A Horseshoe Crab (Limulus polyphemus) Demographic Study
Walls, Elizabeth Ann
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We examined various aspects of horseshoe crab populations in conjunction with BioWhittaker, a biomedical company that bleeds horseshoe crabs (Limulus polyphemus) and utilizes their blood for biomedical purposes. We examined mortality rates of bled and unbled crabs by holding crabs in tanks for 2-week periods. We estimate average differential mortality between bled and unbled horseshoe crabs to be 7.5% (95% CI: 0.14% - 38.1%), significant with p<0.001. We examined the range in amounts of blood extracted from 98 male horseshoe crabs. Mean mass of blood extracted was was 78.3 grams (95% CI: 70.5g - 86.0g) and ranged from 8.2g to 212.3g. We compared gender-, size-, and stage-class distributions of crabs caught in trawls by BioWhittaker in Chincoteague, Virginia and Ocean City, Maryland during the years 1999 - 2000. Significant differences in gender distribution (p=0.0062), size distribution (p=0.0002) and stage-class distribution (p<0.001) were seen between locations, with Chincoteague, Virginia's population comprised of smaller and younger crabs, with greater proportions of females as compared with Ocean City, Maryland. Significant differences in overall gender distributions (p=0.0109) were also seen between years 2000 and 2001, with greater proportions of females present in 2000 than in 2001. We tagged 7,500 bled, adult horseshoe crabs to gain information on horseshoe crab population dynamics. From resight reports (N=121), we examined movement patterns and found average distance traveled was 29.8 miles and maximum distance traveled was 195 miles, suggesting mixing along the Atlantic coast. We found a 1.6% recovery rate of tagged crabs and tags found detached from crabs. 11.6% of our resights consisted of tags found detached from crabs. We use information gained in our study to suggest improvements for future tagging efforts that could lead to further knowledge of horseshoe crab population dynamics.
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