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by Marcia L. Santore
The American horseshoe crab plays important roles in nature, commerce, even medicine. But in recent years their numbers have been declining. Could understanding how they respond to the tides and their own internal “clocks” help reverse that situation?
In August, the National Science Foundation provided a $203,373 grant to Plymouth State University for research on Limulus polyphemus—the American horseshoe crab. Christopher C. Chabot, professor of biology at PSU, is principal investigator on the project, and Winsor H. Watson, professor of zoology at the University of New Hampshire is co-principal investigator. They are studying how the crabs’ internal “clocks” affect their behavior in the wild.
Many species of fish, turtles and migratory birds depend on horseshoe crab eggs for food, including the endangered loggerhead turtle. Commercial fishermen use the crabs for eel and conch bait, harvesting over two million horseshoe crabs in the year 2000 alone. The blood of this species of crab is used to find certain bacteria and toxins in such medical products as dialysis equipment, antibiotics and intravenous solutions. Better understanding how the animals behave in nature could prove crucial to their survival.
Chabot explains, “Circadian rhythms are molecular, physiological or behavioral events that occur about once a day. These clocks are found in animals from single-cell organisms to worms to insects to crustaceans to vertebrates and everything in between! This ubiquity suggests that having a circadian clock helps organisms to anticipate and synchronize to daily environmental changes, and is of tremendous adaptive advantage.
“While Limulus’ circadian rhythms have been documented in the laboratory, not much is known about how these rhythms affect behavior in the wild. This study builds upon recent published findings from our lab that horseshoe crabs also have internal circatidal clocks that may help them to coordinate their seasonal breeding activities. Dr. Watson, our students and I are using both lab and field techniques to investigate the interaction between tidal and circadian rhythms.”
So far, the results are promising. Chabot and Watson use a sophisticated ultrasonic telemetry system to continuously monitor the activity of horseshoe crabs on and around their traditional “mating beaches” for several weeks at a time. Eight crabs (four male, four female) have been tagged with pingers attached to the dorsal carapace (i.e. duct taped or epoxied to their backs) and released back into Great Bay, off Adam’s Point in Newington, N.H. Three buoys out in the bay form a triangle, 300 meters on each side. When a tagged crab is within 100 meters of the triangle, the researchers can tell exactly where they are and whether they are in motion. “So far, winter behavior looks very different from summer behavior, as we predicted,” Chabot says. They have also learned that, contrary to previous assumptions, horseshoe crabs do not appear to travel very far from their breeding areas during the rest of the year. Seven of the crabs are still in the area and Chabot expects to find the last one no farther away than Portsmouth.
Meanwhile, 36 horseshoe crabs in activity cages in the Boyd Science Center are being studied to determine the relative contributions of their internal clocks and environmental factors such as light, temperature and salinity on their behavior. Chabot notes, “If you put them on a ‘summer schedule’ i.e. long light and warmer water, they will begin to show two bouts of activity a day, rather than one, about every 12.4 hours, corresponding to when the tides come in.”
“We are very excited to have received this grant from the Behavioral Systems Program of the National Science Foundation,” Chabot says. “The fact that only 15 percent of the grants submitted to this program were funded suggests that the NSF recognizes the importance of the science that we are conducting here at PSU.”