Horseshoe crabs: “living fossils” among arthropods

Xiphosurans or horseshoe crabs are probably the most ancient living arthropods known nowadays. These prehistoric-like, marine and currently scarce organisms related to arachnids have survived since ancient times without suffering almost any obvious change…until now. In this article, we will introduce you the main traits of these arthropods, as well as their current threats.

What are xiphosurans?

Xiphosurans (from Ancient Greek xíphos ‘sword’ and ourá ‘tail’), commonly known as horseshoe crabs, are a group of marine arthropods dating from as far back as the Ordovician (485,4 ±1,9 – 443,8 ±1,5 myr), in the Palaeozoic Era. Originally, they represented an important fraction of marine fauna; however, their number is extremely scarce nowadays, with only 4 extant species belonging to one order (Limulida) being the sole survivors of a once great radiation. The rest of members are fossils.

To know more about fossils: Knowing fossils and their age“.

Limulus polyphemus or Atlantic horseshoe crab. Source: Public Domain.

Current xiphosuran species are considered ‘living fossils as they haven’t undergone obvious morphological changes in comparison with Carboniferous and Triassic fossil forms. Moreover, they are the only ones that survived different extinction events.

Xiphosurans’ place on the tree of life

Like pycnogonids or sea spiders, xiphosurans’ place on the tree of life has been widely discussed. Until recently, xiphosurans were classified along with the eurypterids or sea scorpions (currently extinct) forming the Merostoma group, because they seemed to share some morphological traits. However, some deeper analyses showed that these organisms weren’t related, so ‘Merostomata’ is considered an artificial group nowadays.

To know more about pycnogonids: ‘Spiders from the deep sea: Pycnogonida’.

Eurypterus, the most common eurypterid fossil and also the first described genus. Author: Obsidian Soul, CC.

Currently, the most accepted stance is that xiphosurans constitute a class of arthropods by themselves (Xiphosura) inside the superclass Chelicerata (subphylum Cheliceromorpha). Moreover, they are classified within the euchelicerates along with two more classes: Arachnida and Eurypterida.

And above everything…despite their appearance and their marine habits, they’re NOT related to crustaceans!

Source: Tree of Life Web Project.

External and internal anatomy

As most of modern cheliceromorphs, the xiphosurans have the body divided in two parts or tagmata (prosoma and opisthosoma), the head not differentiated from the thorax, and antennae and mandibles absent. However, the defining trait of cheliceromorphs is the presence of chelicerae, a pair of modified preoral appendages mostly linked to feeding functions. In spiders, the chelicerae form fangs that most species use to inject venom.

Modern xiphosurans reach up to 60 cm in adult length, but their Paleozoic relatives were usually smaller, sometimes as small as 1 to 3 cm long. Dorsally, their body is covered with a not-segmented tough chitinous cuticle divided in two articulated parts more or less equivalent to the prosoma and the opisthosoma:

Dorsal view. Modified picture, original photography property of Didier Descouens, CC.

Now, let’s see the main anatomical traits of living xiphosurans (Limulida):

Tagmata: prosoma and opisthosoma

In the prosoma, the cuticle has three ridges: one median ridge and two lateral ridges. In the anterior part of the median ridge there are located two small ocelli, while in the external surface of lateral ridges the are located the compound eyes. The cuticle extends laterally towards the opisthosoma forming a kind of wings called genal spines. Ventrally and anteriorly, the cuticle forms a wide triangular area known as hypostome where some sensorial organs are located (e.g. ventral ocelli and frontal organ).

Opisthosomal segments appear fused in the modern xiphosurans (on the contrary, they are differentiated in all specimens of the order ‘Synziphosurina’, currently extinct); however, opisthosomal segments can still been distinguished by the lateral opisthosomal spines and the dorsal fossae (a total of 6 pairs, corresponding to the 6 segments of the opisthosoma). The opisthosoma terminates in a long caudal spine, commonly referred to as a telson.

Dorsal view. Modified picture, original photography property of Didier Descouens, CC.

Appendages

Xiphosurans have 6 pairs of prosomal appendages: one pair of chelicerae to capture preys (or other food particles) and 5 pairs of walking legs. Xiphosurans’ legs have a double function: besides allowing them to walk and swim, the legs’ bases have hard and sharped teeth to grind the food (gnathobase). These special bases make contact medially forming a duct (endostome) through which food is transported to the mouth. All legs end in well-developed pincers, except the first pair in males. In both sexes, the last pair has an organ called flabella they use to analyse water composition.

In the prosoma, we can also see a pair of appendages morphologically related to the first pair of opisthosomal appendages: the chilaria. These appendages, which are thought to be vestiges of the first opisthosomal segment’s legs, act as block for food to not escape behind the last pair of moving legs.

Prosomal appendages (ventral view). Modified picture, original photography property of Wayne marshall, CC on Flickr.

The opisthosoma has 6 pairs of modified appendages: one pair of genital operculum more or less fused and 5 pairs of book gills to breath under water, which are protected by the operculum flaps.

Opisthosomal appendages (ventral view). Modified picture, original photography property of KatzBird, CC on Flickr.

A very special circulatory system

Despite being arthropods, xiphosurans have a well-developed circulatory system with ‘veins’ and ‘arteries’ that resemble of which of more complex organisms. Their blood contains two cellular types: amebocytes, equivalent to leucocytes, and cyanocites, equivalent to erythrocytes but with hemocyanin instead of hemoglobin. When the hemocyanin is attached to oxygen molecules or is exposed to air, xiphosurans’ blood acquires a characteristic blue tonality.

The blue liquid is its blood!. Author: Dan Century, CC on Flickr.

Biology

Reproduction and life cycle

During mating seasons, horseshoe crabs move to shallow waters, shore of beaches and estuaries in massive groups. Males climb onto the back of females, gripping them with their rudimentary first pair of pincers; then, females move to the shore with a male on their backs while looking for a good place to bury the eggs. Females usually lay from 200 to 300 not fertilised eggs. Finally, males cover the eggs with sperm (external fertilization).

Author: U.S. Fish and Wildlife Service Northeast Region, CC on Flickr.

After hatching, xiphosurans go through two pelagic larval stadiums before reaching the adult form linked to the substrate: trilobite larvae, which has the opisthosomal appendages uncompletely developed and a short telson, and prestwiquianela larvae, which has all the appendages completely developed. They have a 20 years life span.

Ecology and distribution

All living horseshoe crabs live in shallow marine waters, despite some of their fossil relatives also inhabited freshwaters and brackish water habitats. They usually live on sandy and silty substrates at 3-9 metres deep. They’re specialized on digging, for what they use both the margins of their though cuticle and the first four pairs of moving legs; at the same time, they use the telson to lift the opisthosoma so that the fifth pair of moving legs could analyse and filter the surrounding water.

When swimming, they do it upside down, as we can see in the following video property of Wayne Brear:

Horseshoe crabs are generally predators of different species of annelids, molluscs, as well as of other groups of benthic invertebrates. However, they can also feed on algae.

Current diversity of xiphosurans is represented by 4 species, all of them belonging to the order Limulida: Limulus polyphemus (Atlantic coast of North America), Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda (Indo-Pacific coast).

Rough distribution of the 4 living xiphosuran species. Source: Charmichael & Brush, 2012.

What is their current conservation status?

Humans have done it again. Despite of been living on Earth since prehistoric eras and even been survived to different extinction events, the horseshoe crabs are now more threatened than ever due to anthropic causes. Among the main threats horseshoe crabs must face we can list the ones that follow:

  • Habitat alteration: water temperature changes due to global warming, contamination and declining of quality of water, and destruction of shores and shallow water environments (essential for these organisms to accomplish mating). This is, probably, the most problematic threat.
  • They are used as baits for fishing industries.
  • They are used for biomedical purposes: xiphosurans’ blood is used to produce a bacterial endotoxin indicator called ‘Limulus amebocyte lysate’ (LAL). The amoebocytes of their blood react against some bacterial endotoxins, forming blood clots. So, the LAL test is used to detected different bacteria in a wide variety of materials. Currently, the way to extract blood from their bodies is quite invasive so, despite returning them to their native habitats after the extraction, they still experience a big mortality rate.
  • They are harvested for being used in researches about vision, endocrinology and other physiological processes.
  • They are captured to be commercialized as food: in some Asian countries, horseshoe crabs are served in traditional dishes and rituals, even though this not seems to be the major threat they must face.
  • They are commercialized as pets.
Blood extraction for LAL. Source: National Geographic/Getty Images.
Dish based on horseshoe crab in Si Racha (Thailand). Author: Marshall Astor, CC.

There exist scarce data about the conservation status of the living species of xiphosurans. The most part of information comes from the American species Limulus polyphemus, which is now classified as ‘Vulnerable‘ and with a decreasing population trend since the last 100 years by the IUCN.

Recently, horseshoe crabs have been recognized as important components of benthic food webs, and their eggs supplement the diet of migratory shorebirds along the Atlantic coast of the USA. So, there is considerable interest in propagating and restoring horseshoe crab populations to support these valuable economic, biomedical, ecological and cultural services.

.           .           .

Solving the phylogenetic relationships of a group mainly composed by extinct organisms is not a piece of cake. But now that we start to glimpse them, we are slowly condemning these organisms to their disappearance. Nor the living fossils are safe from the sixth extinction!

References

  • Carmichael, R. H. & Brush, E. (2012). Three decades of horseshoe crab rearing: a review of conditions for captive growth and survival. Reviews in Aquaculture, 4(1): 32-43.
  • Chacón, M. L. M. & Rivas, P. (2009). Paleontología de invertebrados. IGME.
  • Grimaldi, D. & Engel, M. S. (2005). Evolution of the Insects. Cambridge University Press.
  • Marshall, A. J., & Williams, W. D. (1985). Zoología. Invertebrados (Vol. 1). Reverté.
  • Pujade-Villar, J. & Arlandis, J. S. (2002). Fonaments de zoologia dels artròpodes (Vol. 53). Universitat de València.
  • The IUCN Red List of Threatened Species: Horseshoe crabs.
  • Xifosuros: Animales de la realeza. Boletín Drosophila.

Main photo property of Didier Descouens, CC.

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