Posts Tagged ‘horseshoe crab’


Well, I had a treat last week! I went to watch horseshoe crabs being tagged for research — kind of like birdbanding, but for crabs. The woman with the funny expression above was actually enjoying the whole thing and helping to take notes for the scientist, Kim Gaffett.

Kim, who may be best known to New Shoreham visitors for birdbanding, has been working for some years with Connecticut’s Sacred Heart University on an initiative called the Limulus Project. The idea is to learn more about the amazing horseshoe crab, a species that, depending on whom you ask, has managed to survive between two and five mass extinctions on Planet Earth, including the one that wiped out the dinosaurs.

In spite of their amazing record of survival, the crabs are considered threatened today, so it’s important to study them and try to find out what’s going on. As I wrote a few years ago, their blood has the ability to clot in the presence of bacteria, so it has become invaluable to pharmaceutical companies. Researchers are supposed to draw the blood as one would for a human and then have the fishermen return the crabs to the ocean, but that may not be happening consistently.

I asked Kim how she knew she would find any crabs that particular morning, and she told me that when there is a high tide and a full moon in June (flood tide), the horseshoe crabs come up to the shore to mate. The male has one claw like a boxing glove, with which he attaches to the female’s shell in order to be available when she drops her eggs. The eggs are fertilized outside the body. Sometimes other males are hanging around, and it’s possible for one batch of eggs to get fertilized by more than one male.

All sorts of marine life forms attach themselves to horseshoe crabs — seaweed, barnacles, slipper shells. Kim calls the crabs “their own ecosystem.” The crabs’ fellow travelers don’t usually cause any trouble, but as you can see below, a quahog had snapped onto a claw of one crab. A citizen scientist is shown detaching it.

From Phys.org, I read this about horseshoe crabs’ survival: “They have a special kind of blood, which … coagulates when it encounters bacteria. They can ‘wall up’ any wounds they receive.

“Another key to their survival seems to be their tolerance of habitats that fluctuate in salinity (levels of salt). When environmental changes happen, they can move to safety.

“An ability to live with low levels of oxygen is also important. [Natural History Museum expert Richard Fortey] adds, ‘The horseshoe crab was able to cope with periods of oceanic deoxygenation that were fatal to many marine organisms.’ ”

I was also interested in what Quartz had to say: “These amazing crab species are among the handful of species referred to as ‘living fossils’ because their current form resembles those found in the fossil record. Externally at least, the crab hasn’t changed much in nearly 450 million years. In that time, it has survived all five of Earth’s great mass extinctions, the worst of which killed off an estimated 95% of all marine species, and the most recent of which did away with the dinosaurs. …

“The crab’s blue blood contains a chemical called limulus amebocyte lysate (LAL), which thickens when it comes in contact with toxins produced by bacteria that can cause life-threatening conditions in humans. Labs use LAL to test equipment, implants, and other devices for these toxins.”

Kim says she hopes labs will start using the synthetic version of LAL more and give the horseshoe crab a break.

Note the tag below. Kim found one crab with a tag from a previous year, and she attached another tag to a newer find.




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The humble horseshoe crab is a reminder of prehistoric times. Public Radio International’s Living on Earth recently devoted a segment to this curious character.

From the transcript of the show …

Steve Curwood: “For healthy oceans, it’s not enough to protect just the top of the food chain – the cod or halibut or swordfish we eat. The bottom of the food chain is vital too. That could be the plankton or the tiny forage fish eaten by many species – or it could be the extraordinary prehistoric-looking horseshoe crab.

“These helmet-shaped arthropods have been around for millions of years, and up and down the east coast of the US, volunteers come out to count them as the females come ashore to spawn. On Cape Cod, as Karen Zusi reports, scientists and volunteers are tagging and labeling the crabs to help conserve them.”

Karen Zusi: “There are a lot of reasons why someone might appreciate the lowly horseshoe crab. Eel and conch fishermen use them as bait, and medical companies draw blood from the animals. Horseshoe crab blood will clot in the presence of bacteria, so these companies can use the crab’s blood to make sure vaccines and medical implants are free of germs. Their blood is worth sixty thousand dollars a gallon.

“But horseshoe crab populations are dropping. To preserve them, scientists and volunteers on Cape Cod are wading into the water to count and tag the animals.

“Special labels help them keep track [says] Mark Faherty, the science coordinator at Audubon’s Wellfleet Bay Sanctuary. …

“The Massachusetts Audubon Society just recruited graduate student Michael Long to lead their newest horseshoe crab study. With researchers from the University of Massachusetts, he will be tagging the crabs this summer with a telemetry [label], glued onto the crab’s shell.”

Faherty: “My acoustic study is going to be putting on acoustic receivers out in the bay, and acoustic markers on the crabs. The receivers have about a 600-meter detection radius so anytime a crab that’s marked with an acoustic receiver comes within 600 meters of that receiver, it will mark where it is. So based on where each crab pings, you can kind of track its movements around the bay.”

Zusi: “None of this would be possible without the Audubon Society’s volunteers. They come from all walks of life.

“At an Audubon horseshoe crab conference, Long organizes new volunteers to help him count horseshoe crabs on the beach, and Faherty trains them in the basic survey procedures. …

“Once they got down to business, the volunteers were trained to divide the beach into small sections, count the horseshoe crabs, and record all of their information. The volunteers go out to survey when female crabs are coming to lay their eggs in the sand. Males follow to fertilize the eggs after they’re laid.”

Faherty: “The male crabs you quickly learn to recognize because they’re by themselves. They will mate with a model, if you make a model of a horseshoe crab — the males will congregate around it. They’ll spawn. They’ll spawn with your boot. These are just hormonally-charged animals that are ready to mate with anything. Females are not lonely for long in the horseshoe crab world.”

More here on the effort to study and protect horseshoe crabs.

Photo: Peter Massas, Flickr CC BY-SA 2.0
A horseshoe crab floats by the shore on Union Beach in New Jersey. The species is listed as Near Threatened by the International Union for the Conservation of Nature.

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