|Scientific name||Limulidae||Weight||Size varies across species|
|Pronunciation||Hors-shoo krab||Length||30–79.5 centimeters (11.8–31.2 inches)|
|Classification||Arthropoda, Chelicerata, Xiphosura||Location||Worldwide|
The Horseshoe Crab
Although horseshoe crab fossils are quite rare, paleontologists have managed to establish that they’ve been around for more than 400 million years.
The oldest representatives of the family that includes extant horseshoe crab species date back to 250 million years ago.
The most fascinating aspect of their incredibly long lineages is the fact that these marine and brackish arthropods haven’t changed too much.
Many modern animals whose lineages date back millions of years have undergone multiple diversifications that led to the rise of descendants that were quite different from their ancestors.
Not horseshoe crabs, though – that’s not their story! How awesome can they be to survive multiple extinction events without somehow improving their physiology?
The answer’s simple – they’re awesome! Keep reading to find out why!
Taxonomy and Classification
Horseshoe crabs form the Limulidae family, which is part of the Xiphosura order of arthropods.
At first, one would think that horseshoe crabs are crustaceans.
However, they are not part of the Crustacea subphylum.
Instead, horseshoe crabs are members of the Chelicerata group of arthropods, alongside sea spiders and arachnids (spiders, ticks, scorpions, mites, and others).
Further down the taxonomic tree, horseshoe crabs are in the Xiphosura order and form the Limulidae family, of which they are the only extant members.
All their closest relatives (members of the same family) went extinct approximately 400 million years ago.
Other close relatives are the eurypterids (sea scorpions), but they also disappeared 250 million years ago. Horseshoe crabs were once considered eurypterids!
Some sources also mention that horseshoe crabs are closely related to chasmataspids, a group of chelicerate arthropods.
The horseshoe crab family consists of 15 genera, divided into three major groups.
Most genera are extinct, and many have the status of Incertae sedis, used for problematic taxonomic groups with unknown relationships.
The genera that contain living species are the following:
- Limulus with one species, Limulus polyphemus (Atlantic horseshoe crab)
- Carcinoscorpius, with one species, Carcinoscorpius rotundicauda (mangrove horseshoe crab)
- Tachypleus, with two extant species, Tachypleus gigas (Indo-Pacific horseshoe crab) and Tachypleus tridentatus (Chinese/Japanese/tri-spine horseshoe crab), and two extinct species.
It is believed that horseshoe crabs evolved between 541 and 252 million years ago.
The oldest known member of their family is Lunataspis aurora, which roamed through the Ordovician seas 445 million years ago.
Since then, these creatures have not changed much.
As you’ve probably guessed, horseshoe crabs are completely covered in a hard greenish or gray-brownish carapace.
Their bodies are divided into three parts: the head (prosoma), where the brain and the heart are, the abdomen (opisthosoma), and the tail (telson).
The prosoma is equipped with chelicera (two small appendages that function as mouthparts), pedipalps (two appendages used as walking legs), and four other pairs of appendages, all known as pusher legs.
The opisthosoma is equipped with six other pairs of appendages, of which the first has genital pores, whereas the remaining five turned into flattened cartilaginous plates called book gills, which allow horseshoe crabs to breathe underwater and for short periods on land as well.
Horseshoe crabs possess two compound lateral eyes.
Compound eyes are found in other arthropods as well.
They are composed of ommatidia, which, in short, help the brain form images.
Horseshoe crabs have 1,000 ommatidia in each compound eye.
This is a relatively high number, considering that other arthropods, like the Antarctic isopod, have as few as five.
Nevertheless, other species, particularly moths and dragonflies, have approximately 30,000 ommatidia!
In addition to their compound lateral eyes, horseshoe crabs have two median eyes that detect visible and UV light.
They also have two rudimentary lateral eyes, one parietal eye, two ventral eyes near the mouth, and several photoreceptors on the posterior-most part of their bodies.
Male and female horseshoe crabs are sexually dimorphic – females are roughly 20–30% larger.
Their overall size depends on the species and their geographic distribution.
The largest is T. tridentatus – females grow up to 79.5 centimeters (31.3 inches) long, weigh up to 4 kilograms (8.8 pounds), and have an average carapace width of 27.5–28 centimeters (10.8–11 inches).
The smallest species is C. rotundicauda – the males grow to 30 centimeters (11.8 inches) and reach a carapace width of 14.5–15 centimeters (5.7–5.9 inches).
Habitat and Distribution
Horseshoe crabs are native to the Americas and Asia.
They’re found along the Atlantic U.S. coast, from Maine to Florida.
There’s also a breeding population in the Yucatan peninsula.
Sometimes they’re spotted outside their natural range.
Carcinoscorpius rotundicauda lives around the Indo-West Pacific region in Southeast Asia, India, and Bangladesh.
It prefers tropical and subtropical climates and is sometimes spotted in shallow, sandy, or muddy waters. T. tridentatus and T. gigas are found in Southeast and East Asia.
All species inhabit marine or brackish waters and are found in lagoons, estuaries, mangroves, seagrass meadows, and sandy and muddy shores.
They do not go deeper than 40 meters (feet) under the water, although there’s one exception – the Atlantic horseshoe crab, which can swim deeper than 200 meters (feet) and venture 56 kilometers (miles) away from the shore.
However, like its relatives, it prefers shallow waters.
Behavior and Social Structure
Very little is known about the behavior of horseshoe crabs.
Some studies showed that their activity is influenced primarily by the tidal rhythm, meaning by the rise and fall of sea levels, and secondarily by light and dark cycles.
T. tridentatus’s locomotion is known to be affected by rising seawater temperatures because it’s a poikilotherm, which means that its internal temperature is highly varied.
As such, when the temperatures rise, these horseshoe crabs bury themselves in the sediment.
Sometimes they go into a state of animal dormancy called diapause.
Scientists observed that horseshoe crabs engage in migratory behaviors during the breeding season when they travel to shallow coastal waters.
Diet and Feeding
Horseshoe crabs are omnivores and eat the following:
- Small fish
- Small crabs
- Insect larvae
They do not have jaws to grind up food, so they rely on the bristled legs to grind the food and on the chelicerae to place it inside the mouth.
From there, the food goes through the esophagus to the proventriculus (the stomach).
The stomach consists of two parts – the crop, which expands to fit the food, and the gizzard, responsible for grinding the food until it’s easy to digest.
Sometimes horseshoe crabs may ingest sand and gravel, which aids the gizzard in grinding the food.
If this sounds familiar, you may be thinking about birds, as they’re known to swallow stones for the same reason!
Reproduction and Life Cycle
The beginning of the breeding season for horseshoe crabs depends on their distribution.
For example, those on the U.S. northeastern coast breed during spring or summer, whereas those living in warmer waters breed year-round.
It has also been suggested that the nesting season is influenced by the high tide and the full and new moons.
Males attach themselves to the females using their front claws, and as the female lays the eggs in a nest burrowed in the sand, the male fertilizes them.
Sometimes multiple males hold onto one female, although this depends on the species.
Mangrove horseshoe crabs, for example, are monogamous.
Females lay up to 120,000 eggs in batches of several thousand.
The eggs hatch roughly two weeks after fertilization.
The newly hatched larvae have no tail.
They molt six times over the next year and likely undergo a growth rate of 33% each time they molt.
As they mature, the molting frequency decreases, and they move to deeper water.
Horseshoe crabs become sexually mature at approximately nine years of age and can live up to 20–40 years.
Ecological Role and Interactions
Without a doubt, horseshoe crabs play a significant role in maintaining a balanced ecosystem because their eggs serve as food for at least 11 bird species, among which are ruddy turnstones, red knots, semipalmated sandpipers, dunlins, dowitchers, black-bellied plovers, and semipalmated plovers.
Besides birds, finfish and sea turtles also feed on horseshoe crabs or their eggs.
For example, 42% of a loggerhead turtle’s diet is represented by horseshoe crabs.
Additionally, many creatures, including mussels and oyster spat, attach themselves to horseshoe crabs, either because they’re opportunists or trying to survive.
Horseshoe crabs have been spotted interacting with mud crabs and sand shrimps – or at least, they engaged in something that can be called an interaction.
Conservation Status and Threats
Here’s how each horseshoe crab species is listed on the IUCN Red List:
- Limulus polyphemus – Vulnerable; threatened by harvest for bait and for biomedical industry, collection for marine life fishery, habitat loss, water pollution, and climate change
- Carcinoscorpius rotundicauda – Data Deficient; these horseshoe crabs are caught primarily for their roe, which is a delicacy, and for their blue blood used in biomedical sciences
- Tachypleus gigas – Data Deficient
- Tachypleus tridentatus – Endangered; threatened by loss of tidal flat habitats and loss of ecosystem biodiversity, as well as by overexploitation as material for chitin, for its blood that is used in medical industries, and for its meat that is used as food; some populations are also threatened by illegal trade and habitat loss caused by the construction of seawalls and other coastal defenses alike; the population in Taiwan is affected by the invasive smooth cordgrass, whereas others suffer from climatic changes, primarily from rising sea levels and rising temperatures.
Considering that these creatures survived multiple extinction events and have an evolutionary history of more than 400 million years, the fact that two of the four extant species face so many threats and register a population decline is highly concerning.
Luckily, multiple conservation efforts are in motion, which, hopefully, will help steady their populations.
Unique Adaptations and Survival Strategies
If you ask us, everything a horseshoe crab possesses is a unique adaptation that helps it survive!
Starting from its two compound lateral eyes (and the other eyes!) and ending with the protective carapace, book gills, and multiple pairs of appendages.
Besides this, horseshoe crabs are highly adaptable creatures that thrive in various environments.
Even horseshoe crab larvae can handle harsh conditions; it is believed they’re even more resilient than juveniles and adults!
Another thing these arthropods rely on to ensure their survival and increase their chances of reproducing is their burrowing behavior.
Since they’ve survived for millions of years without changing or diversifying too much, it is obvious that their physiology does its job well!
Cultural Significance and Human Interactions
Since horseshoe crabs have been around for millions of years, it’s only natural that they’re depicted in multiple stories, folklore tales, legends, and works of art.
One ancient Japanese legend says that warriors who sacrificed themselves in battle were reborn as horseshoe crabs and were responsible for protecting the waters surrounding their homes.
Other tales portray horseshoe crabs as protectors against evil.
This belief translates into fact when discussing medicine, as horseshoe crabs are collected for their blood, which is used for creating drugs that treat diseases like gastroenteritis and detect endotoxins.
Additionally, a protein found in their blood may be used for the development of a new antibiotic.
Ever since scientists discovered the properties of horseshoe crab blood, these creatures have been increasingly exploited for various industries.
They are collected from the water, drained of approximately 30% (and possibly more) of their blood, and then released back into the water.
Only a few horseshoe crabs survive this, and even if they do, they may not be able to reproduce at the same rate.
In some countries, people collect horseshoe crabs for their meat and roe.
Future Prospects and Research
The blood of horseshoe crabs is the subject of hundreds of research papers.
Thousands of specialists in the medical and biomedical industries are fascinated by these creatures!
They are ready to develop new drugs using their blood.
We should undoubtedly expect breakthrough discoveries that can change the course of medicine.
Nonetheless, as useful as their blood can be in treating various diseases, let’s not forget that people win in this regard only if the horseshoe crab population declines.
Conservation efforts are a must to avoid their extinction.
It would be a pity to lose one of the world’s oldest creatures!
Needless to mention, a possible horseshoe crab extinction will also affect their marine and brackish ecosystems.
With lineages dating back hundreds of millions of years, horseshoe crabs are now among the most famous chelicerates, not only because of their resilience and unique adaptations but also because of their blue blood, known to have medicinal properties.
Often described as living fossils, horseshoe crabs are found in shallow coastal waters in the Americas and some parts of Asia.
They’re mostly renowned for their strong carapace and long, thin tails.
These arthropods feed primarily on worms and mollusks.
Since many shorebird species prey on horseshoe crab eggs, these creatures have a significant ecological role.
Unfortunately, two of the four horseshoe crab species are threatened by habitat loss, collection for medical and biomedical industries, climate change, pollution, invasive species, and many other factors that affect their populations negatively.
The other two species populations may be declining, too, except that specialists haven’t been able to outline a clear tendency yet.
As such, it is of the essence to spread awareness and acknowledge the beauty and uniqueness of these creatures.
Every small action matters!
Are there horseshoe crabs in Europe?
If we judge by each species’ distribution, we can confirm that there are no horseshoe crabs in Europe.
Historically, however, they were spotted in the North Sea and the Baltic Sea.
Do horseshoe crabs feel pain?
Specialists are now beginning to raise awareness and inform people that there’s a high chance that horseshoe crabs do feel pain.
Are horseshoe crabs violent?
Horseshoe crabs aren’t aggressive. They don’t bite, pinch, or sting.