red knot birds
Several years ago (2010) I wrote an article entitled “The counter-intuitive interconnectedness of species” in which I explained how the red knot (Claidris canutus rufa) bird makes a remarkable, annual journey from Tierra del Fuego (off the southern tip of South America) to the Canadian Arctic where they nest over the short arctic summer and then return home again in the fall. On the way to its arctic stop, the red knot stops along the East coast to feed on eggs laid by the horseshoe crab (Limulus polyphemus), an ancient, primitive-looking crab, that is indigenous to the shorelines of the East coast. The horseshoe crab has been around for 475 million years, so they have sticking power, despite their seemingly clumsy ways and odd shape. The timing of the red knot arrival coincides with the breeding season of the horseshoe crab who come close to the shore to lay their eggs. The red knot birds have about two weeks to feed on the eggs and build up enough body fat and strength to complete their journey. But the horseshoe crabs are becoming scarce. They not only serve as bait for fisherman in the region, but their blood is used in medicine, as the Limulus amebocyte lysate (LAL) is used as a test to detect bacterial endotoxins, for which it is a highly sensitive, unique detection system. Although medical blood-letting is associated with release of the Limulus, a considerable number of the animals die from this experience. A NYT article on the plight of the red knot bird points out that the population of these birds has plummeted by 70 percent since the 1980s. The United States Fish and Wildlife service has proposed to designate the bird as threatened. If the red knot receives this distinction (we apparently will know by Friday), then the government will develop a plan for the bird’s recovery. If so, this will surely involve additional protections for the horseshoe crab, whose diminished numbers have no doubt contributed to the bird’s decline. If use of horseshoe crabs as bait declines through enforcing new limits, the medical use of the crab’s blood is very likely going to increase, given its importance.
- Horseshoe Crab (Limulus polyphemus)
One area where survival improvements could be made is to increase the likelihood that blood letting will be more compatible with crab survival. Some estimates suggest that as many as 20 to 30 percent of the crabs from which blood has been withdrawn do not survive. Since 2004, the demand for horseshoe crab blood has increased by 85 percent.
Given what seems to be a biological event of unique synchronization, biologists worry that global warming mechanisms that may interfere with this dependency (an early spring, such that the birds leave too early, or crabs breed out of sync, the threats of ocean acidity from absorbing carbon dioxide on the life cycle of the crab, changes in the arctic that could effect the breeding grounds of the red knot). One could go on and on with other possibilities. How this interdependency between the horseshoe crab and the red knot got started is itself a remarkable, but unknown story. If crab shortages continue, will the the red knot be able to find alternative sources of food? Given the huge drop in the red knot population that answer appears to be no.
Another animal impacted by the drop in the horseshoe crab population is the Atlantic loggerhead sea turtle that feed on Limulus: their numbers have been dropping. Harvesting crabs was banned in New Jersey in 2008.
loggerhead sea turtle (from Wikipedia)
The horseshoe crab has played a major role in our understanding of visual physiology; studies of this animal led to one Nobel Prize (1967, awarded to H.K. Hartline, Ragnar Granit and George Wald; Hartline was the Limulus guy).
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