Dr. Targett and Kristin Ferarri
by Kirstin Ferrari, Master's Student, and Prof. Nancy Targett
University of Delaware Graduate College of Marine Studies
In the Mid-Atlantic states, the horseshoe crab's future is inextricably linked to the eel and conch fisheries. As we try to balance horseshoe crab conservation with economic concerns, it is important to examine remedies that will address the following questions. Can we sustain the horseshoe crab resource in Delaware Bay without collapsing local fisheries? Can local fisheries persist without collapsing the horseshoe crab population? We are optimistic that the answer to each of these questions is "Yes!" Artificial bait derived from horseshoe crabs may allow a peaceful coexistence.
The first step in understanding this dynamic system is to recognize the ecological value of horseshoe crabs. The eggs and larvae of horseshoe crabs supply fuel for fish, sea turtles, and shorebirds. Gorging on horseshoe crab eggs during their annual stopover in the Delaware Bay, migratory shorebirds can increase their weight by 40-60%.
Horseshoe crabs also have medicinal value. Limulus amoebocyte lysate (LAL), a clotting factor in the horseshoe crab's blood, is invaluable to the biomedical industry. LAL enables the health-care industry to detect bacteria in human blood, to test the purity of clinical drugs, and to verify the purity of all prosthetics before implantation. In addition, chitin from the crab's shell is incorporated into sutures and bandages to decrease the healing period for wounds. Finally, research on the human eye has been greatly improved through studies of the horseshoe crab's optical nerve. This research has resulted in several Nobel awards.
Commercial fisheries in the Delaware Bay also rely heavily on horseshoe crabs. In fact, much of the current debate over horseshoe crab conservation arises from fishery applications. Between 1975 and 1983, a conservative estimate of horseshoe crab bait use was 350,000 crabs per year. By 1997, reported horseshoe crab landings across the eastern seaboard approached 1.5 million crabs. Comparing 1997 landings to early 1980s estimates, the horseshoe crab harvest for the bait fishery has quadrupled.
Why are horseshoe crabs in such high demand? When used as bait, horseshoe crabs support a $2 million conch fishery and a $6 million American eel fishery. In Delaware Bay alone, watermen derive 20-50% of their total fishing income from conch or eel harvests.
Anecdotal evidence from eel and conch fishermen lends testimony to the superior effectiveness of egg-bearing female horseshoe crabs as bait. Interestingly, live horseshoe crabs are not a primary source of food for these species. This being the case, what makes horseshoe crabs such a delicacy for eel and conch?
Research conducted over the last four years at the University of Delaware Graduate College of Marine Studies has shown that a novel chemical cue in horseshoe crab eggs may mediate this predator-prey interaction. Originally suspected to be a compound ubiquitous in the tissue of horseshoe crabs, it is now known that the chemical attractant is concentrated within the horseshoe crab eggs. In laboratory assays, an extract prepared from horseshoe crab eggs is more attractive than either muscle or reproductive tissue from adult horseshoe crabs. Very heat-stable and freeze-tolerant, the water-soluble chemical attractant can be stored with refrigeration for several weeks without a loss of activity.
Given the molecular size and complexity of the attractant, a cost-effective synthesis is unlikely. However, a sustainable source of the naturally occurring chemical cue appears to be available. Anecdotal evidence from eel and conch fishermen in the Northeast and Mid-Atlantic regions has indicated that hemolymph, a component of horseshoe crab blood, might contain the chemical cue. Our lab assays confirmed that hemolymph is an effective attractant, and more importantly, that it contains the same cue that is in the horseshoe crab eggs.
Although the chemical cue is more dilute in the hemolymph, its source is more sustainable, and therefore, has greater potential than the eggs for artificial bait development. As a waste by-product of the LAL industry, hemolymph is available in large quantities year-round. In contrast, the eggs are only available for a short duration (during the horseshoe crab spawning season). Furthermore, concern over a declining stock population precludes egg collection en masse. For these reasons, we believe that hemolymph will provide an excellent, ecologically sustainable, alternative source of attractant for eel and conch fisheries.
How soon can we expect to see an artificial bait product in our fishermen's pots? We have already identified the chemical cue in the hemolymph as being homologous to the attractant in the eggs, and are working to optimize its incorporation into a cost-effective artificial bait. If successful, the artificial bait will provide a long-term, sustainable solution to the current horseshoe crab dilemma.
About the Authors:
Kirstin Ferrari is a master's degree student in the Marine Biology-Biochemistry Program at the University of Delaware Graduate College of Marine Studies. Professor Targett is associate dean of the college and the chief investigator on a Sea Grant-funded project to develop an artificial bait substitute for the horseshoe crab. The authors are based at the University's marine studies complex at the Hugh R. Sharp Campus in Lewes.