Abstract
In the U.S., 525,000 horseshoe crabs (Limulus polyphemus) per year have been captured during 2013–2017, brought to biomedical facilities, and bled to produce Limulus amebocyte lysate (LAL), then mostly released to the area of capture. The Atlantic States Marine Fisheries Commission estimates short-term bleeding-induced mortality to be 15% (4% to 30%), resulting in mortality of approximately 78,750 horseshoe crabs annually in recent years comprising a minor portion (<13%) of the up to one million annual coastwide landings dominated by harvest for bait. However, the long-term effect of bleeding for LAL on annual survival and spawning behavior is unknown; thus, results from short-term studies alone might underestimate bleeding effects at the population level. To address this knowledge gap, we analyzed data from the U.S. Fish and Wildlife horseshoe crab tagging database to estimate the differences in survival and recapture rates of bled and not bled horseshoe crabs tagged in the same years and geographic area. Contrary to expectation, survival was not lower for bled crabs compared to unbled crabs. Differences varied, but survival estimates tended to be higher for bled crabs than for unbled crabs. However, biomedical culling and selection for younger or healthier animals could have resulted in biomedically tagged individuals representing a healthier subset of the overall population with subsequent higher survival. Furthermore, the tagging analysis revealed a post-bleeding reduction in capture probability, which could indicate decreased spawning activity, evident in males more than females. Continued tagging of bled and unbled crabs in the same geographic area while recording age class and sex will contribute to the further resolution of LAL production’s effect on horseshoe crab populations.
Highlights
Horseshoe crabs are the sole remaining representatives of the order of Xiphosura, with the four extant species of horseshoe crabs closely resembling their fossilized relatives from hundreds of million years ago, indicative of extraordinary evolutionary success (Rudkin and Young, 2009; BłaZejowski, 2015)
The International Union for Conservation of Nature (IUCN) Red List ranks are Vulnerable for the American horseshoe crab (Limulus polyphemus), Endangered for the Asian species Tachypleus tridentatus, and Data Deficient for the two remaining Asian species (T. gigas and Carcinoscorpius rotundicauda) (Smith et al, 2016; Laurie et al, 2019)
We found that year-specific survival was not estimable for many years based on zero or uninformative standard errors
Summary
Horseshoe crabs are the sole remaining representatives of the order of Xiphosura, with the four extant species of horseshoe crabs closely resembling their fossilized relatives from hundreds of million years ago, indicative of extraordinary evolutionary success (Rudkin and Young, 2009; BłaZejowski, 2015). Commercial harvest of horseshoe crabs is for bait in the U.S, human consumption in Asia, and biomedical use in the U.S and Asia (Atlantic States Marine Fisheries Commission [ASMFC], 2019; Laurie et al, 2019). Biomedical companies produce an assay from the hemolymph (blood) of adult L. polyphemus in the U.S and T. tridentatus in Asia (Levin et al, 2003). Most bled horseshoe crabs are released, mortality and potential behavioral effects from hemolymph harvest are sources of uncertainty in stock assessments (Atlantic States Marine Fisheries Commission [ASMFC], 2019). The importance of LAL to human health and the need to understand LAL-harvest effects on horseshoe crabs are heightened by the potential demand to test coronavirus vaccines for bacterial contamination (Smithsonian Magazine, 8 June 2020, https://www.smithsonianmag.com/ smart-news/race-coronavirus-vaccine-runs-horseshoecrab-blood-180975048/; National Geographic, 2 July 2020, https://www.nationalgeographic.com/animals/2020/07/covidvaccine-needs-horseshoe-crab-blood/#close)
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