Birds

Red knots and horseshoe crabs: ancient connections, modern peril

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Red knots and horseshoe crabs have a symbiotic relationship. Photo from U.S. Fish and Wildlife Service

By D. Creeksong

One flies to the ends of the earth each year; the other, hatched from an egg barely larger than a grain of sand, never willingly travels outside the waters where it was born.

One is almost totally dependent on the other for survival.  Both are disappearing at an alarming rate.
Yet it wasn’t until an obvious plummet occurred within the Red Knot population that their migratory path–much less their reliance on Delaware Bay horseshoe crab eggs for refueling along the way–was even discovered. 
Calidris canuta rufa, one of six Red Knot sub-species, is a member of the sandpiper family. Known as the “Robin Snipe” in the 1800s, it is slightly larger than a robin. Its pale reddish breast occurs only during the May and June breeding season. Afterwards, a whitish breast returns.

“Rufas,” as they are known, can be distinguished from other sandpipers with these tells: greenish legs with a very short bill, perhaps half the length of other sandpipers with greenish legs.

Although a small number winter in other locations, including Ocracoke, most rufas begin their journey from Tierra del Fuego at the southernmost tip of South America.

Wintering Red knots on Ocracoke. Photo: P. Vankevich

Red knots on Ocracoke. Photo: P. Vankevich

There they have fattened up on whole mussels, eating constantly for three months. This gorging is necessary for bulking up and producing the new flight feathers needed for winging 9,700 miles northward to nest at the Arctic Circle.   Weather permitting, they are known to fly non-stop for as many as six to eight days with no food, water or sleep. Much of this flight takes place over the ocean.

By the time they complete the first leg of their journey, alighting along the Delaware Bay, they are literally famished. All fatty tissue is gone, their muscles now an energy source. Because having their chicks hatched the moment insects begin buzzing in the short Arctic summers is essential, there is no time to waste. They must double their weight within a few days, and only one food will do.

Enter the lowly horseshoe crab. For more than 420 million years, Limulus polyphemus has foraged the ocean floor, chancing seashore appearances only to reproduce. Arrival at shoreline to spawn is in perfect synchronicity with the Red Knots’ stopover to feed. At this point in their journey, the birds are so emaciated that they cannot digest the whole mussels and whole clams they would normally eat.

Horseshoe crabs spawning.

Horseshoe crab harvest for fertilizer production, 1928. Courtesy of the Delaware Public Archives

But like the Red Knots, whose numbers at this stopover have dropped precipitously from 150,000 down to 26,000 the horseshoe crabs have also declined substantially. The resulting decrease in egg production is making it impossible for all Red Knots to put on the weight necessary to complete their journeys.
The answer is not an easy one, as the already exhausted birds cannot just pick up and fly off elsewhere.

This bay shelters the largest horse shoe crab spawning ground in the world. Other spawning areas are not only far smaller, but can be picked clean of these crabs by the fishing industry to use as bait.

Another contributor to the loss of horseshoe crabs is the pharmaceutical industry, which, while it returns the crabs after removing one third of their blood to create a substance invaluable for detecting bacteria, inadvertently kills anywhere from 18 to 30 percent of them. The ability of returned horseshoe crabs to spawn within that same season is also in serious question.

Although many groups are working toward resolutions, much larger issues loom. Rapid acidification of our oceans, rising tide lines, warmer air and water–all brought on by global warming–are wreaking havoc with the Red Knots’ entire journey.

Acidification has decreased by 75 percent the size and weight of the small clams the birds rely on to feed.  A warmer climate is causing insects to hatch earlier in the Arctic.

Since Red Knots are genetically predisposed to migrate according to day length, not temperature change, they cannot arrive any sooner or leave any later. 

Many Red Knot chicks cannot get enough to eat before it is time to run south, so mortality is rising.

In addition, beach development and encroaching tide lines are decreasing shoreline size and accessibility, threatening numerous species that rely upon this habitat.

Those living along the path of Hurricane Matthew in early October got a good taste of what it feels like to watch rising waters threaten our homes, families and lives.

Is it possible for us to pull all of our collective heads out of the sand, stop squabbling and come up with an answer in time? It is not just the Red Knots and the horseshoe crabs that depend on it.

Here’s how to help. Visit http://celebratedebay.org/take-action/

Red Knot in breeding plumage on Ocracoke. Photo by P. Vankevich

Red Knot in breeding plumage on Ocracoke. Photo by P. Vankevich

Horseshoe crabs molt and the shell, called carapace, is a common sight on Ocracoke. Photo by P. Vankevich

Horseshoe crabs molt and the shell, called carapace, is a common sight on Ocracoke. Photo by P. Vankevich

9 replies »

    • Thank-you! It was a tough piece to write–both from the need for accuracy as well as the immediacy of the emotions it stirred.

  1. Note from the author: One thing I couldn’t fit into this article was an important new research discovery re: another of the Red Knot sub-species ‘canutus’. Scientists noted that the birds were now smaller in skeleton and with shorter beaks.

    At first they assumed that this was a marvelous adaptation to climate change.

    Unfortunately, further research showed that the juveniles were now unable to dig down far enough to reach the clams that are a mainstay of their diet, and the population of this sub-species has reduced by 50%.

    If you’re interested, this audubon article will take you deeper:

    http://www.audubon.org/news/the-arctic-shrinking-and-so-are-its-birds

  2. Fabulous writing – sad tale. Hopefully enough of us have learned from the past that we can change the outcome for these animals before it is too late.

    • Thanks for your comment, Rosalyn. Definitely a sad tale. Folks are starting to pay attention, though. The “Just Flip ‘Em” campaign–which teaches folks to tenderly turn over horseshoe crabs when found stuck upside down–counted 78,000 saved horseshoe crabs last year! Hopefully, this will next spur folks into helping with our global warming issues.

  3. This is a wonderfully written article. The author did a superb job of intertwining the stories of two species. The writing really draws the reader in. Hope to see more from her.

    • Thank-you for your praise, Jacqueline! And that’s an interesting idea about Huffington Post. I’ll check with my editors. Yes, continuing to spread stories such as this is crucial!