Pathways & Transformations of Energy and Matter: Horseshoe Crab
Horseshoe crabs are unique organisms with a distinctive anatomy. Horseshoe crabs’ anatomy dictates where they are found and what and how they eat. Horseshoe crabs do not have jaws and therefore use small pinchers named Chelicera to placed food in their mouth, which is a small hole surrounded by appendages. Horseshoe crabs use their hind legs to crush the food before being ingested. Horseshoe crabs have ten eyes and are sensitive to light, and therefore they tend to feed at night. Horseshoe crabs reside mainly in shallow beach waters because of their preference for sand and mud environments. Although horseshoe crabs are mainly found near shorelines, they can also inhabit waters as deep as 200 meters (Botton and Haskin 1984). Due to the beach environment in which most horseshoe crabs inhabit, their feeding preference and behaviors are not only shaped by their anatomy but also their environment.
Horseshoe are omnivores, which means that they eat both plants and animals. Horseshoe crabs eat a variety of organisms native to beaches. A variety of studies have been performed to determine the diversity of organisms consumed by horseshoe crabs and their preference. Two separate studies conducted by Mark L. Botton and colleagues examined the digestive tracts and its contents of horseshoe crabs. The studies revealed that horseshoe crabs eat a plethora of organisms including worms, clams, seagrass, scavenger fish, algae etc. It is not uncommon for horseshoe crabs to eat whole classes of organisms. Although horseshoe crabs have a large range of food sources, they also have preferences. Horseshoe crabs prefer to eat mollusks and more specifically bivalve mollusks (Botton and Ropes 1989). A study done on the Indian horseshoe crab revealed that 24.7% of food contents found in the guts of the horseshoe crabs studied were mollusks and 34.4% of mollusks were bivalve mollusks (Chatterji et al., 1992). The feeding of the Bivalvia in the Indian horseshoe crabs studied was equivalent to 1,538.18 KJ of energy (Chatterji et al., 1992). Female horseshoe crabs are 25% larger than male horseshoe crabs and thus feed at higher rates than male horseshoe crabs. Horseshoe crabs feed more at the bottom of sandy waters, and therefore during breeding season they feed less, due to the ritual of coming on shore to lay eggs. As crabs mature, they molt less and switch their energy use to reproduction.
Horseshoe crabs are predators that often regulate and keep in check populations of mollusks, polychaetes, and amphipods. Although adult horseshoe crabs do not have many predators due to their hard exoskeleton, their eggs often fall prey to a variety of shorebirds. Female horseshoe crabs will bury about 100,000 fertilized eggs in the sand per mating season. Due to wave movements at the shore, the eggs are left exposed and are eaten by shorebirds. Horseshoes are considered secondary consumers, meaning that they are both prey and predators. As, energy flows through the horseshoe crabs’ prey, to the horseshoe crab, to the horseshoe crabs’ predator energy is lost. Still, the horseshoe crab is an essential part of the marine ecosystem.
References
Botton, M.L. 1984. Diet and food preferences of the adult horseshoe crab Limulus Polyphemus in Delaware Bay, New Jersey, USA. Marine Biology 81: 199-207.
Botton, Mark & Haskin, H.H. (1984). Distribution and feeding of the horseshoe crab, Limulus Polyphemus, on the continental shelf off New Jersey. 82:383-389.
Botton, Mark L. & Ropes John W. 1989. Feeding Ecology of Horseshoe Crabs on the Continental Shelf, New Jersey to North Carolina. Bulletin of Marine Science. 45(3): 637-647
Chatterji, A., J.K. Mishra, and A.H. Parulekar 1992. Feeding Behaviour and Food Selection in the Horseshoe Crab, Tachypleus Gigas (Muller). Hydrobiologia 246:41-48.
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