Blue Blood and Shorebirds: Choosing a Plan for Horseshoe Crabs

A Test That Starts on the Beach

1Seventh grader Noor Al-Hashimi got a shot at her checkup last fall, and her doctor mentioned that every batch of that medicine had been tested for endotoxins. Endotoxins are molecules from the outer walls of certain bacteria. Even tiny amounts in a shot, an IV bag or an implant can cause a dangerous fever. Noor's mother runs the testing lab at Lakeside Regional Hospital, so Noor asked to see how the test works.

2The most common test uses cells taken from the blue blood of horseshoe crabs. Each year, collectors gather crabs from the beaches of a bay that this worksheet calls Gull Bay, a lab removes about a third of each crab's blood, and the crabs are returned to the water. Some bled crabs die after they are released. A newer test called rFC uses a copy of the crab's detecting protein, but the copy is made by cells grown in tanks, so no crabs are needed. Before trusting it, Noor's mother ran a side-by-side check: she tested 40 samples with both tests. Twenty samples had endotoxin added and twenty were clean. Both tests flagged all 20 samples with endotoxin and passed all 20 clean samples.

3Horseshoe crabs do more than help hospitals. In late spring they crawl onto the bay's sandy beaches to lay huge numbers of small green eggs. Red knots and other shorebirds stop at the bay on their long flight north and eat huge numbers of eggs to refuel. Visitors also come to watch the birds, which brings money to nearby towns. Benefits that people get from healthy ecosystems, like these, are called ecosystem services. Biologists count spawning crabs, eggs in the sand and red knots each May (Table 2).

4To learn why some bled crabs die, biologists ran a test with 300 crabs from the same beach, put into three groups of 100. One group was not bled. One group was bled and handled the usual way: carried in warm trucks and kept out of water for up to three days. One group was bled but kept cool and damp and returned to the water within one day. Both bled groups gave the same amount of blood. All the crabs were then kept in the same seawater pens for two weeks (Table 1).

5A panel of hospital lab directors, biologists and crab collectors is now comparing plans (Table 3). The panel's limits are that a plan can add no more than $40,000 a year to what the region's hospitals spend on testing, and it must be working within 2 years. Every test that hospitals use must stay as reliable as the test they use now. Among the plans that fit, the panel wants the one that kills the fewest crabs.