Horseshoe Crab: the living fossil

Introduction

Witness to almost 500 million years of Earth's history, horseshoe crabs are the textbook definition of living fossils. Their alien-like primitive forms, featuring the iconic spine-like telson and horseshoe-shaped carapace, have remained virtually unchanged across their existence. While their resemblance to a horseshoe is difficult to challenge, “crab” is a misnomer for the creatures considering they are not even crustaceans. They are, in fact, more closely related to spiders and scorpions. The blue blood of the horseshoe crabs is used for endotoxin detection, making the crabs indispensable for human health. However, their existence is at threat owing to unsustainable harvesting, climate change, habitat loss, pollution, and more.

Figure 1. Front and back views of Limulus polyphemus showing general morphology

Phylogeny and Evolution

Modern-day horseshoe crabs are marine-dwelling chelicerate arthropods of the Limulidae family. Despite their name, they are not crustaceans at all and are, in fact, more closely related to arachnids. Of 88 formally described species, only 4 remain today and are the only surviving members of the order Xiphosura. The extant species include the Atlantic horseshoe crab Limulus polyphemus, and three Asian species: the Indo-Pacific horseshoe crab Tachypleus gigas, the tri-spine horseshoe crab Tachypleus tridentatus, and the mangrove horseshoe crab Carcinoscorpius rotundicauda.

Xiphosurans were considered to have first emerged in the Late Ordovician period of the Paleozoic era, some 450 million years, following the discovery and characterization of the Lunataspis aurora fossil in Canada back in 2007. However, yet undescribed fossils from Morocco, discovered in 2010, established a deeper origin for the Xiphosurans, about 470 million years ago in the Lower Ordovician period of the Paleozoic era. The Limulids, the family of modern horseshoe crabs, first appeared around 250 years ago in the Triassic period. Limulus polyphemus is considered to be the sister taxon, or the closest relative to the three Asian species. The split between the Atlantic and Asian branches of the extant crab species is estimated to have occurred around 135 million years ago and is attributed to a whole-genome duplication event. The Asian species diverged in the Paleogene era, around 45 million years ago, via allopatric speciation. Strikingly, none of the modern species has left a fossil record thus far. Horseshoe crabs are commonly dubbed living fossils owing to the little morphological variation they have undergone across their existence, a phenomenon called evolutionary stasis. Two rounds of evolutionary stasis have been identified in the Xiphosurans, the conservation of prosomal and opisthosomal growth rates near their origin in the Lower Ordivician and the conservation of shape in the Jurassic period owing to the emergence of the Limulids in the Triassic. Notwithstanding physiological changes, present-day crabs are larger in size than their ancestral counterparts and have their abdominal segments fused together.

Whole-genome Duplication

Horseshoe crabs have markedly large genomes for invertebrates; for instance, the species Carcinoscorpius rotundicauda and Tachypleus tridentatus each have genomes approximately 1.72 Gigabase pairs long. This size is attributed to at least three whole-genome duplication events. Whole-genome duplication is a mutation event, relatively rare in animals, which results in the doubling of an entire genome. The redundant copies of genes are often degraded but sometimes, they might acquire novel functions and drive dramatic evolutionary changes. Evidence of whole-genome duplication in horseshoe crabs comes from the existence of multiple fully duplicated hox gene clusters, which, so far, have only been observed in the genomes of species that have undergone whole-genome duplication. Invertebrates typically possess a single hox gene cluster and as such, the number of hox gene clusters in an invertebrate’s genome can be used to predict the number of whole-genome duplication events that might have occurred. Hox genes refer to a class of homeobox genes, which encode transcription factors that specify species shape and regulate development in animals. All hox genes are homologs descended from an ancestral gene. Researchers have identified five Hox clusters in Carcinoscorpius rotundicauda and Tachypleus gigas (the highest among any invertebrates), four in Limulus polyphemus and three in Tachypleus tridentatus. These findings have led researchers to propose three rounds of whole-genome duplication having occurred across the lineage of the crabs.

Whole-genome duplication events are commonly associated with dramatic consequences like speciation and extraordinary morphological diversity. The evolutionary stasis observed in the horseshoe crabs is thus at odds with the several rounds of whole-genome duplication they have undergone. The curious case of the crabs suggests that whole-genome duplication events alone are not sufficient to bring about dramatic morphological changes across various species. As the crabs remained limited to their marine coastal habitats and faced no natural predation, they faced no significant evolutionary pressures which could have caused a shift in their perfectly suitable forms. Regardless, the events have had notable consequences for the crabs’ physiology. Scientists have identified a marked expansion of gene families with important roles in the crabs’ innate immunity, such as carcinolectin-5, carcinolectin-3, CEBP-PI, and FucTC. Horseshoe crabs are constantly exposed to pathogens in their coastal habitats and as such, building up strong defenses was crucial to the animals’ survival.

Morphology

All horseshoe crabs share the same body plan: the cephalothorax or prosoma (head and chest fused together), the opisthosoma (the abdomen with the inclusion of the heart and respiratory organs, a distinctive feature of the chelicerates) and, the iconic feature of the horseshoe crab, the telson (the long tail-like spine jutting out from the abdomen). The prosoma is shrouded in a protective carapace bearing an uncanny resemblance to a horseshoe, giving the horseshoe crab its name. Unlike any other extant chelicerate, horseshoe crabs possess, among its ten eyes, two compound eyes that sit atop its carapace; they also have the largest rod and cone cells of any known animal. All extant species exhibit sexual dimorphism, with female individuals being nearly 1.3 times larger than males. This size difference suits the crabs' particular mating behavior, whereby males attach themselves onto the females' carapaces. While several hypotheses have been proposed for the evolution and maintenance of sexual dimorphism in horseshoe crabs, many researchers believe it is primarily because larger females have the capacity to lay more eggs.

Ecology and Conservation

Present-day horseshoe crabs are all marine dwellers although forays into freshwater have been noted among long extinct groups. The mangrove horseshoe crab, at odds with the other three extant species, also inhabits the brackish waters found near mangrove forests.

Conclusion

While horseshoe crabs no longer possess the range and diversity of former times, their continued existence and survival for nearly 500 million years invite nothing short of marvel and wonder. Yet, their evolutionary success is constantly threatened by human actions. We must become aware as soon as possible of the indispensability of these fascinating creatures, not just to ourselves but also the greater ecosystems of our world, and adopt sustainable practices to sustain their survival.

References

Edited by Ifti Rahman, student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.