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Ecology and Vulnerability Horseshoe Crab
Ecology and Vulnerability
Horseshoe Crab
Background
Atlantic (American) horseshoe crabs are marine invertebrates with large brown shells and hard tails. Unlike true crabs, horseshoes have six sets of legs instead... Read More
Background
Atlantic (American) horseshoe crabs are marine invertebrates with large brown shells and hard tails. Unlike true crabs, horseshoes have six sets of legs instead of five and do not have jaws or antennas. Interestingly, they have 10 eyes distributed throughout their body, located along the top shell (carapace), tail, and near their mouth. Horseshoe crabs inhabit inshore coastal water bodies from the Northern Gulf of Maine to the Gulf of Mexico 9. They occur in the greatest numbers in the Delaware Bay 16. Females grow to about 2 feet in length (including tails), while males are typically smaller 11. Males can be further distinguished from females by a set of legs that resemble boxing gloves, which are used for mating. Horseshoe crabs shed (or molt) and replace their shell multiple times during their lives as they grow. They are a long-lived species, maturing at ~10 years, and living up to 20 years 11. They are currently harvested as bait for American eel and whelk fisheries, as well as for scientific and medical purposes 13 — their blue, copper based blood is harvested for a pharmaceutical product which can be used to detect certain bacteria in vaccines 9.
Spawning migrations typically occur in late spring, and horseshoe crabs are found in Massachusetts sound and bay habitats throughout the summer 1. Spawning occurs along protected sandy beaches 8, peaking on full and new moons during May and June. During migration, horseshoe crabs use incoming tides to carry them to spawning beaches, often burrowing in sand to rest. Eggs are buried in nests made just above mean tide line 12. Factors that may signal spawning include photoperiod (the length of day), water temperature, tide height, and wind direction 9. Horseshoe crabs eat marine invertebrates such as razor clams, surf clams, soft-shelled clams, and various species of worms 1,11. Predators of juvenile horseshoe crabs include crabs, shrimp, and small fish, while larger horseshoe crabs are consumed by sharks, sea turtles, and seagulls 16. Horseshoe crab eggs play an important ecological role by providing food sources for many organisms, including migratory birds such as red knots 6.
Spawning populations of horseshoe crabs have declined throughout their Atlantic range; in particular, populations along Cape Cod, Massachusetts have declined by as much as 80% and spawning activity by 95% since the mid-1980s 16. Although drivers of these declines are not well known, potential causes include loss of habitat and food, changes in water conditions, increased predation, and harvest. Massachusetts has designated areas prohibiting harvest (closures) in an effort to restore populations, however horseshoe crabs continue to decline 10,16.
Climate Impacts
Climatic change, combined with recent human impacts such as overharvesting, has been shown to drastically alter horseshoe crab populations 4. Horseshoe crabs rely on environmental cues, such as water level, temperature, and the cyclical patterns of tides, to initiate important life stages 3. In particular, larval growth and development vary with temperature and salinity 7. Additionally, projected increases in temperature will likely change juvenile movement patterns as well adult spawning success 9. Sea level rise and changes in hydrology from increased precipitation and storm surge will also impact important beach and marsh spawning habitat 15. Horseshoe crabs prefer sand that is at least 4 inches (10 cm) deep for spawning 17.
As sea levels rise, horseshoe crabs will likely have to spawn further upland to successfully reproduce above the mean high tide line. However, the extent to which beach loss and degradation due to sea level rise and erosion will impact horseshoe crabs is still largely unknown 18. In Delaware Bay, horseshoe crab habitat suitability was found to be lower with a 3.9-ft sea level rise compared to a 2-ft sea level rise, although this did not take into account the potential for beach migration 19. Wind and wave activity have also been shown to impact horseshoe crab spawning success, which may change in the future 14. Hurricane Sandy was largely responsible for destroying over 70% of optimal habitat for horseshoe crabs on the New Jersey side of Delaware Bay 20. Horseshoe crabs use a diversity of beach habitats, which may provide options for successful spawning, provided these habitats are not eroding or inundated from sea level rise 14. Coastal infrastructure designed to protect inland areas from sea level rise and storms can prevent beaches from migrating inland and cause erosion to the foreshore of beaches, further reducing spawning habitat 21.
1. Baptist, J.P., Smith, O.R., and Ropes, J.W. 1957. Migrations of the Horseshoe Crab, Limulus Polyphemus, in Plum Island Sound, Massachusetts. U.S. Fish and Wildlife Service, Special Scientific Report-Fisheries No. 220. pp. 15.
2. Baumann, R.H. 1980. Mechanisms of maintaining marsh elevation in a subsiding environment. M.S. Thesis. Louisiana State University, Baton Rouge.
3. Chabot, C.C., J.F. Yelle, C.B. O’Donnell, and W.H. Watson ΙΙΙ. 2011. The effects of water pressure, temperature, and current cycles on circatidal rhythms expressed by the American horseshoe crab, Limulus polyphemus. Marine and Freshwater Behaviour and Physiology 44:43-60.
4. Faurby, S., T.L. King, M. Obst, E.M. Hallerman, C. Pertoldi, and P. Funch. 2010. Population dynamics of American horseshoe crabs-historic climatic events and recent anthropogenic pressures. Molecular Ecology 19:3088-3100.
5. Goodman, J.E., M.E. Wood, and W.R. Gehrels. 2007. A 17-yr record of sediment accretion in the salt marshes of Maine (USA). Marine Geology. Volume 242: 109-121.
6. Karpanty, S.M., J.D. Fraser, J. Berkson, L.J. Niles, A. Dey, and E.P. Smith. 2006. Horseshoe Crab Eggs Determine Red Knot Distribution in Delaware Bay. Journal of Wildlife Management 70:1704-1710.
7. Laughlin, R. 1983. The effects of temperature and salinity on larval growth of the Horseshoe Crab Limulus polyphemus. Biol Bull 164:93-103.
8. Leschen, A.S., S.P. Grady, and I. Valiela. 2006. Fecundity and Spawning of the Atlantic horseshoe crab, Limulus Polyphemus, in Pleasant Bay, Cape Cod, Massachusetts, USA. Marine Ecology 27:54-65.
9. Martinez, Sarah Elizabeth Dorothea. 2012. Spatial Ecology of American Horseshoe Crab (Limulus Polyphemus) in Chatham, Cape Cod, MA: Implications for Conservation and Management. Master’s Thesis. pp. 58.
10. Massachusetts Audubon (Mass Audubon). 2016. Wellfleet Bay Horseshoe Crab Research: Research and Findings. Accessed 06/06/2016.
11. Massachusetts Division of Marine Fisheries (MA DMF). 2016. Programs and Projects: Atlantic Horseshoe Crab. Accessed 06/06/2016.
12. National Park Service (NPS). 2016. Horseshoe Crabs. Wellfleet, MA. Accessed 06/06/2016.
13. Rutecki, D. R.H. Carmichael, and I. Valiela. 2004. Magnitude of Harvest of Atlantic Horseshoe Crabs, Limulus Polyphemus, in Pleasant Bay, Massachusetts. Estuaries 27:179-187.
14. Smith, D.R., N.L. Jackson, K.F. Nordstrom, and R.G. Weber. 2011. Beach characteristics mitigate effects of onshore wind on horseshoe crab spawning: implications for matching with shorebird migration in Delaware Bay. Animal Conservation 14:575-584.
15. Sneddon, L. A., and G. Hammerson. 2014. Climate Change Vulnerability Assessments of Selected Species in the North Atlantic LCC Region. NatureServe, Arlington, VA.
16. Widener, J.W. and R.B. Barlow 1999. Decline of a Horseshoe Crab population on Cape Cod. Biological Bulletin 197:300-302.
17. Titus, J.G., K.E. Anderson, K.R. Cahoon, D.B. Gesch, S.K. Gill, B.T. Gutierrez, E.R. Thieler, and S.J. Williams, 2009: Coastal Sensitivity to Sea-Level Rise: A focus on the Mid-Atlantic region. U.S. Climate Change Science Program, Synthesis and Assessment Product 4.1. 320 p.
18. US Fish and Wildlife Service, NE Region, 2014: Rufa Red Knot Background Information and Threats Assessment. Supplement to: Endangered and Threatened Wildlife and Plants; Final Threatened Status for the Rufa Red Knot (Calidris canutus rufa). 383 p.
19. Czaja, E.H. 2009. Assessing the impact of sea level rise on horseshoe crab spawning habitat in Cape May, N. J.. Unpublished report by the Rutgers University School of Environmental and Biological Sciences, New Brunswick, NJ.
20. Niles, L. J., J. A. M. Smith, D. F. Daly, W. Shadel, T. Dillingham, A. D. Dey, M. S. Danihel, S. Hafner, and D. Wheeler, 2013. Restoration of Horseshoe Crab and Migratory Shorebird Habitat on Five Delaware Bay Beaches Damaged by Superstorm Sandy.
21. Tanacredi, J.T., M.L. Botton, and D.R. Smith [Eds.], 2009: Biology and Conservation of Horeshoe Crabs, Springer, New York, New York, 658 p.
This species was identified as moderately vulnerable to climate change because of the following factors:
- Sea level rise
- Specific hydrological thresholds
- ... Read More
This species was identified as moderately vulnerable to climate change because of the following factors:
- Sea level rise
- Specific hydrological thresholds
- Human disturbance
- Climate change mitigation efforts
Factors that decrease vulnerability include:
- Ability to disperse
Sneddon, L. A., and G. Hammerson. 2014. Climate Change Vulnerability Assessments of Selected Species in the North Atlantic LCC Region. NatureServe, Arlington, VA.
This species was identified as highly vulnerable to climate change because of the following factors:
- Increasing ocean surface temperature
- Ocean acidification ... Read More
This species was identified as highly vulnerable to climate change because of the following factors:
- Increasing ocean surface temperature
- Ocean acidification
- Sea level rise
- Low population growth rate
- Complexity in reproduction
Hare J.A., W.E. Morrison, M.W. Nelson, N.M. Stachura, E.J. Teeters, R.B Griffis, et al. 2016. A Vulnerability Assessment of Fish and Invertebrates to Climate Change on the Northeast U.S. Continental Shelf. PLoS ONE 11: e0146756. doi:10.1371/ journal.pone.0146756
This species was identified as moderately vulnerable to climate change because of the following factors:
- Sea level rise
- Specific hydrological thresholds
- ... Read More
This species was identified as moderately vulnerable to climate change because of the following factors:
- Sea level rise
- Specific hydrological thresholds
- Human disturbance
- Climate change mitigation efforts
Factors that decrease vulnerability include:
- Ability to disperse
Sneddon, L. A., and G. Hammerson. 2014. Climate Change Vulnerability Assessments of Selected Species in the North Atlantic LCC Region. NatureServe, Arlington, VA.
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