On the 1st Day of Ichnology, My Island Gave to Me: 1 Sea Star Gliding

The last of my holiday-inspired series of photos depicting traces from the Georgia barrier islands is of one most people will see only rarely, but is a glorious one to spot. It is the trail left by a lined sea star (Luidia clathrata), best observed on the lower parts of sandy beaches.

Sea-Star-Moving-SapeloA beautifully expressed trail left by a lined sea star (Luidia clathrata). The sea star itself is only about 10 cm (4 in) across, but its trail shows how far it traveled, from its initial “resting” spot to where you see the tracemaker itself. (Photograph by Anthony Martin, taken on Sapelo Island.)

Sea stars make such trails when stranded on the lower part of a beach by a high tide. Once exposed, especially under a summertime sun, they can either dry out quickly or become easy prey for a wandering seagull. If they’re fortunate enough to be on or otherwise near a saturated sand, they’ll bury themselves by moving their hundreds of tube feet underneath them. This makes a sort of localized quicksand around them, and they will sink into the sand, which normally solves their dual problem of dehydration and predation. However, the resulting trace this makes is a star-shaped bump on the sand surface, which to many seagulls still translates as “food.” Many times I have seen and photographed spots on beaches where a gull was practicing its own form of ichnology, where it walked straight to a buried sea star, plucked it from its temporary resting spot, and took it somewhere else to eat.

In this photo, though, the sea star had an even greater challenge when it was left on a sandflat. It was dumped by a high tide onto a part of the beach with a thin layer of wet sand overlying a more firmly packed sand. This meant that the sea star’s tube feet could only get it so far down into the sand, having been stopped by the hard, packed layer underneath. So its only other choice was to move laterally along the wet sand, which it accomplished through a combination of tube feet and arms, causing it to glide through and on top of the sand.

My interpretation of this behavior is that the sea star was desperately seeking moisture – whether a softer, wetter sand or a submerged area – and that this journey was more likely to ensure its survival than to simply sit and wait for the next tidal cycle. Considering that sea stars have been around for more than 450 million years, I can only assume this behavior worked quite well for at least of few of this species’ ancestors. Thus I would not be surprised at all by the discovery of trace fossils matching the form and intent of the modern traces shown here.

Meanwhile, let’s give thanks for how lucky we are to see them and understand the meaning of these and other traces being made by Georgia-coast animals every day. Each vestige is a lesson in natural history, beckoning us to learn more about the evolutionary processes that led to what we observe now.

Further Information

Luidia clathrata: Lined Sea Star. Encyclopedia of Life.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

On the 4th Day of Ichnology, My Island Gave to Me: 4 ‘Gators Denning

 On the 3rd Day of Ichnology, My Island Gave to Me: 3 Ghost Shrimp Pooping

On the 2nd Day of Ichnology, My Island Gave to Me: 2 Otters Running

On the 2nd Day of Ichnology, My Island Gave to Me: 2 Otters Running

On this Christmas of 2013, I thought that the second-to-last post of my “On the __th Day of Ichnology” series would be a gift, one speaking of the beautiful harmony we sometimes are so fortunate to see recorded in the sands of the Georgia barrier islands. The traces composing this gift are the tracks of a male-female pair of river otters (Lutra canadensis).

Otter-Tracks-St-CatherinesSynchronicity expressed in traces: a pair of river otters, running and turning together along a Georgia beach. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale is about 10 cm (4 in) long.)

A normal gait for river otters is a lope, which registers as a 1-2-1 pattern, in which one rear foot is in front, a rear and front foot are next to one another, and a front foot is behind. However, in this instance, I think both otters were galloping, as it looks like both rear feet exceeded their front feet, and a well-defined space is in between each set of four tracks.

What really struck me about these tracks, and made me gasp with joy when I saw them, was their near-perfect symmetry and how they hint of one otter reacting to the other otter’s movement. I can’t say for sure right now what evidence lends to my discerning the following interpretation (sorry, fellow scientists). But my hunch is that the otter on the left was running just in front of the other, maybe separated by a body length at this point, and then turned just slightly to her/his left. The otter on the right was galloping to catch up, saw its partner turn to the left, and decided to turn her/his body in response to this change in direction. Notice how the gap between their trackways is narrowed just a bit, and how the tail of the second otter left an arc-like impression on the sand that points directly to the next set of tracks.

Such a gorgeous set of traces, left by a species we humans often revere (or envy) for its love of play! But I also found these tracks even more gratifying for how they told of two otters linked to one another, perhaps through play, but certainly through their mirrored behaviors, and how this in turn held up a mirror to ourselves. What interactive traces do we similarly leave in our lives? In which instances are we the otter on the left, leading the way and making decisions to change course? In which instances do we follow just behind and to the side of others, and run to catch up? Why do we sometimes lead, why do we sometimes follow, and what makes us come together? Thoughts for Christmas, thoughts for the end of this year, and thoughts for the start of a new year, bestowed by the symbolism of these traces.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

On the 4th Day of Ichnology, My Island Gave to Me: 4 ‘Gators Denning

 On the 3rd Day of Ichnology, My Island Gave to Me: 3 Ghost Shrimp Pooping

On the 3rd Day of Ichnology, My Island Gave to Me: 3 Ghost Shrimp Pooping

Everyone poops. More specifically, every animal has to eat, converting this food into energy and otherwise applying it to bodily functions. As we also know through daily experience, this conversion is never 100% efficient. Thus waste is produced and excreted from all animal bodies, sometimes liquid, sometimes solid, or a mixture of the two.

And on the Georgia barrier islands, few animals are more visibly productive with their poop than ghost shrimp. So today’s photo and explanation celebrates those wondrous poopers of the Georgia coast.

Ghost-Shrimp-Pellets-Burrows-JekyllWe three burrows of Georgia coast are adorned with feces, showing that each of us is actively occupied by a ghost shrimp. In each burrow, the shrimp is probably just below the narrow aperture, doing a little housecleaning. (Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

I’ve written previously about ghost shrimp – otherwise known as callianassid shrimp – and the significance of their burrows to ecologists, geologists, and  paleontologists (linked under “Further Reading”). But I haven’t focused on one of their most important roles as ecosystem engineers, which is their prolific pooping of pellets.

These pellets are small, dark, perfectly shaped cylinders that, because of their resemblance to “chocolate sprinkles,” never fail to capture the attention of cupcake lovers as they stroll along Georgia beaches. (Now that you know what they are, please don’t eat them. Unless you like them, in which case, I’m never buying a cupcake from you.) However, aside from inspiring confectionery allusions, these pellets are extremely important in Georgia beach environments as sources of mud.

Only two species of ghost shrimp are responsible for all of this mud dumping, the Georgia ghost shrimp (Biffarius biformis) and Carolina ghost shrimp (Callichirus major). Nonetheless, they make up for their lack of diversity through sheer numbers; look closely at most Georgia beaches at low tide and you will see thousands of little “sand volcanoes,” most with pellets. Nearly all of these represent a live ghost shrimp, down below your feet, burrowing, feeding, mating, and pooping.

After feeding on mud-rich organics in their burrows, these shrimp make and emit mud-rich fecal pellets, neatly shrink-wrapped by mucus. The shrimp can then collect these packets of poop and pump them out the tops of their burrows, an efficient form of waste disposal that keeps their homes clean. These pellets become the hydrodynamic equivalent of sand grains, rolling with the tides and waves and are commonly deposited in ripple troughs and other low spots on a sandy beach.

Eventually their mucus coverings break down and release the mud particles (silt and clay), but at least these sediments were deposited. This would almost never happen on its own because of tides and waves keeping it suspended in the water, and means that the mud would be much less likely to get recycled into coastal sediments, and Georgia coastal waters would be even muddier than normal.

So take note, geologists: those thin layers of mudstone you see in the troughs of a rippled sandstone that you might just label “flaser bedding” in your field notebook, then promptly forget? Those beds probably got there by something pooping in the ancient past. And for everyone else, give thanks for these gift-wrapped feces, and for what they do for Georgia coastal environments.

Further Reading

The Lost Barrier Islands of Georgia. Written by me, posted October 3, 2011.

Ghost Shrimp Whisperer. Written by me, posted May 20, 2013.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

On the 4th Day of Ichnology, My Island Gave to Me: 4 ‘Gators Denning

On the 4th Day of Ichnology, My Island Gave to Me: 4 ‘Gators Denning

For today’s photo and explanation of traces of the Georgia barrier islands that beguile, I’ll turn to one of the more charismatic and well-known of  tracemakers, and what are among the largest traces of any animals on the Georgia coast. These would be alligators (Alligator mississippiensis) and alligator dens, respectively.

Juvenile-Alligators-Denning-SapeloSee those two big holes just above the shoreline of this freshwater pond? Those are alligator dens, large burrows that benefit them in many ways. And just to prove this point, these two dens have a pair of alligators hanging out at their entrances. Both alligators are only about 1 meter (3.3 feet) long, though, which means they’re way too small to have been the alligators that made these dens. So what’s going on here? (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

I’ve written several times before about alligator dens on the Georgia barrier islands, and these are a subject of on-going research for me and several colleagues. So I won’t go on about them here, and instead will just focus on what this specific photo of these dens and alligators tells us.

The picture was taken in March, 2011, at the start of spring on the Georgia coast. Hence the alligators might have been just then coming out of dens after overwintering in them. However, notice the mismatch in sizes of the alligators compared to the den entrances. The dens are much too large for their denizens, implying that these are not their original makers, but instead are secondary occupiers, reusing these dens. I was also surprised to see five alligators – all about the same size – sharing dens. Yeah, I know, the title of this post says “four ‘gators denning,’ but you’re only seeing four of them in the photo; the one on the right had at least three I saw that day.

A little bit of background might help with understanding what was happening there and then. I’ve been revisiting this freshwater pond on Sapelo Island for nearly 15 years, and can confirm that these are the same dens. Sometimes I’ll see evidence of alligators actively using them, as in, I see alligators lying at their entrances, and alligators that retreat into these dens if they get too shy from all of the enthusiastic ichnologically inspired love I’m sending their way.

Sometimes those alligators have been large, full-sized adults with body widths only slightly smaller than den widths. Other times the alligators will be most modestly sized, like these. Regardless, this shows that once a den is made, it can be used by many alligators of varying sizes, over more than a decade, and possibly over generations.

Something else interesting about this photo? All four of the visible alligators – and the one you don’t see that’s in the den to the right – were about the same length. Along with their congregating in the same location, this is a hint that they might have been siblings, having hatched from the same egg clutch and grown up together in this pond. Even better, their mother might have raised and protected them there by using one or both of these dens. This means that alligator dens might be passed down in families and occasionally shared out of necessity by family members. You know, just like us. Amazed? If so, thank ichnology for inducing that sense of wonder.

Further Reading

Into the Dragon’s Lair: Alligator Burrows as Traces. Written by me, published on this blog March 15, 2012.

Deconstructing an Ichnology Abstract, with Alligators. Written by me, published on this blog October 19, 2012.

What a Big Momma Alligator in Her Burrow Tells Us about Dinosaurs. Written by me, published on the BBC’s Walking With Dinosaurs site November 20, 2013.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

Today’s photo of Georgia-coast traces – similar to yesterday’s about sea turtles connecting to the land through their traces – shows how other marine animals depend on landward environments of the Georgia barrier islands for their livelihoods. In this instance, marine clams require trees to give them homes, and these clams leave marks of their dependency for us to see on formerly forest flotsam that made its way back to land.

Marine-Bivalve-Bored-DriftwoodA piece of driftwood rendered holey by abundant and active wood-drilling marine bivalves, some of which also left their bodies in their former homes. These borings are probably all the work of wedge piddocks (Martesia cuneiformis), which settled onto the wood as wee little clams (larvae, actually), then started drilling.(Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

After the larvae of these clams latched onto these woody substrates – whether these were floating on ocean currents or sunken on sea bottoms – they then lived out their lives drilling into the wood. They drill into wood by rotating or otherwise moving their ridged shells against the hard substrate, like a self-propelled screw.

A few species of wood-drilling clams – sometimes nicknamed “shipworms,” despite their molluscan heritage – actually eat the wood for food. But others, including wedge piddocks, are just making tight, secure homes, similar to how some animals make snug burrows for themselves. Once in a while we get to see the handiwork of these clams in pieces of wood that wash up on Georgia shorelines, a special delivery brought to us by tides and waves.

Wood-boring clams probably evolved about 150-200 million years ago during the Mesozoic Era, and their trace fossils are common in fossil driftwood from the Jurassic Period to just recently. For marine clams to start drilling into wood – whether for food, homes, or both – is pretty remarkable as a behavior, when you think about it evolving in response to the growth of forests on land. After all, bivalves lived in the world’s oceanscapes long before forests spread across landscapes, with the former starting in the Cambrian Period (more than 500 million years ago) and the latter starting in the Devonian Period (about 350 million years ago).

But it’s also interesting to think about how marine clams apparently did not take advantage of these terrestrial tissues for several hundred million years after wood first started floated out to sea. In contrast, mites, insects, and other land-dwelling invertebrates began chewing wood right away, and consequently left their own distinctive traces (mentioned last week with beetle borings). But thanks to trace fossils, we can better tell when terrestrial animals commenced wood-eating behaviors, and when certain marine clams began mixing their traces with those of their land-lubbing compatriots.

Further Reading

Martesia cuneiformis (Say, 1822) Wedge Piddock. Jaxshells.org, by Bill Frank, images by Joel Wooster.

The Second World That Forms on Sunken Trees. Ed Yong, Not Exactly Rocket Science, National Geographic Phenomena.

Wood: It’s What’s For Dinner. Craig McClain, Deep Sea News.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

 

 

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

For today’s photo of Georgia-coast traces and explanation of their meaning, we’ll look at some that are made only very briefly in the first moments of active life for their tracemakers, and in an environment very few of them will ever revisit. Moreover, those exceptional individuals who do make it back to the same environment – sometimes the same place where they took their baby steps – may take as long as three decades to do so. The traces are the trackways made by hatchling sea turtles, in this instance loggerhead turtles (Caretta caretta).

Hatchling-Turtle-Trackways-St-CatherinesYeah, I know, there are a lot more than just six sea-turtle hatchling trackways here, but it’s at least six. Regardless, they’re still in the spirit of the holiday season. These tracks were made only moments after the hatchlings emerged from a buried hole nest, which was behind coastal dunes on St. Catherines Island, Georgia. They hatched early in the morning of July 31, 2011 and promptly began making tracks. (Photograph by Anthony Martin; scale in centimeters.)

The trackways are relatively simple, consisting of alternating front-back flipper impressions and central body drag marks that are vaguely defined in dry sand (like in the photo) but become gorgeously expressed in wet sand. Overall trackway patterns tend to loop and intersect one another close to the nest as they tried to get oriented toward the sea, then become more linear and cross one another less often once they find the beach and waddle down slope. Trackways may be tens of meters long, depending on how far hatchlings must travel from their nests to the surf zone.

As an ichnologist, what I find remarkable conceptually about hatchling traces is knowing that their makers only leave traces on land for a few minutes after they’re born. All successfully hatched sea-turtle eggs are located in nests above the high-tide mark, and on the Georgia coast these are normally behind the first line of coastal dunes along a sandy shoreline. Then, assuming the hatchlings don’t die during their brief and vulnerable time on land (raccoons and herons and ghost crabs – oh my!) and that they do make it into the sea, almost all of their remaining tracemaking behaviors will be done in the Atlantic Ocean.

In other words, you’ll have to be very patient and relatively young to see traces made by those same individual turtles on Georgia beaches and dunes again, and these will only be from adult females. About 15-30 years pass before sea turtles reach sexual maturity, meaning it will take at least that long for pregnant mother turtles to come ashore. Even then, they do this seasonally, from about May through August, so you will only see adult female and hatchling tracks in the middle of any given year. Thus loggerheads and other sea turtles collectively are important tracemakers on the Georgia coast, but individually are rare.

For paleontologists, the huge trackways, covering pits, hole nests, and other marks of sea turtles comprise a trace assemblage with a great potential for preservation in the fossil record. Sure enough, sea turtle trace fossils have been reported from Cretaceous Period rocks in the western U.S. More are likely out there, and I have little doubt that these will be found by applying the right search images. Who will recognize the first trace fossils of sea-turtle hatchlings? My bet is that it will be someone who saw a lot of modern ones, perhaps even some from the Georgia coast. Good luck!

Further Information

St. Catherines Island Sea Turtle Conservation Program. (Mostly done by Gale Bishop.)

Georgia Sea Turtle Center. Jekyll Island, Georgia.

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

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

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

Continuing the theme of “On the __th Day of Ichnology, My Island Gave to Me,” today’s photo and explanation constitute a veritable cornucopia of ichnological goodness. It not only shows traces from two Georgia-coast species that differ greatly in size, but also three types of traces. The tracemakers are small lizards – probably ground skinks (Scincella
lateralis) – and feral horses (Equus caballus), and the traces are trackways, feces, and a burrow, all of which are connected to one another.

Lizard-Tracks-Burrow-Horse-Dung-CumberlandLizard trackways looping around feral-horse dung piles in dunes near Lake Whitney on Cumberland Island, Georgia. But wait, what’s that little hole underneath one of the piles? Why, would that be a lizard burrow? Why, yes indeed, which means that this lizard has horse excrement as the roof to its home. Please feel free to make your own joke about shingles. (Photograph by Anthony Martin, scale in centimeters.)

As far as I know, all of the skinks and other lizards on the Georgia barrier islands are native to the islands and identical to those on the mainland. In contrast, the horses are not native to the Georgia coast, and would not be considered “native” even if they were descended from Spanish horses, which they’re not. Anyway, the horses, which only live a free-manging, er, I mean, free-ranging life on Cumberland Island, leave sufficient quantities of feces that the dung beetles cannot keep up with them. As a result, old and dried horse dung is easily spotted on sand dunes, telling of a former horse presence that can sometimes last for weeks before it breaks down.

Even though people like me might get tired of this crap, the lizards take it all in stride, as in, they just stride around it like any other obstacles they might have in their ecosystems. Think of the horse-dung piles as little hills, and the lizards avoiding them by staying in the valleys. In this instance, their trackways and the old dung piles were in a sand dune near Lake Whitney, the largest freshwater lake of any of the Georgia barrier islands.

The big bonus in this photo, though, is the skink-sized burrow (top center) underneath one of the dung balls, with tracks leading into and out of it. So at least one lizard thought this was this was a splendid place to site its home, enjoying the luxury of a solid, prefab roof in a place with lots of soft, shifting sand.

How to apply these observations to the fossil record? Well, surely animals that were smaller than the dung piles of large land vertebrates in the geologic past must have likewise moved around these as objects that got in their way. Hence trackways reflecting such behaviors might show looping patterns, instead of straight or simply turning to the right or left.

But did any of these small animals also take advantage of these droppings as shelters, in which they decided that they should not go to waste? If so, a coprolite with a small burrow underneath it would be a most fortuitous two-for-one trace fossil, one that could only be made better if the burrow-maker’s tracks were also preserved going into and out of the burrow. Imagine finding something like this in the fossil record: tracks, feces, and a burrow, all together and from two different species and demonstrating that the feces influenced the other two traces. Be still, my ichnologically inspired heart!

Further Reading

Ground skink (Scincella lateralis). Savannah River Ecology Laboratory, University of Georgia.

Feral Animals of Cumberland Island. Wild Cumberland (by Hal Wright and Rhett Lawrence).

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

On the 8th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

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

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

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

For today’s entry in the holiday-and-ichnology inspired countdown of Georgia-coast traces, we will move from the maritime forest to the shoreline, where a trace made through the behavior of one species of molluscan – the knobbed whelk (Busycon carica) – influenced the behavior (and hence traces) of another molluscan – the dwarf surf clam (Mulinia lateralis).

These traces then attracted shorebirds, which added their tracks and beak probe marks to the molluscan traces. This is a excellent modern example of how the interaction of one species of animal with a sediment can affect the interactions of other species with that same sediment, leading to their creation of composites traces.

Whelks-Dwarf-Surf-Clams-Burrowing-JekyllSee all of the knobbed whelks (Busycon carica) in this photo? I know, you don’t actually see their shells because they buried themselves, but you see their outlines on this sandy beach surface because of the many dwarf surf clams (Mulinia lateralis) that burrowed around them. Also look for all of the bird tracks and probe marks around the whelks. (Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

I’ve already written about these composite traces and the ecological story they tell, so for those details, go to this link and this link. But the summary version goes like this:

  • Low tide stranded the whelks and clams on the beach.
  • Whelks used their muscular feet to pull themselves into the still-wet sand to avoid desiccation and predation.
  • Clams took advantage of disturbed (liquified) sand around the whelks and buried themselves, also to avoid predation and desiccation.
  • Shorebirds saw whelk-shaped concentrations of clams, chowed down on them.

The story gets more complicated in places, especially when seagulls decided they also wanted to eat the whelks, but that’s most of it. So next time you’re on a beach and you see a triangular-shaped concentration of small clams, take a second look to see whether there’s a live whelk underneath it: traces begetting traces.

Further Reading

Busycon carica: Knobbed Whelk. Smithsonian Marine Station at Fort Pierce.

Mulinia lateralis: Dwarf Surf Clam. Smithsonian Marine Station at Fort Pierce.

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

Continuing the theme this week of celebrating the traces of the Georgia barrier islands, we this time move inland to the maritime forests, where a myriad of traces await us, most of them made by the true rulers of our planet, insects. In this instance, let’s focus on those insects that leave many well-defined and easily visible traces in so-called “dead” trees in those forests.

Beetle-Borings-SapeloA fortuitous exposure of insect tunnels bored into the wood of a “dead” pine tree (probably a loblolly, Pinus taeda). Most of these traces are probably from beetles, although other borings in any given tree could also be from carpenter ants, carpenter bees,and termites. And what made the big, deep holes? Just some insect-eating dinosaurs that some people insist on calling “woodpeckers.” (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Although I’ll admit that I’m just an enthusiastic novice when it comes to insect traces, I can’t help but notice and stop to admire those left in wood. Although termites, carpenter bees, and carpenters ants are arguably more famous (or infamous) for their wood-drilling abilities, by far the most intriguing traces for me are those left by wood-boring beetles.

Often nicknamed “bark beetles,” they belonging to an evolutionarily related group represented by thousands of species, Scotylinae. Bark-beetle traces consist of shallow tunnels made just below the bark surface of a tree, sometimes back-filled with chewed woody material, called frass. Their pathways can be smooth, simple, meandering tunnels, but also commonly branch into complicated patterns. My favorites of these are the galleries made by beetle larvae that, upon hatching from their eggs, drill outward from a central tunnel, rendering a centipede-like form. Incidentally, this behavior has also been around for a very long time: I’ve seen trace fossils in Late Triassic petrified trees – from nearly 200 million years ago – nearly identical to those made by modern beetles.

On the Georgia coast and elsewhere in the southeastern U.S., the most likely makers of these tunnels are Southern pine beetles (Dendroctonus frontalis). However, invasive species of beetles are also leaving their marks on native trees, such as the red bay ambrosia beetle (Xyleborus glabratus), which is quite rightly blamed for the decline of an important plant species on the Georgia coast, red bay (Persea borbonia). The overall trace this beetle leaves is a dying red bay, its once green and aromatic leaves reduced to brown, odorless husks.

These insect traces serve as a dual reminder during our walks through the maritime forests of the Georgia barrier islands. One is that none of these trees we label as “dead” actually fulfill such a glib description: in fact, they are teeming with thousands of insect lives. A second note to make is that the larger, deeper holes preserved in the tree – alongside and intersecting the insect borings – tell of their ecological and co-evolutionary connection with living dinosaurs, the insectivorous birds that specialize in pounding wood. When did such dinosaur-on-insect-in-wood behavior first start? As far as I know, we don’t know. Nevertheless, I have every confidence that trace fossils will eventually guide us to a much more satisfying answer than our current collective shrug.

Further Reading

Scolytinae – Bark and Ambrosia Beetles: BugGuide

Bark Beetles: University of California (Davis) Integrated Pest Management

Bark and Wood-Boring Beetles of the World: Barkbeetles.org

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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