The Way-Back Creature

Visit South County, RI this summer and you may catch a glimpse of these prehistoric creatures looking for a little love in our oceans and salt ponds. Don't be afraid, these hard-shell critters are completely harmless. In fact, watching their mating ritual every spring and summer under a full moon is a fun educational activity. Contact us for more information on when and where to watch horseshoe crabs spawning or for a tour of our salt ponds. This month, guest blogger and naturalist, Mark Bullinger shares his knowledge and photos of the horseshoe crabs spawning.

Solo Horseshoe Crab, photo by Mark Bullinger
It’s pretty unusual to see a female without a mate, but we see many of the smaller males motoring around looking for a female to literally hook up with.

For more than 400 million years it has wandered the oceans.  It was around in recognizable form 350 million years before T-Rex and 270 million years before Stegosaurus.  It has survived multiple extinction events and so successfully filled its niche that it has barely evolved through the immensity of years since it first appeared in the fossil record.  And today, when you visit Rhode Island, you may encounter this creature – a living dinosaur going about its business in our coastal estuaries – engaging in routines and rituals that have seen it through the extremes of time.  Who is our Ordovician friend?  Why Limulus Polyphemus, a.k.a. the horseshoe crab.

The horseshoe crab is a common visitor to our coastal salt ponds.  Like all crabs it is an arthropod, with jointed legs and an exoskeleton, but is more closely related to spiders and scorpions than to common crab species.  It is a close cousin to trilobites, which are one of the oldest fossilized species known.  Perhaps you have seen a horseshoe crab, and perhaps you recoiled at the sight of its spiny shell and impressive sword-like tail.  If so, it would have been a false alarm.  The horseshoe crab is a benign creature, and although it isn’t exactly warm and cuddly it doesn’t have the capacity to harm a person.  It is not venomous; the tail (called the telson) is not sharp nor is it a stinger.  The main purpose of the appendage is to help right the animal if it is flipped over.  They can’t bite and even the pincher claws lack the strength to inflict pain.

In fact, the horseshoe crab is a fascinating creature and as long as you watch your fingers around the joints of its hard shell it is safe to pick up and examine.  The top of the creature presents a helmet-like shell with two compound eyes towards the front.  Flip it over and you can examine the business end of the animal.  Start with the legs: horseshoe crabs have ten of them and they are set up for different tasks.  On females, the front four on either side have small pincher claws.  Males have a similar setup, except the foremost legs have a boxing glove like appendage that is used to hook onto a female’s shell during mating.  Females are generally much larger than males, but checking for the difference in the front foot is the surefire way to distinguish between the sexes.  The rearmost legs on both male and female are called the pusher legs and they are used for propulsion.  The foot configuration on the pusher leg features flower-petal-like appendages that fan out to give the crab purchase on loose sand and propel it along at a smart pace.

The underside of a horseshoe crab

Ahead of the ten legs, each horseshoe crab has a pair of small appendages that feed food into the mouth.  It is the presence of these small arm-like feeders that puts the horseshoe crab in the same family as arachnids and scorpions.  Behind the legs, and nestled in the hinged rear section of the body, you will find the book gills, so named because they resemble the pages of a book.  They extract oxygen from the water, and as long as they are kept moist a horseshoe crab stranded by the tide can easily survive to the next high tide.  The book gills can also be waved like paddles to help the crab swim through the water column.  Please take care to place the horseshoe crab back in the water right side up.  It takes time for it to right itself, during which it is susceptible to being eaten by gulls.

This is a fine example of a male and female going about their business.  The large female is in front and the smaller male is latched on her back.  When she finds her spot, she will dig in and lay her eggs and the male will contribute his part.  Fertilization of the eggs takes place in the sand. Horsehoe crabs mating, photo Mark Bullinger

Horseshoe crabs mate throughout the late spring and a good part of the summer, but they are most active under the full moon in late May or early June.  In the case of Limulus it’s not the romantic qualities of the full moon that put them in an amorous mood, it’s that the tides run extra high at this time and the crabs can easily gain access to what are normally exposed beaches to lay their eggs. During this time - at around nine or ten p.m. and in the right locations - you can see dozens or even hundreds of crabs motoring around the shallows looking for love and just the right place to dig in and lay their eggs.  Warmed by the sun, the eggs will incubate for several weeks and then, when the beaches are again inundated by a full moon tide, the tiny new horseshoe crabs will join the water column.

Horseshoe crab pair, photo by Mark Bullinger
When she finds the right spot, the female horseshoe crab digs herself into the sand and starts laying her eggs – maybe up to twenty thousand in a night.  The male senses that the eggs are being laid and releases his contribution as they are deposited in the sand.

A female horseshoe crab will lay hundreds of thousands of eggs, but a large percentage will be eaten by migrating shore birds.  The most amazing example involves a medium-sized shore bird called a Red Knot.  Red Knots winter over near the very southern tip of South America and start migrating north in May.  They work their way up to Brazil, feed heavily to build up fat reserves, and then take off on an epic two-and-one-half-day non-stop flight to the mid-Atlantic coast – often around Deleware Bay.  Depleted of energy, as well as 50% of their body mass, they practically crash land on the shore just as the horseshoe crabs are laying their eggs.  By feasting on these tiny pellets of pure energy they double their body weight and then, after a week or two, continue on to their nesting grounds above the Artic Circle.  A journey of over 9,300 miles each way and only possible because of the horseshoe crab!

Large Group Horseshoe Crabs, photo by Mark Bullinger
Because fertilization takes place externally, multiple males can fertilize eggs from a single female

But the life-saving value of the horseshoe crab isn’t limited to migrating birds.  The blood of the horseshoe crab, which is blue and based on copper, has some very unique properties.  Because Limulus is such a primitive creature, it does not have an immune system based on antibodies.  Instead, it relies on an agent in its blood called Limulus Amebocyte Lysate (LAL), which is highly sensitive to a very common form of bacteria known as endotoxin.  When endotoxin comes in contact with the animal’s bloodstream, the LAL quickly clots, and binds to and isolates the contamination.  Pharmaceutical manufacturers utilize this unique component of horseshoe crab blood to detect bacterial contamination in intravenous products, which must not only be free of living bacteria, but also toxins that can be produced when bacteria are killed via a sterilization process.  An LAL solution, derived from the blue blood of our prehistoric friends, is placed in a test tube and a sample from a batch of pharmaceuticals is injected into the solution.  If there are bacterial impurities in the drug, the solution in the tube quickly gels and the batch is rejected.  LAL has also been used in the development of bandages and sutures and a variety of other medical advancements.

Horseshoe crab viewing, photo by Mark Bullinger
Full-moon viewing of spawning horseshoe crabs can make for a fun outing.  Groups equipped with boots and bright lights can patrol protected tidal shallows during the June or even early July nights and regularly find mating crabs.  Just take care to stay away from areas where the depth drops off quickly and where running tides might be a danger.

Horseshoe crabs can live several decades and older ones will often be covered with barnacles and/or slipper snails.  As they grow they will shed their shells many times and often what appear to be dead crabs are really molted shells that have been cast off and dried in the sun.  The horseshoe crab can also cover some territory.  Several years ago we found a mating crab that had been tagged.  I called the 800 number to report the details of my sighting and a few days later US Fish & Wildlife emailed a report on when and where the crab was tagged.  This individual had been tagged five-years earlier in Moriches Inlet – over 50-miles away on the south shore of Long Island.   Funny to think of it randomly bumping along the bottom, crawling over obstructions and setting off in random new directions and ultimately finding its way into our local salt pond.  What an amazing creature: a survivor, a savior, a living relic from ancient Earth - and right here in our coastal waters!  Keep your eyes open when you visit a southern Rhode Island beach and you might be lucky enough spot one, and to wade alongside and through a short stretch of history with a real live dinosaur!