Published on July 31st, 2020 | by David Marshall

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Episode 114: Horseshoe Crabs

The horseshoe crabs (Xiphosura) are a group of large aquatic arthropods known from the East coast of the USA, and the Southern and Eastern coasts of Asia. Despite their name, they are not actually crabs at all, but are chelicerates (the group containing spiders and scorpions). As a group, the horseshoe crabs possess an extremely long fossil record, reaching as far back as the Ordovician Period, some 480 million years ago. Since that time, they would appear to have undergone very little change, leading the horseshoe crabs to become the archetypal ‘living fossils’.

Joining us for this two-part episode is Dr Russell Bicknell, University of New England, Australia. We discuss what makes a horseshoe crab, before taking questions from our listeners as to all aspects of horseshoe crab ecology and what we can infer from them about other extinct arthropods.

Paper: Bicknell RDC and Pates S (2020) Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida. Front. Earth Sci. 8:98.

Horseshoe crabs are named after the shape of their head (prosoma/cephalothorax). Their bodies are roughly divided into three functional units: the head, abdomen (opisthosoma/thoracetron) and the tail spine (telson).

Only four species of horseshoe crab exist today. Pictured are the two species of Tachypleus: T. tridentatus, the Chinese/Japanese horseshoe crab and T. gigas, the Indo-Pacific horseshoe crab.

Image: (A,B) Male T. tridentatus. (A) Dorsal view. (B) Ventral view. (C,D) Male T. gigas. (C) Dorsal view. (D) Ventral view. (E,F) Female T. tridentatus. (E) Dorsal view. (F) Ventral view. (G,H) Female T. gigas. (G) Dorsal view. (H) Ventral view.

That’s it! Those are all the horseshoe crabs, male and female, that you can see alive today.
Pictured are Limulus polyphemus, the American horseshoe crab and Carcinoscorpius rotundicauda, the mangrove horseshoe crab.

Image: (A,B) Male C. rotundicauda. (A) Dorsal view. (B) Ventral view. (C,D) Male L. polyphemus. (C) Ventral view. (D) Dorsal view. (E,F) Female C. rotundicauda. (E) Dorsal view. (F)Ventral view. (G,H) Female L. polyphemus. (G) Ventral view. (H) Dorsal view.

Whilst just four species are alive today, the evolutionary history of the horseshoe crabs reveals several periods, notably the Triassic and Carboniferous when the number of different species, their diversity, was much higher.

Some of the earliest horseshoe crabs, such as Belinurus carterae, are instantly recognisable as horseshoe crabs. This is because there isn’t a huge amount of morphological disparity (differences in shape) between the species, however important differences still exist. Here, it is easy to spot that the abdomen is composed of individual segments. In later species, these would have fused together, functionally forming one big segment we call the thoracetron today.

During the Carboniferous period, a relatively large number of horseshoe crab species are known. The reason for this might be due to horseshoe crabs experimenting with more freshwater environments and living in places where they might stand a better chance of being preserved. It may be due to the fact more Carboniferous rocks were excavated in the search of coal, or it might simply be an actual increase in the number of species alive at that time.

Image: Examples of the Carboniferous horseshoe crab Euproops danae.

Another period during which there was incrased diversity and disparity was the Triassic. Here wonderful forms such as the pick axe-shaped Austrolimulus fletcheri existed.

Image: Austrolimulids from Australia. (A) Austrolimulus fletcheri from the Triassic-aged Beacon Hill Shale, NSW, Australia. (B) Tasmaniolimulus patersoni from the Permian-aged Jackey Shale, Tasmania, Australia. (C) Dubbolimulus peetae from the Triassic-aged Ballimore Formation, NSW, Australia.

Reconstruction of Tasmaniolimulus patersoni.

The Triassic is also host to the famous Vaderlimulus tricki whose pointed head resembles the helmet of Darth Vader. Image: Joschua Knüppe.

Even by the Triassic, species belong to the modern genera Tachypleus (A) and Limulus (E) were known. That is a duration of over 200 million years for these genera!

In the Jurassic, fantastic specimens can be found in the Solnhofen Limestone. These fossils are so well-preserved that they are often shown side by side with modern horseshoe crabs to illustrate how little they have evolved over time, or even to try and disprove evolution.
Examples of the iconic Jurassic-aged Mesolimulus walchi from Germany

It’s true that horseshoe crabs have changed very little in superficial shape over 480 million years and they can still be found on beaches, probably living their lives not too dissimilar to how they have been doing for almost half a billion years!

Throughout their evolutionary history they have survived nearly every mass extinction event, however humans pose perhaps their greatest threat. Horseshoe crabs gathering on the beaches represent a fantastic source of free fertiliser.

Unfortunately, the blood of the horseshoe crab is a vital component of the pharmaceutical industry and live individuals are captured and their blood harvested. This is obviously a traumatic procedure and is having a negative effect on horseshoe crab populations.

Dr Russell Bicknell with a horseshoe crab.