A horseshoe crab (Limulus polyphemus) demographic study

Walls, Elizabeth

Abstract: 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. The mean differential mortality between bled and unbled horseshoe crabs was 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 78.3 grams (95% CI: 70.5g - 86.0g) and ranged from 8.2g to 212.3g. We compared sex-, size-, and stage-class distributions of crabs caught in trawls by BioWhittaker in Chincoteague, Virginia and Ocean City, Maryland during the years 1999 – 2000. Sex distribution (P = 0.0062), size distribution (P = 0.0002), and stage-class distribution (P < 0.001) were significantly different between locations. Chincoteague, Virginia's population was comprised of smaller and younger crabs, with greater proportions of females than the population in Ocean City, Maryland. Between years 2000 and 2001, overall sex distributions were also significantly different (P = 0.0109), 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 47.7 km and maximum distance traveled was 312 km, suggesting mixing along the Atlantic coast. We found a 1.6% recovery rate of tagged crabs and tags found detached from crabs. We document tag loss, as 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.