Horseshoe crabs, described as “living fossils”, have been around for approximately 455 million years, which is quite astonishing considering there are only four species of Horseshoe crabs, and in their history there never appears to be more than about nine species at any one time. Originally thought to be related to crabs due to their external appearance, they are actually more closely related to the extant spiders and scorpions and the extinct trilobites and ancient sea scorpions.
Depiction of an ancient Sea scorpion
Currently there are four extant species of Horseshoe crabs. Limulus Polyphemus found on the Atlantic coast of North America, Tachypleus tridentatus, T. gigas, and Carcinoscoprius rotundicauda are found in the waters around S.E. Asia and India. The ancient ancestors of the Horseshoe crabs are thought to first appear about 445 million years ago, with the lineage of the current species appearing about 220 million years ago. In that time, the appearance of the horseshoe crabs have changed remarkably little. Although outwards they don’t seem to be much different from their ancestors, their internal physiology is said to be “up to date”.
By domdomegg (Own work) [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
The Horseshoe crab’s exoskeleton can be divided into three sections: the prosoma which is the ‘shield’ that covers the main body and most of the internal organs, the opisthosoma which is the mid-piece that contains the book gills (their respiratory organ), and the telson which is a tail like spike. When moving through the water column, the Horseshoe crabs swim upside down, when on land they are unable to walk backwards! Living up to 20 years, they will moult at least 16-17 times before reaching sexual maturity at about 9-10 years old. It is believed that after they do, they will stop growing and moulting.
By Breese Greg, U.S. Fish and Wildlife Service [Public domain], via Wikimedia Commons
Horseshoe crabs have a diverse diet and have been known to prey on molluscs such as clams and mussels, worms, and some plants. Predators are numerous and include birds (such as gulls, sandpipers, and knots), arthropods (such as shrimps and crabs), fish (perch, flounder, leopard shark, swordfish), and sea turtles. Their breeding grounds are important staging areas for migratory birds in the USA who prey on their eggs to fuel their long flights. In the Delaware spawning area, an estimated 425,000 – 1,000,000 birds pass through these areas. Each adult female Horseshoe crab can produce about 88,000 mature eggs, and it is estimated the migratory birds require 1,820,000 females to feed them.
By Hayden (Horseshoe Crabs mating) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
Horseshoe crabs provide humans with a very important product, their blood contains Limulus amebocyte lysate (LAL). It is important for pharmaceuticals and surgical equipment as it can detect the presence of a bacterial contamination. The blood clots when exposed to bacterial endotoxin, and can even detect quantities as small as one millionth of a billionth of a gram. Despite their importance to us, Horseshoe crabs are not very well protected and are threatened by coastal erosion and pollution of key spawning grounds, shoreline development, and overexploitation.
By Andrea Westmoreland from DeLand, United States (Horseshoe Crab at Smyrna Dunes Park) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
As they are few in number, have been around for so long, and are generally not well known, it can be hard to appreciate these fantastic species. I think this quote sums up the wonder of Horseshoe crabs quite well.
“Gone are the ammonites, the trilobites, and numerous other once abundant creatures with whom they shared the ancient oceans. The Atlantic ocean has opened, closed, and formed again during their time, Ice ages have come and gone as the planted repeatedly cooled, warmed, and cooled again. Horseshoe crabs were witness to the arrival and departure of great ocean dwelling reptiles and terrestrial dinosaurs and remained through waves of extinction and numerous new arrivals: insects, spiders, birds, mammals, salamanders and frogs, trees, flowers, and eventually, Homo sapiens.” – Sylvia A. Earle.