Salt Marsh Part 3: Mr. and Mrs. Crab

(Read part 1 here and part 2 here.)

Walk through the marsh on a cool day, look closely at the surface of the salt pan, and you’ll notice that it’s riddled with small round holes.

These are the burrows of fiddler crabs (Uca sp.).  The little balls of sand surrounding them have significance as well – they’re the results of the crabs’ excavation efforts.  In some places you can see two distinct size classes of sand balls, big and little.  My understanding is that the big ones are from crabs digging their burrows, and the little ones are from their feeding activity.  They get a mouthful of sand, swish it around to extract the tasty microbes, detritus, etc., and spit the sand back out as a compact ball.

Come back on a warmer day and you’ll find the marsh crawling with the crabs themselves.  If you move fast, it’s fairly easy to pin them and pick them up.

This is a female – you can tell by her two tiny claws.  She’s on the small side, they get bigger than this.  Actually there are several different species of fiddler crab of different sizes and colors found in our marsh, but I’m not an expert at distinguishing them from each other.

This is a male.  Full disclosure: he’s dead.  On the day I took my camera with me to the marsh, I had a surprisingly hard time finding and catching a decent-sized live male, so when I found this dead one at the edge of a salt pan I picked him up and snapped some photos of him instead.  Obviously the big difference between him and the female is his one big claw, the main purpose of which is communication.  The males wave their big claws back and forth as a display to attract females (you should hear fifth graders giggle when I wave my arm back and forth in the air and tell them it’s fiddler crab for “heeyyyyy ladies!”).  If the big claw is broken off, the remaining little claw grows into a big one and a little one grows back in place of the big one that was lost.  Mammals are pretty much the only group of animals without any significant ability to regrow lost limbs.  Hardly seems fair, does it?

Hope you’ve enjoyed my guided tour of the salt marsh and its little critters.  It’s also full of ibises, storks, spoonbills, herons, and egrets, of course, but birds are a lot harder to photograph with my point-and-shoot camera.  One of these days I need to learn how to use an SLR…


A Ghost (Crab) Story

Halloween is tomorrow, so I thought this would be a good time to do a post on ghost crabs.  Ghost crabs!  Halloween!  Get it?  Get it?

On the beach here, up near the base of the dunes, one can find many small (up to a few inches across) round holes in the sand, entrances to burrows leading down out of sight.  The tracks that surround them make it clear that these are not the homes of mammals, nor (as my students sometimes guess) are they “snake holes.”

These are actually the burrows of ghost crabs, Ocypode quadrata.  They get their name from their pale coloration, ability vanish almost instantly into their holes (“Ocypode” is Greek for “swift foot”), and nocturnal habits.  Younger ones dig their burrows closer to the waves, while older ones venture farther back into the dunes.  They’re almost completely terrestrial, only going into the water to moisten their gills occasionally and lay their eggs – their larvae are planktonic, drifting in the ocean until they’re ready to return to land as adults.

Despite their reputation as being nocturnal, we sometimes see these guys abroad during the day, and if you’re very lucky you can get between one and its burrow and get a close look at it before it disappears.

Look how cool!  Those big black eyes swivel to give it excellent, three-hundred-sixty-degree vision.  They’re very impressive-looking, and very different in structure from crabs that spend most of their time in the water.  While this guy has four pairs of legs well-suited for walking on sand, the rear pair of legs in ocean-going crabs, such as blue or speckled crabs, is modified into a pair of round flippers for swimming.

Like most crabs, these guys will eat just about anything they can find, and that includes newly-hatched sea turtles.  Apparently when the sea turtle patrol folks here on the island are out checking nests they try to run over as many ghost crabs as they can with their golf cart.

Anyway, happy Halloween!  Check back tomorrow for photos of the costume I wore to teach in Friday morning.  I was a marine invertebrate, but not a crab…


International Rock Flipping Day

Today is International Rock Flipping Day 2010!  I heard about this celebration of things that live under rocks a couple weeks ago and really wanted to participate, but there was just one problem: I live on an island made entirely of sand.  Where was I going to find a rock to flip?

Then I remembered: there is one place on the island that’s rocky, although it’s not that way naturally.  The beaches on the north end of the island are eroding, thanks to the longshore current, and the powers that be have seen fit to deposit rip rap on them in an attempt to stop this process.  “Rip rap” is a technical term for “chunks of rock.”  Perfect (although I won’t go into whether rip rap actually works, or whether beach armoring is wise)!

The first few rocks I flipped yielded only wet sand.

However, I persevered, switching to an area where the rocks were sitting in about an inch of standing water, a sort of minimalist tide pool.  There I started having better luck, turning up this small crab, species unknown…

…and a couple tiny brittle stars, flexible-limbed cousins of the sea stars.

At this point thunder started to rumble in the distance, and since I wasn’t dressed for rain and didn’t particularly want to be caught on the beach in a thunderstorm regardless I returned to my car.  See you next year for International Rock Flipping Day 2011!