Exploring Low Tide

As you might know, I went on a road trip over my Winter Break. Much of it was spent exploring the beautiful South Carolina Lowcountry. I spent 4 days of my time in the Lowcountry camping on Hunting Island State Park on the southern tip of South Carolina.

Driftwood on the beach at Hunting Island.

This isn't the first time I've been to Hunting Island State Park; the ornithology class I was taking last semester stayed here while on a birding trip (Link). It's an absolutely amazing place that I wanted to revisit, so I decided to come back on my break. I spent most of my time birding around the area, but whenever I wasn't birding, I was exploring the island's beach. First, some background on the setting. Hunting Island is a subtropical barrier island in the Atlantic Ocean. The natural beach is approximately 4 miles long. As with elsewhere, Hunting Island experiences 3 types of tidal cycles: daily, monthly, and annual. These are all caused by different types of interactions between the Earth, Moon, and Sun. The most obvious tidal cycle is the daily cycle. This cycle is caused by the interplay between the Earth's rotation and the Moon's gravity. This results in 2 daily high tides and 2 subsequent low tides. The tides can make the beach on Hunting Island go from only 10 or 20 yards wide during high tide to 100+ yards wide during low tide. I opted to explore during low tide, as many "goodies" are visible once the intertidal zone is exposed.

Until this trip, I've never had the chance to really explore a beach. I honestly didn't know what kinds of things to expect, especially in South Carolina. I was absolutely flabbergasted to see Sea Stars. One part of the beach during low tide had some rocky tide pools, and these tide pools were teeming with dozens of sea stars and various other life. All the sea stars were one species, the Forbes Sea Star (Asterias forbesi). The Forbes Sea Star can be found in Atlantic coast intertidal zones ranging from Maine down to the Gulf of Mexico. It has a characteristic orange madreporite. The madreporite (the round orange thing near the center) is a specialized opening which all sea stars have. The madreporite is essentially a valve which allows sea water into the specialized water vascular system which run throughout the sea star's body. Sea stars actually use sea water essentially as a means of locomotion in a rather complex process. You can read more about this process and the madreporite at The Echinoblog.

The Forbes Sea Star is a molluscivore, meaning it feeds on mollusks. Specifically, the Forbes Sea Star feeds on bivalve mollusks, which are mollusks that are enclosed by two shell parts which are attached by a hinge. Bivalves include commonly-known mollusks like clams, oysters, mussels, etc. The Forbes Sea Star tracks down bivalves by honing in on the chemical "smell" of bivalves. Once it senses a bivalve, the sea star will crawl toward it. Upon reaching it, the sea star will then grab the bivalve with its tube feet (which you can see in the photo above). The sea star then uses its tube feet to pry the bivalve's shell open, exposing the helpless mollusk inside. This is when things get a little strange. Instead of "swallowing" the squishy mollusk body like you might think it would, the Forbes Sea Star does things a bit differently. It pushes its stomach out of its body and up against the body of the mollusk where it then excretes digestive enzymes that begin to break the mollusk down. The stomach then absorbs the resulting nutrients before getting pulled back into the sea star's body. Pretty crazy, isn't it?

Another common sight in the tide pools I explored was the Warty Sea Anemone, Bunodosoma cavernata. Sea anemones are actually animals in the Order Actiniaria. The sea anemones are Cnidarians (Phylum Cnidaria), and as such they are related to jellyfish, corals, tube-anemones, and the like. The Warty Sea Anemone is a more southerly species in the US, ranging from North Carolina down throughout the Caribbean. The venom of the Warty Sea Anemone isn't toxic to humans like some other sea anemone venom can be, and so they pose essentially no threat to the beach goer. If you decide to gently poke one, you can witness their reactionary defensive behavior which consists of quickly pulling its tentacles in to a tight ball.

Sand Dollars, with a live individual on the left, and a dead individual on the right.

As I moved away from the tide pools and walked along the beach, I often came across individuals of a species of Sand Dollar called Mellita isometra. Common names for this species include the Keyhole Sand Dollar and the Five-Holed Sand Dollar. Sand Dollars are actually a type of sea urchin that have a flattened body which helps them easily burrow under sand. Mellita isometra is a species which can be found in soft intertidal zones to shallow depths of ~100 meters from Massachusetts down to Florida and the Bahamas. Most of the Sand Dollars that people find along the beach are actually the "skeletons" of dead individuals, called "tests." To see the difference between a live Sand Dollar and a dead one, look at the photo above. The Sand Dollar on the left is alive; notice its purple coloration and fuzzy appearance. On the right you have the test of a dead Sand Dollar. Notice how it's smooth and a pale creamy color; it's much different looking than a live individual!

Of course, one of the most common sights on the beach are various mollusk shells. There are, however, few shells in South Carolina as large and eye-catching as the Knobbed Whelk, Busycon carica. I was wading through a shallow channel on the north end of Hunting Island when I came across my lifer Knobbed Whelk. The large shell sitting in the sand easily grabbed my attention. Amazingly, this is a small Knobbed Whelk. A full grown adult will be between 5 and 12 inches long, while this one was only about 4 inches.

So what exactly is a whelk? A whelk is essentially just a type of sea snail. More specifically, they are sea snails in the family Buccinidae (the true whelks), but some sea snails in other families have "whelk" as part of their common name (which is one of the many issues with common names). The Knobbed Whelk can be found in near-shore environments ranging from the intertidal zone to 50 or so meters deep. Their range extends from Massachusetts to northern Florida, and in South Carolina they are the most common of the whelk species present (there are 4 total in South Carolina). They are a predatory snail that feeds on bivalves such as oysters and clams.

As with all creatures, Knobbed Whelks eventually die. After their soft body decomposes, a nice empty shell is left over. That shell makes for a great hermit crab home. In fact, the hermit crab in the photo above is using a small Knobbed Whelk shell as its temporary home. Hermit crabs are a type of crustacean, but they aren't actually true crabs. Hermit crabs are kind of their own thing. Most crustaceans have a completely hard body due to it being nearly all calcified, but the hermit crabs have a non-calcified, and subsequently soft, abdomen. This soft abdomen is a very big weak spot for the hermits, and they have come up with an easy fix: crawl into an empty shell! Most often a hermit crab will choose a snail shell of some sort, like the individual above. By using this shell as a mobile protected "home," the hermit crab can better prevent becoming an easy meal for a hungry predator.

There are several species of hermit crabs in South Carolina, and I have essentially no knowledge when it comes to identifying crustaceans. I think this is a Long-Clawed Hermit Crab, Pagurus longicarpus, however I could be totally wrong. Another common species is the Thinstripe Hermit Crab, but this individual lacks the characteristic pinstripes. Another possibility is the Broad-Clawed Hermit Crab, but its claw doesn't look nearly large enough. If anyone can help me positively ID this species, please leave a comment!

Of course, if I'm going to talk about hermit crabs, I should probably talk about the true crabs! This guy was a surprise find. I was in one of the tide pools looking around when I saw something just barely sticking out of the sand. At that point in time, all I could see was the outline of a round object. I thought it might be some sort of bivalve, so I put my hand under it and attempted to pull it out of the sand for a closer look. Suddenly, the object erupted in a flurry of movement, stirring up the sand and clouding the water. I jumped, and then felt a pinch on my hand. Oops, it was a crab. The sand settled to reveal the scene above - a now-obvious blue crab sitting half buried in the sand. My colleague Alayna Tokash identified this as the Atlantic Blue Crab, Callinectes sapidus, which is often just referred to as the "Blue Crab" (although there are several species of blue crabs in the Atlantic).


The Atlantic Blue Crab is a commonly consumed crab, and is also a very economically important species for the Atlantic Coast. However, there's some trouble. The Atlantic Blue Crab population has been declining for some time. It is estimated that the population declined from 900 million individuals to approximately 300 million individuals from the mid-1990's to 2004. In the Chesapeake Bay region, where the Atlantic Blue Crab is most heavily harvested, the population is estimated to have decreased by nearly 70% over the past few decades. Overharvesting is, as you might have guessed, a major problem, but several other factors, including diseases and habitat degradation, are to blame as well. These declining trends suggest an upcoming battle between conservationists advocating for the protection of the Atlantic Blue Crab and the crabbers who rely on this species to make a living. Hopefully it doesn't get to that point, but this is a scenario that is played out much too often nowadays...

I know this is a very different subject matter than what I normally get the chance to write about, so hopefully you enjoyed it! Thanks for reading!