Atlantic Horseshoe Crab
Limulus polyphemus
Atlantic Horseshoe Crab (Limulus polyphemus) is a species in the genus Limulus, family Limulidae, order Xiphosurida.
Scientific classification
| Rank | Taxon |
|---|---|
| Domain | Eukaryota |
| Kingdom | Animalia |
| Phylum | Arthropoda |
| Class | Merostomata |
| Order | Xiphosurida |
| Family | Limulidae |
| Genus | Limulus |
| Species | Limulus polyphemus |
Description
The Atlantic horseshoe crab is not a crab but a chelicerate, more closely related to spiders and scorpions than to any crustacean. The body is covered by a horseshoe-shaped carapace, with a long spine-like telson used to right the animal when overturned rather than as a weapon.
It has ten eyes, including a pair of large compound eyes whose unusually big photoreceptors made them the preparation of choice for early work on lateral inhibition in vision — research that earned H. K. Hartline a share of the 1967 Nobel Prize.
A living fossil
Horseshoe crabs have retained essentially the same body plan for well over 400 million years, predating the dinosaurs by a wide margin. The term "living fossil" flatters them slightly: the lineage has continued to evolve, but the external form has changed remarkably little.
Medicine
Horseshoe crab blood is blue — it uses copper-based haemocyanin rather than iron-based haemoglobin — and contains amoebocytes that clot violently in the presence of bacterial endotoxin. The resulting Limulus amoebocyte lysate assay has been the standard test for endotoxin contamination in injectable drugs and implanted devices for decades, and every such product is tested with it or a synthetic replacement.
Animals are bled and returned to the sea; mortality estimates vary considerably, and the practice together with harvesting for bait has driven declines that recombinant Factor C alternatives are now intended to relieve.
Spawning
On spring high tides, particularly around the full moon, adults come ashore in enormous numbers to spawn. Delaware Bay is the largest such aggregation and is a critical refuelling stop for migrating red knots, which feed on the eggs — one of the tighter couplings between an invertebrate and a migratory bird anywhere.
References
- Smith, D. R. et al. (2016). Limulus polyphemus. IUCN Red List assessment 11987.
- Shuster, C. N. et al., eds. (2003). The American Horseshoe Crab. Harvard University Press.