"HORSESHOE CRABS"

The extant animal Limulus polyphemus is commonly called the “horseshoe crab,” but it is misnamed. It is neither a horseshoe, nor a crab. It belongs to Phylum Arthropoda, Class Merostomata, and the order Xiphosurida, which is more closely related to spiders and scorpions than to crabs. Xiphos means “sword” and ura refers to the “spike-like tail” of the “horseshoe crab.” In addition to its helmet-like/tanklike shell (typically 50 to 75 mm long and 40 to 50 mm wide), this complex animal has ten legs: eight of which have  sharp pincers at their end; the other two “legs” are used for feeding purposes. Its "spike-like tail" is not used for defense; rather, it is used to “right” the animal, if it gets turned onto its back by a strong wave. Gills are on the underside of its body. Limulus polyphemus swims on its back (at a 30° angle) and is active at night, when in digs into mud, in search of worms and bivalves. It burrows in sediment during the day. Its preferred depth in the ocean is less than 50 m, but it can tolerate a wide range of salinities, including brackish-water and estuaries. Limulus polyphemus can even use its legs to crawl on land for short distances (Prothero, 2004). As shown in my sketch and photographs given below, this ancient animal has two well developed compound eyes.

Three views (dorsal, ventral, and right side) of Limulus polyphemus.

The fossil record of "horseshoe crabs" ranges, with a fair degree of certainly, as far back as the Silurian Period during Paleozoic time (about 425 million years ago). The animal has hardly changed its shape through this time because changes were not necessary. It is and has been well suited for its environment. In a “nutshell,” it was built to last (Prothero, 2004).

Although “horseshoe crabs” have survived several mass extinctions (e.g., the terminal Permian, the terminal Triassic, and the much-publicized terminal Cretaceous). However, it now faces a crisis. It has lost more than half of its population in the past 60 years. These animals are collected by humans today for the purpose of extracting some of their blue blood, which is copper-based, rather than red blood, which is iron-based. The blue blood of “horseshoe crabs” contains a rare clotting enzyme critical for the development of safe vaccines. Their copper-based blood has played a role in COVID-19 vaccine development. Unfortunately, many “horsecrabs crabs” do not survive the “blood-letting” process because “they are drained of all their blood, rather than just a portion of it” (McKeever, A. and L. Ballesta. 2022).    

Four living species of “horseshoe crabs” are known. The most one most commonly reported on in magazine articles is Limulus polyphemus. It lives today along the east coast of North America from southern Canada/northern Maine to Florida and the Gulf Coast (Alabama, Mississippi), as well as in the waters off the Yucatan Peninsula of eastern Mexico. One of the other species is Carcinoscorpius rotundicauda, found along the coast of the Bay of Bengal in India, as well as along the eastern coast of Malaysia and Indonesia. The other two species, Tachypleus gigas and Tachypleus tridentatus, are found in localized in Malaysia, Indonesia, and a few places in the Philippine Islands.

References:

McKeever, A. and L. Ballesta. 2022. Under the Big Top. National Geographic, Aug. 2022, pp. 74–83.

Proothero, D. 2004. Bringing fossils to life: An introduction to Paleontology. 2^nd ed. McGraw Hill Co., 503 pp.