Limulus - the Horseshoe crab

The Horseshoe Crab

Jonathan Lowrie

Calling a horseshoe crab a crab is a misnomer, since this distinctive arthropod is more closely related to spiders and other arachnids than to crustaceans. The horseshoe crab is truly a living fossil.  Its only living relatives are found in the East Indies, China and Japan.  But its earliest relatives lived in the Devonian seas more than 350 million years ago.  The genus Limulus, to which the modern horseshoe crab belongs dates back to the Triassic period, the same time as the first dinosaurs. .  Because they are so unique, they are cannot be confused with that of any other creature.

The classification of the Horseshoe crab is as follows: Phylum Arthropoda, Class Merostomata,  Family Limulidae, Genus Limulus, Species polyphemus . An adult female Limulus will attain lengths of 24 inches.  Most first time encounters can be rather scary, because they also have a very long spiked tail. Contrary to public opinion, the tail is quite harmless and the horseshoe crab should never be picked up this way. This unique creature lives on sandy or muddy bottoms.  Because of its propensity to burrow, it prefers a softer sediment.  It frequents intertidal and sub-tidal regions, rarely going deeper than 75 feet.  The Atlantic Horseshoe crab may be found from the Gulf of Maine all the way to the Gulf of Mexico. They start of life very light in color.  When small, they are a sand color and, as they molt and grow older, they darken.  After the terminal molt, they are a deep brown color.

The horseshoe crab has six pairs of appendages.  The first pair is called the chelicerae, followed by 5 pairs of legs, the first of those being called pedipalpi.  The pedipalpi also act as modified claspers in male horseshoe crabs.  The last pair has a special adaptation to facilitate digging into the substrate.  The mouth has no appendages of its own, quite different from true crabs. The chewing mill is the opening of the mouth, and is located at the base of the legs. The only other appendages are the 5 pairs of book gills, so named because they are large, and sheet like and resemble the pages of a book.  The eyes of Limulus are two lateral bump like protrusions and are not stalked.  There is also a third median eye, which is located beneath the foremost spine on the horseshoe-shaped prosoma.

The typical diet of Limulus is clams, worms, and other invertebrates which it grinds with the burr-like bases of the walking legs, which surround the mouth. They collect their food via foraging.  These animals are an incredible sanitizer of the sea, as they will forage in the muck and consume both healthy animals and sick and injured animals. 

Limulus are also known for their seasonal migrations and breeding.  During the first full moon and high tide of spring, thousands of horseshoe crabs return to shore to mate.  The females dig small burrows in the sand to deposit the eggs, after which the male will release sperm to fertilize them.  The timing is such that the females dig and deposit eggs, and the males, further out in the water, release sperm as the waves come in to wash it over the deposits. One can find the many eggs masses along the beach in late spring.  The masses are found as clumps of greenish eggs, about 3 mm in diameter.  It takes only a few weeks for them to mature and be washed back out to sea, where the juveniles spend time as a miniature adults living a benthic lifestyle.

Horseshoe crabs travel mostly on the surface of the seafloor, but they can also swim to escape predation or to move around an obstacle.  They do this by swimming upside down; using the large carapace as a wing, they point their long tails in the direction they wish to go, and beat their kegs frantically.  Although not the most efficient means of propulsion, they can get off the surface enough to catch some waves and travel quite a distance down the beach.  Since they live in the tremulous region of the intertidal zone, they must have a means to right themselves when flipped by a wave.  Fortunately, Limulus has such a means - its tail.  The tail is a long dagger-like projection that may be 12 inches long.  It is covered with many smaller protrusions and spikes.  They will use this tail as a cantilever to right themselves when turned over.  Because of its sinister appearance, the tail sometimes has the reputation of being venomous or capable of stinging.  Neither is true - the worst injury once could sustain from a Horseshoe crab is stepping on the tail, an action which would probably not even break the skin.

Limulus has an interesting history as well.  It was once used en masse as a fertilizer.  Tens of thousands were harvested and spread on fields to fertilize for the summer harvests.  It was a cheap source of fertilizer, since the flesh of the Limulus is inedible to humans and they are considered a nuisance species.  Most clam and oyster farmers dislike the presence of Limulus because they can disrupt their beds and, in the summertime, the beaches are clogged with thousands of crabs.  Trawlers also dislike the tons of crabs that take up valuable net space each year.  So, these animals were collected with little regard.  It was not until the 1970’s that scientists found a special use for Limulus.  Their blood.  Because of the unique properties of their blood, they make an excellent biomedical specimen.  In fact, they have a unique immune system. Because of this, their blood reacts very strongly to certain pathogens.  So, each year, small amounts of blood are collected harmlessly from these animals and used to test the purity of many vaccines, antibiotics and other injectible medications.  It is now illegal to purposely kill Limulus in most coastal states.

Limulus can sometimes be found for sale at  local fish stores.  Once one takes into account the reasons they are being sold and then looks at their natural lifestyle, it is apparent that this species is just not suitable for the home aquarium.  Often they are sold as ‘sand sifters’, and they do just that.  As mentioned, Limulus is an excellent burrower and will adeptly crawl though the substrate.  Herein lies the problem - they do this almost constantly.  Unless provided with a huge tank, they will soon be under the live rock formations, and they will easily topple over corals, and rock.  There are also dietary problems.  We keep live sand, trying to keep the bacteria, and infauna heavily populated.  By confining a creature that constantly sifts through sand for food, eating molluscs, worms, and such, it can easily wipe out a live sand bed of its beneficial populations.  In nature, they feed over many square feet, rarely returning to an area for many days.  But, in a 75 gallon tank, the small area will force Limulus to completely clean the sand bed of food.  Sadly, many times Limulus will slowly starve to death because it cannot forage enough food from an aquarium.  And since they are always offered as beige young animals, few people will ever realize they will reach 24 inches in length.

If one is already purchased, or if the resources for the proper habitat can be provided, the next obstacle is temperature.  Any organism that is found from the Gulf of Maine to the Gulf of Mexico can tolerate a wide range of temperatures.  Interestingly, the horseshoe crab cannot easily adjust to these large changes.  There are many separate and distinct populations of Limulus along the Eastern coast, so its critical to know the origin of the horseshoe crab and to what temeperature range it is best suited.  In the Gulf of Mexico, they can easily withstand 76 to 80 degrees.  Yet, the Limulus off the Carolina coast have a tolerance for the mid 70’s only, while those from further north require 60 degrees or cooler waters to survive. Because of their habits, movement, and propensity for growth, Limulus is simply not a species that should be kept in an aquarium. 

It is always fascinating to observe a true living fossil, but these creatures are best left in nature.  If one is fortunate enough to see one at the beach, please keep a few things in mind.  They are 100% harmless.  Never pick them up by the tail, but grasp firmly onto the carapace and hold them.  They will thrash about , ‘close up,” and try for form a ball, but eventually they will relax,  and one can then observe the book gills, chelicerae - and even tell if it’s a boy or girl.  In males, the first pair of appendages, the chelicarae, have a thickened claw, much like boxing gloves.  The females do not have this.  And if, by chance, on a spring time full moon night at an East coast beach, you happen to be walking along the sand, keep your eyes open for one of the most spectacular events you will ever see.