One of two horseshoe crabs, Limulus polyphemus (Linnaeus, 1758), seen on a recent trip to the beach. The horseshoe crab is an ancient resident of the shores of Eastern North America, having changed very little since the early Paleozoic Era some 450 million years ago.
The name horseshoe crab is a bit of a misnomer as they are not crustaceans as crabs are, but are chelicerata like spiders, scorpions and ticks. All chelicerata lack the mandibles of crustaceans and other arthropods to chew food, but have pointed appendages called chelicerae (think spider fangs) to bring food to the mouth. Because the lack chewing structures though most chelicerates drink blood (ticks & mites) or inject digestive enzymes into their prey to pre-digest they food so they can then drink it. Horseshoe crabs are scavengers, eating molluscs, worms and bits of flesh. Lacking jaws they use their legs to tear food up and a gizzard filled with gravel and sand to further process the food.
Horseshoe crabs are primarily used as bait for fishermen. Unfortunately they are hacking up enough of them to significantly contribute to recent declines in the number of horseshoe crabs. Another contributing problem is beachfront development and traffic. Horseshoe crabs make their nests on beaches on the spring tide. There females make 15-20cm depressions in the sand just below the high water mark and lay a batch of eggs in the nest before dragging a male, clinging to her tail with special claspers, across the nest to fertilize the eggs. In total a large female may lay 60,000 eggs. On the next spring tide the eggs hatch and the larvae emerge. They will swim in the plankton for 5 to 7 days before settling to the bottom and spending a dozen years molting and growing before becoming sexually mature. They can live another 10 15 years as adults.
The egg nests are important for a number of migratory shorebird species which time their return to be on the beaches of the Del-Mar-Va peninsula and especially Delaware Bay during the horseshoe crab nesting season. Unfortunately with decreasing number of adults breeding there are significantly fewer nests. The cascading impact was enough to prompt New Jersey to ban taking horseshoe crabs for any reason other than for medical purposes.
The medical industry uses horseshoe crab blood to test serums for contamination as the blood of horseshoe crabs has a unique substance that clots in the presence of harmful bacteria. Horsecrabs are gathered and “donate” blood for use, then are returned to the sea where their blood volume quickly returns to normal levels. Oh, like cephalopods (and other molluscs) these are true blue bloods, since their blood does not contain iron based hemoglobin (like ours) but instead copper based hemocyanin which turns blue when exposed to oxygen.
Kingdom Animalia
Phylum Arthopoda
Subphylum Chelicerata
Class Merostomata
Order Xiphosura
Family Limulidae
Genus Limulus
Species Limulus polyphemus (Linnaeus, 1758)
