Holes, Bumps and Trails in the Sand
by Dr. Joe Richardson – Tybee Beach Ecology Trips
When you are walking along Tybee’s beach, you are actually traversing across a wide, dynamic habitat for a number of animals that mostly go unnoticed. The marine biological term for animals that live “in” the bottom is in fauna. Some infauna species excavate holes and live fairly sedentary lives inside their holes and chambers. Others are mobile and dig or burrow through the sand. By paying attention to a little detail, you can learn to spot subtleties in the wet sand and find some of these animals that may be just a few inches below your feet.
As you move from the boardwalk across the dunes and onto the dry loose sand heading toward the high tide line, you might see some obvious holes in the dry sand. They can be half-an-inch to a couple of inches in diameter. These holes and burrows are the protective tunnels of Ghost Crabs. Look closely and you can often see the scratchy foot-prints and excavated sand deposits left by the crab when it was digging. Ghost Crabs are practically a land animal, and they only occasionally need to run down to the water’s edge and take a quick dip to get their gills wet. They spend most of the day time down in their burrows, protected from drying out; but during the evening, they come out and scavenge along the beach and high tide line for food. Ghost Crabs are about the same color as the dry sand, and they have strong claws, fast legs, and eyes up on stalks. So as they move quickly in the dim light, it really is like you think you are seeing a small ghost darting around on the beach.
As you move across the wet intertidal sand toward the water line, you will probably start seeing holes in the sand. Most look like somebody poked a pencil down into the sand. These holes are the opening to long cylindrical open channels created by a couple of different kinds of animals. These animals make a lot of sticky slime, so as they dig and move water up and down the channels, the sand along the inner lining sticks together and doesn’t cave in. So the deep holes stay open throughout the sand. Many of the holes belong to Polychaete Worms. These segmented worms have short, thin paddle-like bristles sticking out from each segment. Polychaete means “many bristles,” and the worms use the bristles for digging and crawling. Most of the worms in these holes are very thin and look more like a red string that may be a few inches to a foot or more long. Usually they are 2 to 3 feet deep below the sand surface. When the tide comes in and covers the hole opening, the worm pulls seawater down through the open channel and eats plankton from the seawater.
You might see that some holes have a ring or pile of tiny, dark-brown cylindrical “sprinkles” around them. These holes are the openings for the burrows excavated and inhabited by Ghost Shrimp. Ghost Shrimp are very odd-looking animals with claws and legs at the front, but soft, mushy bodies. With such soft bodies, they have practically no defense against drying out or predators, so Ghost Shrimp stay deep (4-5 feet) down inside their branched tunnel-like burrows. They also pull water down to feed on seawater plankton. When the tide is back out and there is no water to pull down, the Ghost Shrimp uses it’s paddle-like legs to push water back up the hole and cleans out its burrow. So those “sprinkles” you see around the hole are the small fecal pellets that have been flushed out by the Ghost Shrimp below.
If you happen to see 2, 3, 4, or 5 small holes close together, you might want to very carefully scrape away some sand around the holes. Be gentle, because it might be a Sand Dollar buried just below the surface. Sand Dollars are actually a type of very flat sea urchin with very short spines. As a Sand Dollar plows through the sand, it uses those “key-holes” to move water and sand through, to assist in moving below the sand.
Sometimes as you walk along the intertidal sand you might see tracks or trails that look like somebody dragged a stick, or their finger, across the surface of the sand. Check to see if there is a “bump” at one end of the plow-line and you might discover a Lettered Olive Snail in the bump. Lettered Olives have beautiful, polished, cylindrical shells with markings that look like letters, but no two of them have the same pattern of markings. These snails are plowing around through the sand looking for other animals to smother and eat. The Lettered Olive Snail shell is South Carolina’s state seashell.
Another snails burrows through the sand, but it is usually a bit deeper so its trail is much more subtle and a little harder to distinguish at first. But after seeing a couple of their trails and bumps, you will be able to spot where a Moon Snail is digging and hunting for other mollusks to drill into and eat. Its trail is often a very subtle furrow on the sand surface, often slightly curved, with a broad, slight bump at one end. It reminds me of a comma on the sand surface. Under the bump, the Moon Snail might be an inch or more below. If you have a bucket or something you can put some seawater in, place the Moon Snail in it and give it a few minutes. Hopefully the snail will come out and start crawling around and you will be amazed at how big its body is compared to the size of its shell.
I will sometimes get fooled by smaller, thinner furrows in the sand (like you can make with your fingernail dragging lightly across the sand) thinking I’m going to find a small Moon Snail. Instead, at one end or the other of the trail, I’ll pull up a small clam from just below the surface. These small, multi-colored clams are Coquinas. Coquinas usually live in the swash zone where the water washes back and forth after each wave, but sometimes they don’t follow the tide going out and so they burrow into the sand and have to wait till the tide comes back in.
Occasionally you might see a low “D-shaped” hump on the sand, and often there will be a piece of a shell sticking up out of the sand at the narrow end of the “D.” Do a little digging with your finger around that hump and you might find it to be a rather large Whelk that has buried into the sand waiting for the tide to come back up. We find Knobbed, Lightning, Channel, and Pear or Fig Whelks on Tybee’s beach. The Knobbed Whelk is Georgia’s state seashell.
So as you walk along the wet sand on Tybee, keep an eye out for holes, bumps and humps. And don’t be afraid to do a little careful digging with your fingers to see what might be under there. Tybee protects its live beach animals, so after a little observation and some pictures, just place it back where you discovered it. Oh, and don’t forget to look out at the water every now and then. There might be some dolphins out there, and you don’t want to miss them!
“Dr. Joe” Richardson is a retired marine science professor who has studied, researched and taught marine biology/science along Georgia’s coast and Bahamas for more than 35 years. Dr. Joe conducts Tybee Beach Ecology Trips (www.TybeeBeachEcology.com) year-round for families, schools, scouts, and other groups. He regularly reports his findings on his “Tybee Beach Ecology Trips” facebook page. If you find something you don’t recognize, get a picture and post it on his page and ask “what is this?” And for more information and pictures about Tybee’s marine animals, biogeography and seasonality, see his webpage.