhost-shrimp fecal pellets, each about 5 mm long, and recently ejected by a ghost shrimp through the top of the burrow, which is the little hole just to the right. If you use them with any cupcake recipes, let me know how that worked for you. (Photo taken by Anthony Martin on St. Catherines Island, Georgia.)

Geologists love ghost shrimp, too, because of how their burrows are so numerous, fossilize easily, and are sensitive shoreline indicators. I wrote about this before with regard to how geologists in the 1960s were able to map ancient barrier islands of the Georgia coastal plain by looking for trace fossils of these burrows. Since then, geologists and paleontologists have identified and applied these sorts of trace fossils worldwide, and in rocks from the Permian Period to the Pleistocene Epoch.

I could prattle on about ghost shrimp and their ichnological incredibleness for the rest of the year, but will spare you of that, gentle reader, and instead will get to the point of this post. Just when I thought I’d learned nearly everything I needed to know about ghost-shrimp ichnology, one shrimp decided I needed to have my eyes opened to some traces I had never seen them make before just a few months ago. I mentioned these traces briefly in a previous blog post, when I was teaching undergraduate students from my barrier-islands class on Jekyll Island (Georgia) in mid-March. They were tracks and a shallow horizontal burrow made on the surface on the northernmost beach of Jekyll Island, and they were made by a ghost shrimp. How do I know they were made by a ghost shrimp? Well, maybe because they had a ghost shrimp attached to them, but that’s beside the point.

Ghost-Shrimp-Tracks-Burrow-Left-JekyllA close-up of the left side of the trackway shows more clearly how it definitely is connected to the funneled top of a burrow. The trackway shows small pointed impressions and a central groove in places, showing that this is an animal with legs and a tail, respectively. The irregular path of the trackway is a record of pauses, where the trackmaker stopped briefly before moving on. The body length of the tracemaker is subtly revealed along the way too, but explaining that would require a more advanced lesson in ichnology. So maybe another time. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-Tracks-Burrow-Right-JekyllThe right side of the trackway, ending in a short and shallow horizontal tunnel, just under the sandy beach surface. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-Tracks-Burrow-Closeup-JekyllThe trackway and tail-trail ends in a tunnel with a thin roof of sand. The bilobed pattern was made by the claws and other legs on either side moving sand up and around the body of the tracemaker. Notice the roof collapsed a little on the right, and that its tail is sticking out on the left: kind of like hiding under a too-short blanket. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-JekyllTa-da – the tracemaker revealed! I’m fairly sure this is a Georgia ghost shrimp (Biffarius biformis), but would appreciate all of those marine biologists out there to correct me if I’m wrong. (And not those fake marine biologists, either.) Rest assured, after showing it to my students and allowing them to photograph it, I put it back in the ocean, where it burrowed happily ever after. Unless it died, that is. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

What truly amazed me about these traces, though, was their rarity. As I shared with my students, in more than 15 years of field work on the Georgia coast, I had never seen anything like this sequence of traces. Even better, the tracemaker was right there, and like the period at the end of a sentence in the story.

Furthermore, the story told by these traces was that something must have threatened the life of this shrimp to cause it to behave in such an unusual way. These shrimp almost never see the light of day, and prefer to stay deep in their burrows, away from the prying eyes and beaks of shorebirds, fish, and other predators. Consequently, they remain largely invisible to humans; hence the “ghost” part of their nickname. This means something very bad must have happened to this one in its burrow, prompting it to abandon its refuge and expose itself so vulnerably. It would be like a fire forcing people out of their fortified underground bunkers, but when they know tyrannosaurs are lurking just outside. Damned if you do, damned if you don’t, but something in this ghost shrimp’s evolutionary program made it take the path of the lesser damned.

What happened? Did a predator find its way into the burrow and chase it out? Was it a chemical cue of some sort, like oxygen-poor water flooding into the bottom of its burrow? Was it competition from another ghost shrimp, evicting it from its home? Was it a mate that decided it had enough of sharing this burrow and needed some “alone time,” or took up with another more comely shrimp? I don’t know, but it made for a good little mystery, yet another posed by life traces on a Georgia beach, and one I was delighted to discover and share with my students on Jekyll Island.

Doing Field Work on a Developed Barrier Island

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The second day of our Barrier Islands class field trip (Sunday, March 10), which is taking place along the Georgia coast all through this week, involved moving one island north of Cumberland (mentioned in this previous post), to Jekyll Island. I’ve been to Jekyll many times, but none of my students had, so they didn’t quite know what to expect other than what I had told them.

For one, I warned the students that Jekyll was not at all like Cumberland, which is under the authority of the U.S. National Park Service as a National Seashore. Consequently, it has a few residents, but is limited to less than 300 visitors a day. In contrast, many more people visit or live on Jekyll, and people have modified it considerably more. For example, Jekyll has a new convention center, regularly sized and miniature golf courses, a water park, restaurants, bars, and other such items absent during most of its Pleistocene-Holocene history. Another difference is that a ferry was need to get onto Cumberland, whereas we could drive onto Jekyll and stay overnight there in a hotel.

So why go there at all with a class that is supposed to emphasize the geology, ecology, and natural history of the Georgia barrier islands? The main reason for why I chose Jekyll as a destination for these students was so they could see for themselves the balance (or imbalance) between preserving natural areas and human development of barrier islands. Jekyll is one of those islands that is “in between,” where much of its land and coastal areas have been modified by people, but patches of it retain potentially valuable natural-history lessons for my students.

So what you’ll see in the following photos will focus on those more natural parts of Jekyll island, with some of the wonders they hold. However, this series of photos will end with one that will shock and horrify all. Actually, you’ll probably just shake your head and sigh with rueful resignation at the occasional folly of mankind, especially when it comes to managing developed barrier islands.

We started our morning like every day should start, with ichnology. Here, tracks of a gray fox, showing direct register (rear foot stepping almost exactly into the front-foot impression) cut between coastal dunes on the south end of Jekyll Island. The presence of gray foxes on Jekyll has caused some curiosity and concern among residents, with the latter emotion evoked because these canids are potential predators of ground-nesting birds, like the Wilson’s plover. Also, people have no idea how many foxes are on the island. If only we had some cost-effective method for detecting their presence, estimating their numbers, and interpreting their behavior. You know, like tracking.

My students show keen interest in the gray fox tracks, especially after I tell them to show keen interest as I take a photo of them. Funny how that works sometimes.

A Wilson’s plover! At least, I think it is.( Birders of the world, please correct me if this is wrong. And I know you will.) We spotted a pair of these birds traveling together on the south end of the island, causing much excitement among the photographers in our group blessed with adequate zoom capabilities on their cameras. Wilson’s plovers are ground-nesting birds, and with both gray foxes and feral cats on the island, their chicks are at risk from these predators. Again, if only we had some cost-effective method for discerning plover-cat-fox interactions. Tracking, maybe?

Here’s a little secret for shorebird lovers visiting Jekyll Island. Walk around the southwest corner of the island, and you are almost assured of seeing some cool-looking shorebirds along the, well, shore, such as these American oystercatchers, looking coy while synchronizing their head turns. These three were part of a flock of about twenty oystercatchers all traveling together, which I had never seen before on any of the islands. If you go walking on Jekyll, and know where to walk, you’ll see some amazing sights like this.

You were probably all wondering what American oystercatcher tracks look like, especially those made by ones that are just standing still. Guess this is your lucky day. Also notice the right foot was draped over the left one, causing an incomplete toe impression on the right-foot one. Wouldn’t it be nice to find a trace fossil just like this?

Black skimmers! We didn’t get to see them skim, but we still marveled at this flock of gorgeous shorebirds. These were in front of the oystercatchers, with an occasional royal tern slipping into the party, uninvited but seemingly tolerated.

Yeah, I know, you also wanted to know what black skimmer tracks look like. So here they are. Now you don’t need to use a bird book to identify this species: just look at their tracks instead!

You think you’re bored? Try being driftwood, with marine clams out there adapted for drilling into your dead, woody tissue. This beach example prompted a nice little lesson in how this ecological niche for clams has been around since at least the Jurassic Period, which we know thanks to ichnology. You’re welcome (again).

Beach erosion at the southernmost end of Jekyll gave us an opportunity to see the root systems of the main tree species there, such as this salt cedar (actually, it’s a juniper, not a cedar, but that’s why scientists use those fancy Latinized names, such as Juniperus virginiana). My students are also happily learning to become the scale in my photos, although I suspect they will soon tire of this.

Look at this beautiful maritime forest! This is what I’m talking about when I say “…patches of it [Jekyll Island] retain potentially valuable lessons in natural history.” This is on the south end of the island, and this view is made possible by walking just a few minutes on a trail into the interior.

Few modern predators, invertebrate or vertebrate, provoke as much pure unadulterated giddiness in me as mantis shrimp. So imagine how I felt when, through sheer coincidence, a couple walked into the 4-H Tidelands Nature Center on Jekyll, while I was there with my class, and asked if I identify this animal they found on a local beach. The following are direct quotations from me: “Wow – that’s a mantis shrimp!! Squilla empusa!! It’s incredible!!” I had never seen a live one on the Georgia coast, and it was a pleasure to share my enthusiasm for this badass little critter with my students (P.S. It makes great burrows, too.)

A stop at the Georgia Sea Turtle Center on Jekyll was important for my students to learn about the role of the Georgia barrier islands as places for sea turtles to nest. But I had been there enough times that I had to find a way to get excited about being there yet again. Which is why I took a photo of their cast of the Late Cretaceous Archelon, the largest known sea turtle. I never get tired thinking about the size of the nests and crawlways this turtle must have made during the Cretaceous Period, perhaps while watched by nareby dinosaurs.

At the north end of Jekyll, shoreline erosion has caused the beach and maritime forest to meet, and the forest is losing to the beach. This has caused the forest to become what is often nicknamed a “tree boneyard,” in which trees die and either stay upright or fall in the same spot where they once practiced their photosynthetic ways.

Quantify it! Whenever we encountered dead trees with root systems exposed, I asked the students to measure the vertical distance from beach surface to the topmost horizontal roots. This gave an estimate of the minimum amount of erosion that took place along the beach.

Perhaps a more personal way to convey the amount of beach erosion that happened here was to see how it related to the students’ heights. It was a great teaching method, well worth the risk of being photobombed.

Are you ready? Here it is, in three parts, what was without a doubt the traces of the day. Start from the lower left with that collapsed burrow, follow the tracks from left to right, and look at that raised area on the right.

A close-up of the raised area shows a chevron-like pattern, implying that this was an animal that had legs, and knew how to use them. Wait, is that a small part of its tail sticking out of the left side?

Violá! It was a ghost shrimp! I almost never see these magnificent burrowers alive and outside of their burrows, and just the day before on Cumberland Island, the students had just learned about their prodigious burrowing abilities (the ghost shrimp, that is, not the students). I had also never before seen a ghost shrimp trackway, let alone one connected to a shallow tunnel on a beach. An epic win for ichnology!

This may look like soft-serve ice cream, but I suspect that it’s not nearly as tasty. It’s the fecal casting of an acorn worm (Balanoglossus sp.), and is composed mostly of quartz sand, but still. These piles were common on the same beach at the north end of Jekyll, but apparently absent from the south-end beach. Why? I’m guessing there was more food (organics) provided by a nearby tidal creek at the north end. But I’d appreciate all of those experts on acorn worms out there to augment or modify that hypothesis.

This is how dunes normally form on Georgia barrier-island beaches: start with a rackline of dead smooth cordgrass (Spartina alterniflora), then windblown sand begins to accumulate in, on, and around these. Throw in a few windblown seeds of sea oats and a few other dune-loving species of plants, and next thing you know, you got dunes. Dude.

In contrast, here is how not to form dunes on Georgia barrier-islands beaches. Build a concrete seawall on the middle part of the island, truck in thousands of tons of metamorphic rock from the Piedmont province of Georgia, place the rocks in front of the seawall, and watch the beach shrink. So sad to see all of that dune-building smooth cordgrass going to waste. Anyway, the contrast and comparison you just saw is also what my students experienced by standing in both places the same day.

Jekyll Island gave us many lessons, but we only had a day there. Which islands were next? St. Simons and Little St. Simons, with emphasis on the latter. So look for those photos in a couple of days, in between new exploits and learning opportunities.

 

 

 

 

Descent with Modification

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At this time last year, Fernbank Museum of Natural History was hosting the Darwin exhibit. On loan from the American Museum of Natural History, this exhibit was a major coup for the museum and the Atlanta area, which has enjoyed a growing culture of celebrating science during the past few years. Along with this exhibit, the museum also planned and concurrently displayed an evolution-themed art show, appropriately titled Selections, which I wrote about then here.*

Descent with Modification (2011), mixed media (colored pencils and ink) on paper, 24″ X 36.” Although this artwork might at first look like a tentacled creature infested with crustaceans and living on a sea bottom, its main form actually mimics a typical burrow system made by ten-legged crustaceans (decapods). Yet it’s also an evolutionary hypothesis. Intrigued? If so, please read on. If not, there are plenty of funny cat-themed Web sites that otherwise require your attention. (Artwork and photograph of the artwork by Anthony Martin.)

One unusual feature of this art show was that five of the eight artists were also scientists (full confession: I was one of them). Furthemore, one of the other artists was married to a scientist (fuller confession: that would be my wife Ruth). The show stayed up for more than three months, which was also as long as the Darwin exhibit resided at Fernbank. Thus we like to think it successfully exposed thousands of museum visitors to the concept that scientists, like many other humans, have artistic inspirations and abilities, neatly refuting the stereotype that not all of us are joyless, left-brained automatons and misanthropes.

Last week I was reminded of this anniversary and further connections between science and art during a campus visit last week by marine biologist and crustacean expert Joel Martin (no relation). Dr. Martin was invited to Emory University to give a public lecture with the provocative title God or Darwin? A Marine Biologist’s Take on the Compatibility of Faith and Evolution. His lecture was the first of several on campus this year about the intersections between matters of faith and science, the Nature of Knowledge Seminar Series. This series was organized as a direct response to the university inviting a commencement speaker this past May who held decidedly strong and publicly expressed anti-science views.

Dr. Martin, who is also an ordained elder in his Presbyterian church and has taught Sunday school to teenagers in his church for more than 20 years, gave an informative, organized, congenial, and otherwise well-delivered presentation to an audience of more than 200 students, staff, faculty, and other people from the Atlanta community. In his talk, Martin effectively explored the false “either-or” choice often presented to Americans who are challenged by those who unknowingly misunderstand or deliberately misrepresent evolutionary theory in favor of their beliefs. Much of what he mentioned, he said, is summarized in a book he wrote for teenagers and their parents, titled The Prism and the Rainbow: A Christian Explains Why Evolution is Not a Threat.

I purposefully won’t mention any of the labels that have been applied to the people and organizations who promote this divisiveness between evolutionary theory and faith. After all, words have power, especially when backed up by Internet search engines. Moreover, it is an old and tired subject, of which I grow weary discussing when there is so much more to learn from the natural world. Better to just say that Martin persuasively conveyed his personal wonder for the insights provided by evolutionary theory, how science informs and melds with his faith, and otherwise showed how science and faith are completely compatible with one another. You know, kind of like science and art.

Previous to his arrival, his host in the Department of Biology asked Emory science faculty via e-mail if any of us would like to have a one-on-one meeting with Dr. Martin during his time here. I leaped at the chance, and was lucky enough to secure a half-hour slot in his schedule. When he and I met in my office, we had an enjoyable chat on a wide range of topics, but mostly on our shared enthusiasm for the evolution of burrowing crustaceans, and particularly marine crustaceans.

Ophiomorpha nodosa, a burrow network in a Pleistocene limestone of San Salvador, Bahamas. In this instance, the burrows were probably made by callianassid shrimp, otherwise known as “ghost shrimp,” and are preserved in what was a sandy patch next to a once-thriving reef from 125,000 years ago. (Photograph by Anthony Martin.)

Interestingly, during this conversation we also touched on on how art and science work together, and I was pleasantly surprised to find out that Dr. Martin is a talented artist, too. It turns out he has illustrated many of his articles with exquisite line drawings of his beloved subjects, marine crustaceans. Yes, I realize that some artists like to draw a line (get it?) between being an “artist” and an “illustrator,” with the latter being held in some sort of disdain for merely “copying” what is seen in nature. If you’re one of those, sorry, I don’t have the time or inclination to argue about this with you. (Now go back to putting a red dot on a white canvas and leave us alone.)

Cover art of branchiopod Lepidurus packardi from California, drawn by Joel W. Martin, for An Updated Classification of the Recent Crustacea, also co-authored by Joel W. Martin and George E. Davis: No. 39, Science Series, Natural History Museum of Los Angeles County, Los Angeles, California.

During our discussion in my office, I pointed out a enlarged reproduction of a drawing of mine depicting the burrow complex of an Atlantic mud crab (Panopeus herbstii). He immediately recognized it as a crustacean burrow, for which I was glad, because it is an illustration of just that in my upcoming book, Life Traces of the Georgia Coast.

Burrow complex made by Atlantic mud crab (Panopeus herbstii), originally credited to a snapping shrimp (Alpheus heterochaelis). Scale = 5 cm (2 in). (Illustration by Anthony Martin, based on epoxy resin cast figured by Basan and Frey (1977).

After his campus visit, though, I realized that an even more appropriate artistic work to have shown him was the following one made for the Selections art exhibit last fall, titled Descent with Modification. This title in honor of the phrase used by Charles Darwin to describe the evolutionary process, but also is a play on words connecting to the evolution of burrowing crustaceans.

Descent with Modification again, but this time look at it as an evolutionary chart, where the burrow junctions in the burrow system reflect divergence points (nodes) from common ancestors. For example, from left to right, the ghost shrimp is more closely related to a mud shrimp, and both of these are more closely related to the ghost crab (middle) than they are to the lobster and freshwater crayfish (right). The main vertical burrow shaft represents their common ancestry from a “first decapod,” which may have been as far back as the Ordovician Period, about 450 million years ago.

The image shows five burrowing crustaceans, or to be more specific, ten-legged crustaceans called decapods. Along with these is a structure, which has a burrow entrance surrounded by a conical mound of excavated and pelleted sediment, a vertical shaft connecting to the main burrow network, and branching tunnels that lead to terminal chambers. A burrowing crustacean occupies each chamber, and these are, from left to right: a ghost shrimp (Callichirus major), a mud shrimp (Upogebia pusilla), a ghost crab (Ocypode quadrata), a marine lobster (Homarus gammarus), and a freshwater crayfish (Procambarus clarkii).

Here’s the cool part (or at least I think so): this burrow system also serves as an evolutionary chart – kind of a cladogram – depicting the ancestral relationships of these modern burrowing decapods. For example, lobsters and crayfish are more closely related to one another (share a more recent common ancestor) than lobsters are related to crabs. Mud shrimp are more closely related to crabs than ghost shrimp. Accordingly, the burrow junctions show where these decapod lineages diverged. So the title of the artwork is a double entendre with reference to Darwin’s phrase describing evolution as a process of “descent with modification,” along with burrowing decapods undergoing change through time as they descend in the sediment.

Modern decapod burrows and trace fossils of probable decapod burrows support both the science and the artwork, too. Here are a few examples to whet your ichnological and aesthetic appetites:

Thalassinoides, a trace fossil of horizontally oriented and branching burrow systems made by decapods in Early Cretaceous rocks (about 115 mya) of Victoria, Australia. In this case, these burrows were likely by freshwater decapods, such as crayfish, which had probably diverged from a common ancestor with marine lobsters more than 100 million years before then. Scale = 10 cm (4 in). (Photograph by Anthony Martin.)

Thalassinoides again, but this time in limestones formed originally in marine environments, from the Miocene of Argentina. Note the convergence in forms of the burrows with those of the freshwater crayfish ones in Australia. Think that might be related to some sort of evolutionary heritage? Scale = 15 cm (6 in). (Photograph by Anthony Martin.)

Horizontally oriented burrow junction of a modern ghost shrimp – probably made by a Carolina ghost shrimp (Callichirus major) – exposed along the shoreline of Sapelo Island, Georgia. Note the pelleted exterior, which is also visible on the burrow networks of the fossil ones in the Bahamas, pictured earlier. So if fossilized, this would be classified as the trace fossil Ophiomorpha nodosa. Scale in centimeters. (Photograph by Anthony Martin.)

Two ghost-shrimp burrow entrances on a beach of Sapelo Island, Georgia, with the one on the right showing evidence of its occupant expelling water from its burrow. No scale, but burrow mound on right is about 5 cm (2 in) wide. (Photograph by Anthony Martin.)

Burrow entrance and conical, pelleted mound made by a freshwater crayfish (probably a species of Procambarus) in the interior of Jekyll Island, Georgia. Scale = 1 cm (0.4 in). (Photograph by Anthony Martin.)

So the take-away message of all of these musings and visual depictions is that evolution, faith, science, art, trace fossils, modern burrows, and burrowing decapods can all co-exist and be celebrated, regardless of whether we sing Kumbaya or not. So let’s stop dividing one another, get out there, and learn.

*I’m also proud to say that my post from October 17, 2011, Georgia Life Traces as Art and Science, was nominated for possible inclusion in Open Laboratory 2013. Thank you!

Further Reading

Basan, P.B., and Frey, R.W. 1977. Actual-palaeontology and neoichnology of salt marshes near Sapelo Island, Georgia. In Crimes, T.P., and Harper, J.C. (editors), Trace Fossils 2. Liverpool, Seel House Press: 41-70.

Martin, A.J. In press. Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press, Bloomington, IN: 680 p.

Martin, A.J., Rich, T.H., Poore, G.C.B., Schultz, M.B., Austin, C.M., Kool, L., and Vickers-Rich, P. 2008. Fossil evidence from Australia for oldest known freshwater crayfish in Gondwana. Gondwana Research, 14: 287-296.

Martin, J.W. 2010. The Prism and the Rainbow: A Christian Explains Why Evolution is Not a Threat. Johns Hopkins University Press, Baltimore, MD: 192 p.

Martin, J.W., and Davis. G.E. 2001. An Updated Classification of the Recent Crustacea, No. 39, Science Series, Natural History Museum of Los Angeles County, Los Angeles, California: 132 p.

 

The Lost Barrier Islands of Georgia

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The Georgia coast is well known for its historic role in the development of modern ecology, starting in the 1950s and ongoing today. But what about geologists? Fortunately, they were not long behind the ecologists, starting their research projects on Sapelo Island and other Georgia barrier islands in the early 1960s. Indeed, through that seminal work and investigations afterwards, these islands are now renown for the insights they bestowed on our understanding of sedimentary geology.

Why would geologists be attracted to these islands made of shifting sand and mud that were nearly devoid of anything resembling a rock? Well, before sedimentary rocks can be made, sediments are needed, and those sediments must get deposited before solidifying into rock. So these geologists were interested in learning how the modern sands and muds of the barrier islands were deposited, eroded, or otherwise moved in coastal environments, a dynamism that can be watched and studied every day along any Georgia shoreline. The products of this sediment movement were sedimentary structures, which were either from physical processes – such as wind, waves, or tides – or biological processes, such as burrowing. Hence sedimentary structures can be classified as either physical or biogenic, respectively.

Cabretta Beach on Sapelo Island at low tide, its sandflat adorned with beautiful ripples and many traces of animal life. Sand is abundant here because of a nearby tidal channel and strong ebb-tide currents that tend to deposit more sand than in other places around the island. This sand, in turn, provides lots of places for animals that live on or in the sand, making trails and burrows, demonstrating how ecology and geology intersect through ichnology, the study of traces.  Speaking of traces, what are all of those dark “pipes” sticking out of that sandy surface? Hmmm… (Photograph by Anthony Martin.)

These geologists in the 1960s were among the first people in North America to apply what they observed in modern environments to ancient sedimentary deposits, and just like the ecologists, they did this right here in Georgia. For example, in 1964, a few of these geologists – John H. Hoyt, Robert J. Weimer, and V.J. (“Jim”) Henry – used a combination of: geology, which involved looking at physical sedimentary structures and the sediments themselves; modern traces made by coastal Georgia animals; and trace fossils. Through this integrated approach, they successfully showed that the long, linear sand ridges in southeastern Georgia were actually former dunes and beaches of ancient barrier islands.

These sand ridges, barely discernible rises on a mostly flat coastal plain, are southwest-northeast trending and more-or-less parallel to the present-day shoreline. Remarkably, these ridges denote the positions of sea-level highs during the last few million years on the Georgia coastal plain. The geologists applied colorful Native American and colonial names to each of these island systems – Wicomico, Penholoway, Talbot, Pamlico, Princess Anne, and Silver Bluff – with the most inland system reflecting the highest sea level. So how did these geologists figure out that a bunch of sand hills were actually lost barrier islands? And what does this all of this have to do with traces and trace fossils?

Map showing positions of sand ridges that represent ancient barrier islands, with each ridge marking the fomer position of the seashore. The one farthest west (Wicomico) represents the highest sea level reached in the past few million years, whereas the current barrier islands reflect an overlapping of two positions of sea level, one from about 40,000 years ago (Silver Bluff), and the other happening now. (Photograph by Anthony Martin, taken of a display at the Sapelo Island Visitor Center.)

Here’s how they did it. They first observed modern traces on Georgia shorelines that were burrows made by ghost shrimp, also known by biologists as callianassid shrimp. On a sandy beach surface, the tops of these burrows look like small shield volcanoes, and a burrow occupied by a ghost shrimp will complete that allusion by “erupting” water and fecal pellets through a narrow aperture.

Top of a typical callianassid shrimp burrow, looking much like a little volcano and adorned by fecal pellets, which coincidentally resemble “chocolate sprinkles,” but will likely disappoint if you do a taste test. (Photograph by Anthony Martin, taken on St. Catherines Island.)

A couple of ghost shrimp, which are either a male-female pair of Carolina ghost shrimp (Callichirus major) or a Carolina ghost shrimp and a Georgia ghost shrimp (Biffarius biformis). Sorry I can’t be more accurate, but I’m an ichnologist, not a biologist (although I could easily play either role on TV). Regardless, notice they have big claws, which they use as their main “digging tools.” The tracemakers look a little displeased about being outside of their protective burrow environments, but be assured I thanked them for their contribution to science, and promptly threw them back in the water so they could burrow again. Scale = 1 cm (0.4 in) (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Just below the beach surface, these interior shafts widen considerably, making these burrows look more like wine bottles than volcanoes. This widening accommodates the ghost shrimp, which moves up and down the shaft to irrigate its burrow by pumping out its unwanted feces (understandable, that) and circulating oxygenated water into the burrow. Balls of muddy sand reinforce the burrow walls like bricks in a house, stuck together by shrimp spit, and the burrow interior is lined with a smooth wall of packed mud.

A small portion of a ghost-shrimp burrow, showing its wall reinforced by rounded pellets of sand and stuck together with that field-tested and all-natural adhesive, shrimp  spit. Photograph by Anthony Martin, taken on Sapelo Island.

Amazingly, these shafts descend vertically far below the beach, as much as 2-3 meters (6.5-10 feet) deep. Here they turn horizontal, oblique, and vertical, and tunnels intersect, branch, and otherwise look like a complex tangle of piping, perhaps reminding baby-boomers of “jungle gyms” that they used to enjoy as children in a pre-litigation world. Who knows what goes on down there in such adjoining ghost-shrimp burrow complexes, away from prying human eyes?

The deeper part of a modern ghost-shrimp burrow, exposed by erosion along a shoreline and revealing the more complex horizontally oriented and branching networks. Gee, do you think these burrows might have good fossilization potential? (Photograph by Anthony Martin, taken on Sapelo Island.)

See all of those burrow entrances on this sandy beach? Now imagine them all connecting in complex networks below your feet the next time you’re walking along a beach. Feels a little different knowing that, doesn’t it? (Photograph by Anthony Martin, taken on Sapelo Island.)

Interestingly, these burrows are definitely restricted to the shallow intertidal and subtidal environments of the Georgia coast, and their openings are visible at low tide on nearly every Georgia beach. Hence if you found similar burrows in the geologic record, you could reasonably infer where you were with respect to the ancient shoreline.

I think you now know where this is going, and how the geologists figured out what geologic processes were responsible for the sand ridges on the Georgia coastal plain. Before doing field work in those area, the geologists may have already suspected that these sandhills were associated with former shorelines. So with such a hypothesis in mind, they must have been thrilled to find fossil burrows preserved in the ancient sand deposits that matched modern ghost-shrimp burrows they had seen on the Georgia coast. They also found these fossil burrows in Pleistocene-age deposits on Sapelo Island, which helped them to know where the shoreline was located about 40,000 years ago with respect to the present-day one. This is when geologists started realizing that the Georgia barrier islands were made of both Pleistocene and modern sediments as amalgams of two shorelines, and hence unlike any other known barrier islands in the world.

Vertical shaft of a modern ghost-shrimp burrow eroding out of a shoreline on Cabretta Beach, Sapelo Island. Scale in centimeters. (Photograph by Anthony Martin.)

Vertical shaft of a fossil ghost-shrimp burrow eroding out of an outcrop in what is now maritime forest on Sapelo Island, but we know used to be a shoreline because of the presence of this trace fossil. Scale in centimeters. (Photograph by Anthony Martin.)

Geology and ecology combined further later in the 1960s, when paleontologists who also were well trained in biology began looking at how organisms, such as ghost shrimp, ghost crabs, marine worms, and many other animals changed coastal sediments through their behavior. So were these scientists considered geologists, biologists, or ecologists? They were actually greater than the sum of their parts: they were ichnologists. And what they found through their studies of modern traces on the Georgia barrier islands made them even more scientifically famous, and these places became recognized worldwide as among the best for comparing modern traces with trace fossils.

Further Reading:

Hoyt, J.H., and Hails, J.R. 1967. Pleistocene shoreline sediments in coastal Georgia: deposition and modification. Science, 155: 1541-1543.

Hoyt, J.H., Weimer, R.J., and Henry, V.J., Jr. 1964. Late Pleistocene and recent sedimentation on the central Georgia coast, U.S.A. In van Straaten, L.M.J.U. (editor), Deltaic and Shallow Marine Deposits, Developments in Sedimentology I. Elsevier, Amsterdam: 170-176.

Weimer, R.J., and Hoyt, J.H. 1964. Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments. Journal of Paleontology, 38: 761-767.

Why Study Traces in Georgia? A Celebration of the Familiar

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For those of us who live in Georgia, we either forget or don’t know about the ecological and geological specialness of this part of the U.S. For example, my undergraduate students here in Atlanta often talk dreamily about their desire to visit the Amazon River basin, Costa Rica, Kenya, Australia, or other places far removed from Georgia, beguiled as they are by the exotic “other” qualities of those places with their biota and landscapes. On the other hand, almost none of these students have been to the Okefenokee Swamp, the Blue Ridge Mountains, the Cumberland Plateau, the long-leaf pine forests of Ichauway, or the Georgia barrier islands, unless my colleagues or I have taken them there on field trips. Yet these places, especially those with freshwater ecosystems, collectively hold a biodiversity nearly matching that of the Amazon River basin, an evolutionary consequence of the long geologic history of the Appalachian Mountains.

To be fair, I have likewise found myself succumbing to such place-based deflection and lack of appreciation for what is more-or-less in my backyard. In 2001, I realized that I had been to Brazil (three times) more often than Fernbank Forest (two times), even though Fernbank was only a five-minute bicycle ride from home in Decatur, Georgia. This imbalance was soon corrected, though, and many visits later, I learned to appreciate how this old-growth southern Appalachian forest in the middle of metropolitan Atlanta is a gem of biodiversity, every native species of plant and animal a facet testifying to their long evolutionary histories. Still, I wonder why we often ignore what is nearby, even if it is extraordinary?

Related to this quandary is one of the most common questions I encountered from friends, family, and colleagues while writing my book – Life Traces of the Georgia Coast – which was, “Why are you, a paleontologist and geologist, writing about the traces of modern plants and animals in Georgia?” This is a legitimate inquiry, but my answer surprises most people. I tell them that my main reason for staying here in Georgia to study the tracks, trails, burrows, nests, and other traces of its barrier islands is because these traces and their islands are world-class and world-famous. This high quality is directly linked to the biodiversity of the Georgia barrier islands, but also their unique geological histories compared to other barrier-island systems. Furthermore, these islands have inspired more than a few major scientific discoveries related to modern ecology and geology, some of which, made nearly 50 years ago, are still applicable to diagnosing the fossil record and the earth’s geologic history. In short, the Georgia barrier islands and their traces also reflect a legacy recognized by scientists far outside the confines of Georgia.

How so? I’ll explain in upcoming posts, and hope to demonstrate how the marvelous ecosystems of the Georgia coast and its geological processes are the proverbial gift that keeps on giving, continually helping us to better understanding the earth’s geologic past. Now that’s special!

Burrows at dawn: a partial view of the thousands of ghost-shrimp burrows dotting a Georgia beach at low tide, their entrances looking like tiny volcanoes. What makes these burrows so important, scientifically speaking, and why are they something that would cause scientists from outside of Georgia to travel and see in person? Photo by Anthony Martin and taken on Sapelo Island, Georgia.