Horseshoe crab


Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.
Only four species of horseshoe crab are extant today, the Atlantic horseshoe crab, native to the eastern coast of North and Central America, as well as the mangrove horseshoe crab, tri-spine horseshoe crab and Indo-Pacific horseshoe crab, which are native to South, South East, and East Asia.
Most horseshoe crabs are marine, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems such as the Delaware and Chesapeake bays. Additionally, certain extinct species transitioned to living solely in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed.
Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.

Phylogeny and evolution

The fossil record of Xiphosura, the broader group that includes horseshoe crabs and their extinct relatives, extends back to the Early Ordovician, around 480 million years ago. Ordovician xiphosurans, such as Lunataspis, already bear a close resemblance to living horseshoe crabs. For modern horseshoe crabs, their earliest appearance was approximately 250 million years ago during the Early Triassic. Because they have seen little morphological change since then, extant forms have been described as "living fossils".
Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata. Horseshoe crabs are closely related to the extinct eurypterids, which include some of the largest arthropods ever to have existed, and the two may be sister groups. The difficult-to-classify chasmataspidids are also thought to be closely related to horseshoe crabs.
The radiation of horseshoe crabs resulted in 22 known species, of which only 4 remain. The Atlantic species is sister to the three Asian species, the latter of which are likely the result of two divergences relatively close in time. The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in the Cretaceous.
Limulidae is the only extant family of the order Xiphosura, and contains all four living species of horseshoe crabs:
After Bicknell et al. 2021 and Lamsdell et al. 2020
  • Incertae sedis
  • * †Albalimulus? Bicknell & Pates, 2019 Ballagan Formation, Scotland, Early Carboniferous
  • *†Casterolimulus Holland, Erickson & O'Brien, 1975 Late Cretaceous Fox Hills Formation, North Dakota, USA
  • * †Heterolimulus gadeai Vıa & Villalta, 1966 Alcover Limestone Formation, Spain, Middle Triassic
  • * †Limulitella? Størmer, 1952 Middle-Upper Triassic, France, Germany, Tunisia, Russia
  • * †Sloveniolimulus Bicknell et al., 2019 Strelovec Formation, Slovenia Middle Triassic
  • * †Tarracolimulus Romero & Vıa Boada, 1977 Alcover Limestone Formation, Spain, Middle Triassic
  • * †Victalimulus Riek & Gill, 1971 Lower Cretaceous Korumburra Group, NSW, Australia
  • * †Yunnanolimulus Zhang et al., 2009 Middle Triassic, Guanling Formation, Yunnan, China
  • * †Mesolimulus Middle Triassic-Late Cretaceous England, Spain, Siberia, Germany, Morocco
  • *†Ostenolimulus Lamsdell et al. 2021 Early Jurassic Moltrasio Limestone, Italy
  • *†Volanalimulus Lamsdell, 2020 Early Triassic, Madagascar.
  • Subfamily Limulinae Leach, 1819
  • * †Crenatolimulus Feldmann et al., 2011 Upper Jurassic Kcynia Formation, Poland. Lower Cretaceous Glen Rose Formation, Texas, USA
  • * Limulus O. F. Müller, 1785 Pierre Shale, United States, Late Cretaceous, Atlantic North America, Recent
  • Subfamily Tachypleinae Pocock, 1902
  • * Carcinoscorpius Pocock, 1902, Asia, Recent
  • * Tachypleus Leach, 1819 Upper Cretaceous Haqel and Hjoula Konservat-Lagerstatten, Lebanon, Upper Eocene Domsen Sands, Germany, Asia, Recent

    Phylogeny

The horseshoe crab's position within Chelicerata is complicated. However, most morphological analyses have placed them outside the Arachnida. This assumption was challenged when a genetics-based phylogeny found horseshoe crabs to be the sister group to the ricinuleids, thereby making them an arachnid. In response, a more recent paper has again placed horseshoe crabs as separate from the arachnids. This new study utilized both new and more complete sequencing data while also sampling a larger number of taxa.
Below is a cladogram showing the internal relationships of Limulidae based on morphology. It contains both extant and extinct members.

Whole genome duplication

The common ancestor of arachnids and xiphosurans underwent a whole-genome duplication event. This was followed by at least two, possibly three, WGDs in a common ancestor of the living horseshoe crabs. This gives them unusually large genomes for invertebrates. Evidence for the duplication events includes similarity in structure between chromosomes, and clustering of homeobox genes. Over time, many of the duplicated genes have changed through processes of neofunctionalization or subfunctionalization, meaning their functions are different from what they originally were.

Evolution of sexual size dimorphism

Several hypotheses have been given as possible reasons why a size difference exists between male and female horseshoe crabs. This phenomenon is known as sexual size dimorphism and results in the females having a larger average size than males. The existence of this trend is likely due to a combination of two things:
  1. First, females take a year longer to mature and undergo an additional molt, giving them a larger average body size.
  2. Second, larger female horseshoe crabs can house more eggs within their bodies. This lets them pass on more genetic material than smaller females during each mating cycle, making larger females more prevalent.

    Anatomy and physiology

General body plan

Like all arthropods, horseshoe crabs have segmented bodies with jointed limbs, which are covered by a protective cuticle made of chitin. They have heads composed of several segments, which eventually fuse as an embryo.
Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts : the cephalothorax and the opisthosoma. The first tagma, the cephalothorax or prosoma, is a fusion of the head and thorax. This tagma is also covered by a large, semicircular carapace that acts as a shield around the animal's body. It is shaped like the hoof of a horse, giving this animal its common name. In addition to the two main tagmata, the horseshoe crab also possesses a long tail-like section known as the telson.
In total, horseshoe crabs have 6 pairs of appendages on their cephalothorax. The first of these are the chelicerae, which give chelicerates their name. In horseshoe crabs, these look like tiny pincers in front of the mouth. Behind the chelicerae are the pedipalps, which are primarily used as legs. In the final molt of males, the ends of the pedipalps are modified into specialized, grasping claws used in mating. Following the pedipalps are three pairs of walking legs and a set of pusher legs for moving through soft sediment. Each of these pusher legs is biramous or divided into two separate branches. The branch closest to the front bears a flat end that looks like a leaf. This end is called the flabellum. The branch towards the back is far longer and looks similar to a walking leg. However, rather than ending in just a claw, the back branch has four leaf-like ends that are arranged like a petal. The final segment of the cephalothorax was originally part of the abdomen but fused in the embryo. On it are two flap-like appendages known as chilaria. If severed from the body, lost legs or the telson may slowly regenerate, and cracks in the body shell can heal.
The opisthosoma or abdomen of a horseshoe crab is composed of several fused segments. Similar to a trilobite, the abdomen is made up of three lobes: a medial lobe in the middle, and a pleural lobe on either side. Attached to the perimeter of each pleural lobe is a flat, serrated structure known as the flange. The flange on either side is connected by the telson embayment, which itself is attached to the medial lobe. Along the line where these lobes meet are six sets of indentations known as apodeme. Each of these serves as a muscle attachment point for the animal's twelve movable spines.
On the underside of the abdomen are several biramous limbs. The branches closest to the outside are flat and broad, while the ones on the inside are more narrow. Closest to the front is a plate-like structure made of two fused appendages. This is the genital operculum and is where horseshoe crabs keep their reproductive organs. Following the operculum are five pairs of book gills. While mainly used for breathing, horseshoe crabs can also use their book gills to swim. At the end of a horseshoe crab's abdomen is a long, tail-like spine known as a telson. It is highly mobile and serves a variety of functions.