Nest site selection minimizes environmental stressor exposure in the American horseshoe crab, Limulus polyphemus (L.)

Abstract

The American horseshoe crab, Limulus polyphemus, migrates to beaches at Seahorse Key, FL, on the highest-high tides to mate and deposit clutches of fertilized eggs. By nesting at high beach elevations, females may increase hatching success by minimizing exposure of the developing eggs to hypoxia and hydrogen sulfide, although at the potential added cost of increased exposure to harmful high temperatures. To test this, we took a two-fold approach: 1) we determined microclimate conditions experienced by developing horseshoe crab eggs at a nesting site on the Gulf Coast of Florida, and 2) in laboratory-rearing experiments we determined whether developmental success was affected by exposure to temperature, ambient O2 and sulfide conditions modeled on values from potential nesting sites. We found that most horseshoe crabs nested in the high and extreme-high tidal height zones above the high tide line. These high-tidal areas were characterized by normoxia, temperatures exceeding 35°C, and minimal exposure to hydrogen sulfide. When fertilized eggs and larvae were exposed to similar “control” conditions in the laboratory, eggs and larvae exhibited 80–90% developmental success. The mid-tidal height zone was characterized by temperatures exceeding 35°C, 21–17% ambient O2, and minimal exposure to hydrogen sulfide. When exposed to similar mid-tidal conditions in the laboratory, development success was equivalent to eggs exposed to control conditions. In contrast, the low tidal height zone was characterized by temperatures never exceeding 35°C, ambient O2 as low as 5.9%, and significantly greater exposure to hydrogen sulfide. In the laboratory, exposure to conditions similar to low-tidal zone resulted in developmental success that was reduced by up to 25% compared to the eggs and larvae exposed to control conditions. We conclude that horseshoe crabs choose nesting sites that maximize developmental success by avoiding sites containing hypoxia and hydrogen sulfide, although this increases exposure to high temperature.