Photo: Red Knots feeding on Horseshoe Crabs, Greg Breese/USFWS
Tom Blackburn
What has nine eyes, light-sensitive organs in its tail, lays up to 120,000 eggs a year, and is an important food source for shorebirds? Answer: the horseshoe crab. The horseshoe crab is not actually a crab but is most closely related to spiders and scorpions. It is a perfect example of complex inter-species relationships and the impact of climate change and other human-caused impacts on bird populations.
Red Knots feeding on Horseshoe Crabs, Greg Breese/USFWS
Horseshoe crabs come ashore by the millions in the spring, and particularly during the full moon high tides, to lay and fertilize their eggs. However, they do not spawn until water temperatures reach 55 degrees. While climate change is resulting in higher average temperatures, it also causes wider short-term variations in temperature. This spring’s unusual cold spells kept ocean temperatures below average, leading to concern that the water would not be warm enough to support spawning during the May full moon. Fortunately, water temperatures warmed up and the horseshoe crabs came ashore to lay their eggs on schedule.
The pinhead-sized eggs take about two weeks to hatch. During that time, huge flocks of shorebirds and gulls converge on the beaches to feast on them; on a trip to Cape May, New Jersey last week, I was astonished by the deafening cries of tens of thousands of Laughing Gulls gathered on the beaches of the Delaware Bay to eat horseshoe crab eggs. In particular, the eggs are a critical part of the diet of endangered Red Knots. They time their 9.000-mile migration from the southern tip of South America to nesting grounds above the Arctic Circle to take advantage of the protein-rich eggs to fuel their trip northward.
Horseshoe crab populations have undergone steep declines in the past fifty years, leading to concern that their reduced numbers are affecting the populations of Red Knots and other shorebirds that depend on them. Hundreds of thousands of horseshoe crabs are harvested yearly for bait. The medical industry uses the blood of horseshoe crabs to test for toxic bacteria in vaccines. Thankfully, several states have banned the use of horseshoe crabs as bait, and the medical industry has improved the ways it takes the blood of horseshoe crabs, leading to fewer deaths from the process. Eighteen months ago, ASNV joined other Audubon Society chapters in urging the Food and Drug Administration to recognize a synthetic alternative to horseshoe crab blood for medical purposes. These efforts have helped to stem the loss of horseshoe crab populations. It remains to be seen whether they will be sufficient to protect Red Knots and other shorebirds from further declines. But until then, ASNV will continue advocating for causes that will allow birds to flourish.