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Atlantic Horseshoe Crab (1984-12-22 0001_01)

Atlantic Horseshoe Crab (1984-12-22 0001_01)

Although they look like crustaceans, horseshoe crabs belong to the subphylum Chelicerata and are more closely related to spiders and harvestmen (AKA Daddy-Long-Legs). They have been assigned to the order Xiphosura and its only living family Limulidae. There are four living species:

  • Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda) which is found in South and Southeast Asia
  • Atlantic (AKA American) Horseshoe Crab (Limulus polyphemus) which can be found along the Atlantic coast of the USA and in the Gulf of Mexico.
  • Indo-Pacific (AKA Indonesian, Indian or Southern) Horseshoe Crab (Tachypleus gigas) from South and Southeast Asia.
  • Chinese (AKA Japanese or Tri-spine) Horseshoe Crab (Tachypleus tridentatus) which is found in Southeast and East Asia

All species have a hard shell (carapace) on top and are loaded with eyes. There are two compound eyes located toward the sides, each of which has about 1,000 ommatidia. Toward the center there is a pair of eyes which can see ultraviolet as well as visible (to us) light. There is also an endoparietal eye which helps regulate circadian rhythms and a pair of rudimentary eyes on the top which become functional just before the embryo hatches.crabs have  Not finished yet there is a pair of near the mouth, as well as a cluster of photoreceptors on the end of its tail. In spite of having all these eyes, it doesn’t see all that well, except at night. Horseshoe Despite have the largest rods and cones of any known animal, roughly 100 times the size of ours making them a million times more sensitive to light at night than during the day.

Its mouth is between its legs. There are two small legs which push food into the mouth and five large pairs of legs for locomotion. The male also uses the front pair to hold onto the female while mating. The other pairs of legs have a weak claw at the tip. Lost legs can regenerate.

Their blood, which is blue in color, contains amebocytes which can act like the white blood cells in vertebrates including human beings. The animals are harvested when they come ashore to breed and a portion of their blood is drained. Between 3 and 30% of the animals do not survive.

Horseshoe crabs were also used by eel fisherman as bait. Some areas in the United States have banned this practice because it was resulting in a shortage of food for the Red Knot, an endangered bird species.


22 Dec 1984

Shedd Aquarium, Chicago, Illinois

Pentax MX, SMC-Pentax 50/1.4, Auto-flash

Fujichrome 100 60/6.7The name horseshoe Crab is a misnomer. They are not actually crabs. (Atlantic Horseshoe Crab – Phylum Arthropoda : Subphylum Chelicerata : Order Xiphosura : Suborder Xiphosurida : Family Limulidae : Limulus polyphemus)

Editing was done with ACDSee Ultimate 2019. I adjusted the lighting, boosted the contrast, adjusted the levels and used the SmartErase tool to get rid of some dust spots.

Photo © 1984 Gary J. Sibio. All rights reserved.

Text © 2019 Gary J. Sibio. All rights reserved.

#nature #oceans #oceanlife #conservation #aquatic #seas #ACDSee

  • Should more efforts be made to protect the horseshoe crab?

    • Yes
    • No


What do you think?

10 points

Written by Gary J Sibio

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    • I tend to agree with you but I’m not unsympathetic to those who need them to put food on their tables. (Not that I’m saying you’re not sympathetic.) If they have other options to use for bait, then they should use them. One thing that needs to be done is to find ways of collecting their blood without losing so many of them in the process. I’ve also heard that scientists are working on a process that would allow the blood to be synthesized.


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