Category Archives: Evolution

Darwin, Wallace, and Natural Selection – A Short Film

Below is an HHMI BioInteractive short film on the epic voyages of Darwin and Wallace that led each to independently discover the natural origin of species and to formulate the theory of evolution by natural selection.

Many textbooks are smothered with Charles Darwin as being the “father” of natural selection,  rarely mentioning Alfred Wallace at all. In scientific circles, both men are credited with arriving at natural selection theory independently.

It is important to note that their theories were not entirely identical. Darwin’s theory is more focused on individual struggle while Wallace’s tends to concern itself with populations and groups. Wallace possessed more combined field time than Darwin, and in my opinion, Wallace explored more smaller isolated lands than Darwin which led to the Wallace Line – an imaginary boundary line separating ecozones that explains the differentiation of species among groups.

Wallace Line
Wallace Line

There is a richer backstory to both Darwin and Wallace, but to end this blog post, be comforted in knowing that they remained friends, despite minor disagreements. For example, Darwin believed that animal breeding could demonstrate natural selection, but Wallace always disagreed, arguing that such demonstrations are examples of artificial selection rather than natural selection (Wallace was correct). The short film below highlights how both men arrived at their theories.

The Environmental Adaptations of Limulus polyphemus

The Environmental Adaptations of Limulus polyphemus 

by antonio kuilan



Limulus polyphemus, or the American horseshoe crab, has survived millions of years residing in its aquatic habitat along the Eastern coast of the United States. The organism resides in sandy, murky, and/or muddy waters. Its book gills, appendages, and its pair of pusher legs are direct adaptations to this environment. The distal ends of the pusher legs have leaf-like processes that are adapted to push its way through substrate and used for another adaptation: burrowing. The burrowing behavioral adaptation allows L.  polyphemus to protect itself from desiccation, hunt for subsistence, and to excavate nests to lay eggs. Laying eggs on land is a behavioral adaptation that allows eggs to be protected from sea predators and for quicker development due to warmer variations exhibited on land. Females communicate with males using pheromones and males have adapted its pedipalps to clasp on the female’s carapace during mating. L. polyphemus has survived in microbe rich environment with its powerful blood that coagulates when encountering bacteria and L. polyphemus is adaptively tolerant to climate and salinity. It grows by the process of molting, going through 16-17 molts by the age of ten years – emerging 25% to 30% bigger after each molt. Its multiple eyes have adapted to see through murky waters, assist in mate finding, and even synchronizing its circadian


The American horseshoe crab, known by its scientific binomial name Limulus polyphemus, has had a legendary journey through geologic time. Its early ancestors first appeared early in the Ordovician Period, blooming into the Silurian Period, giving rise to the first early horseshoe crabs. Several species arose and became extinct from the early warm Carboniferous through the great tectonic activities of the Devonian. The Permian pounded many organisms into extinction, but the horseshoe crab survived and flourished. Limulus polyphemus has forged its presence, continuing to coast through the Mesozoic and Cenozoic era – still standing alongside man today. This arthropodal warrior has earned the nickname the “living fossil.”

We will take a short voyage into the life of this ancient survivor and discover its morphology, physiology, behavior and adaptations that have allowed it to thrive in its environment.

The Taxonomy of Limulus polyphemus

A brief taxonomic classification of the extant species of Limulus polyphemus is as follows:

Kingdom: Animalia
Phylum: Arthropoda – Animals with hard exoskeleton and jointed legs.
Subphylum: Chelicerata – From greek; “cheli” meaning claw and “cerata” meaning horn. Animals with no jaws.
Class: Merostomata – Animals with their legs surrounding a ventral mouth.
Subclass: Xiphosura – From Greek; “xiphos” meaning sword and “ura” meaning tail.
Order: Xiphosurida – Animals with sword-like tails.
Family: Limulidae
Genus: Limulus – Meaning somewhat sideways. Referring to the side compound eyes.
Species: polyphemus – From Greek, the one-eye giant. Referring to the median eyes.

Morphology & Physiology


L. polyphemus has ten eyes in total (see Figure 1), used for mate finding and sensing light. The 2 immediate noticeable eyes are the lateral compound eyes, which are used in mate finding during the spawning season.


Two eyes known as the median ocelli are located on the midline near the outer carapace. The medians are most sensitive to the ultraviolet light from the moon and stars, which increases the efficiency and sensitivity of the lateral compound eyes by sending signals to the brain. This makes its lateral eyes more sensitive at night, making L. polyphemus vision significantly better at night instead of the day. The only eyes of Limulus that have lenses and form images are the median ocelli and the lateral compound eyes.

Closeup of the lateral eyes
Closeup of the lateral eyes

The rest of the “eyes” on L. polyphemus are primitive photosensitive organs. The rudimentary lateral and endoparietal eyes are prominent in Limulus less than a year old and thought to provide them with an ability to orient light before the median ocelli and lateral compound eyes are fully developed.

Two ventral eyes are located in the ventral side of L. polyphemus near its mouth. It is suggested that they send neural messages to the brain about light intensity underneath it, being useful in signaling whether L. polyphemus is right side up or upside down. It is important to note that the functions of the ventral eyes and the other primitive photosensitive organs are not well understood  and the ventral eyes may be vestigial eyes.

The last tenth eye is actually an array of photosensitive organs along the telson (tail) which have an important role in the species life. The output of the photosensitive organs to the brain helps synchronize its circadian clock, regulating certain biochemical, physiological and behavioral processes to the day/night cycle.


Due to its intimidating appearance, the telson, or tail, is often mistaken as a defensive mechanism. Besides the telson being home to the photosensitive organs, its true function is to turn L. polyphemus over should it find itself upside down. If upside down, L. polyphemus will ball up and use the telson to dig in the sand, attempting to push itself over.


L. polyphemus has six paired appendages on its ventral side (see Figure 1). The chelicera is the first pair and are the two most centered over the mouth area (see Figure 1.2). L. polyphemus uses this pair of pincers to place food in its mouth.

Horseshoe crab ventral view close up showing appendages_700

Pedipalps are the second pair and first ambulatory legs. On a female, the pedipalps possess typical claws on the end, but for a male they contain a grasping claw  (see Figure 1.5). These grasping claws allow the male to clasp itself to the two areas on the rear of a female’s opisthosoma during spawning.

The next three pairs are used for digging and walking. The last pair of appendages is used for locomotion. The legs can spread outward, giving L. polyphemus an advantageous leverage when pushing itself across sandy bottoms. Sometimes called the pusher legs, they do not have claws; instead they possess four leaf-like processes (see Figure 1.6).



Our friend Limulus possesses five pairs of gill plates called branchia (see Figure 1.4), which is composed of 150 to 200 leaflets referred to as lamellae. The 150-100 lamellae on the branchia resemble pages in a book, therefore are commonly called “book gills.” Gaseous exchange occurs on the surface of the lamellae when the book gills are actively irrigated by seawater.



Though its primary function is for breathing, the book gills can also be used for swimming (see Figure 1.7). It can flap the gills while upside down, allowing it to be propelled through water. However, it can do so for only short distances and usually done by the cover of darkness or in emergencies – such as a predator.


L. polyphemus can survive on land, given that it manages to maintain the gills wet, but only up to four days.


Limulus polyphemus inhabits the western Atlantic coastline and Gulf coasts, from Nova Scotia, Maine to Florida, down to the Yucatan Peninsula (see Figure 2.0).

They are particularly bounteous in the coastal areas between New Jersey and Virginia, and most notably in the Delaware Bay. In fact, the largest population of L. polyphemus can be found at the Delaware Bay (see Figure 2.1). They populate within a radius of 50 miles from the mouth of the bay, because the bay is a lengthy 140 miles of uninterrupted sandy beaches – an area secluded from heavy rains and strong winds from the raw ocean.

The benthic arthropods frequent the sandy shallow bottoms of these areas for most of the year. As fall approaches, adults may stay in bay areas or migrate to the Atlantic to depths at under thirty meters, where they will spend the winter on the continental shelf.



L. polyphemus generally is a heterotrophic scavenger. Though the young do not feed until their first molt, adults will plow through sediments eating almost any food item they encounter. Some of its food include worms, decaying organisms, mollusks, and algae. They scrape the algae organisms off the bottom rocks.

Since L. polyphemus lacks jaws, it uses its legs to grab, crush, and chew its food. It uses the chelicera to place food in the gnathobase, which grinds it up before entering the mouth (see Figure 1.2).

A fascinating aspect of L. polyphemus is that they can also find their prey by using chemical receptors located on their claws and gnathobases. There are three million receptors on the chelae and one million receptors on the gnathobases which provides a sensory input into its brain. Using its legs and gills, it can stir up sediment and water, allowing them to “sniff” chemicals in the water with its receptors, capturing any prey with its legs.


Females communicate with males (using pheromones (natural chemicals) – signaling that it is time to mate.


mating hcdelawarebayEvening mating is preferential  with peak spawning occurring at high times with the full and new moons overhead. As the moon cast its glow on the evening coasts, adults travel from deeper waters to shorelines to mate. Thousands arrive on inlets and bays, such as the Delaware Bay area, using the light receptors on their tails to determine movement and changes in moonlight.


horseshoe-crab-eggs-9016Using their compound eyes, they find mating partners. The female oozes a jelly-like substance, chemically signaling the male. The male attaches to the female’s carapace using the pedipalps from its second pair of appendages. Every few feet, the female pauses to excavate a nest to deposit as many as 20,000 eggs (see Figure 2.4). With the male still on the back, the female pulls him over the nest, where it fertilizes the eggs with its sperm.

After a few more episodes of this ritual, the spawning is completed. Both parties depart, and the waves brushes sand over the nests. The mating rituals lasts from spring to early summer, only to repeat itself every year, until the death of the organism.


 larvaeL. polyphemus chooses beach habitats to nest, notably those around inlets and bays because of its porous, well-oxygenated sediments. This provides a perfect environment for the eggs’ survival and development.  After developing in the sediment and molting 4 times within the embryo, the eggs begin to hatch around 4 to 5 weeks later (see Figure 2.5). The freshly hatched buried larvae may stay in the sand for weeks before emerging – which then begin swimming for hours in a frenzied state.

Molting and its Process

hs-crab-molt peconic esutary long islandL. polyphemus grows by molting (see Figures 3.0 & 3.1). Twenty days after hatching, the larvae will molt for the first time. During the early molting stages of its life, it will reside in the intertidal flats, continuing to molt 5 or 6 times in its first year of life, and molt about once a year as it ages thereafter. L. polyphemus will have gone through 16 or 17 molts during its developmental years. Roughly at 10 years of age, it has reached adulthood and will start to move out to deeper waters.


Thus far, adults are not known to molt.

Prior to shedding their old exoskeleton, L. polyphemus generally burrows itself in a wet area. It pulls in and engorges itself with water through the book gills. This process causes the exoskeleton to split along the exuviation suture line (see Figure 1), allowing L. polyphemus to emerge (see Figure 3.0) with a new soft shell. In about 24 hours, the soft shell will harden. Larvae can emerge within 15 minutes after the suture line has opened, but the molting process will take more time and become difficult with each succeeding molt. L. polyphemus grows an estimated 25% to 30% after each molt.

Other Adaptations


Burrowing (see Figure 3.2) is an adaptation that is favorable in many aspects in the life of L. polyphemus, including seeking food, migrating, excavating nests, or to prevent desiccation when exposed by an ebbing tide. Burrowing during the mating ritual allows its eggs to rest in the sediment, providing more protection for the eggs and a higher chance of survival.

16 - Horseshoe Crab

L. polyphemus starts burrowing by moving the sand centrally and backward with its first four pairs of legs – this begins to create a depression. It then starts to push the collected matter away from its rear by using the last pair of legs, typically called the pusher legs. As we recall from earlier, the pusher legs do not possess claws, but instead four leaf-like processes (see Figure 1.6), which allow for easier pushing of material. With its rear prosoma slightly sinking into the depression, it straightens its body and backward thrusts with its legs, pushing the front of the prosoma into the substrate. The front of the prosoma is slightly arched; when L. polyphemus flexes and thrusts the prosoma forward into any substrate, the edge acts like a bulldozer.  This process is repeated a few times until it reaches its desired depth, which varies. But according to another source, L. polyphemus commonly stops when the substrate reaches below the lateral compound eyes or completely cover. That source also states that the receptors on the appendages enable it to gauge the extent of the burrowing.

Being interested in the amount of time it takes for a burial and not finding the answer in my sources, I viewed four videos of L. polyphemus on the Internet regarding its burrowing behavior. It is a relatively slow process, taking three to five minutes for a near complete burial. Here is one burrowing captured on video:

With its environment being sandy and muddy, the prosoma and pusher legs have adapted to this environment, an advantage to L. polyphemus.


The blood of L. polyphemus possesses a unique adaptive characteristic that is one reason they have survived for millions of years.  Unlike mammals, L. polyphemus do not have hemoglobin in their blood, but instead contains the copper-based compound hemocyanin to carry oxygen. The blood contains amoebocyte lysate and helpful enzymes that work together to create a powerful immune system. The enzymes within the blood detect intruding bacteria and the amoebocytes quickly surround them, creating a coagulating barrier that protects L. polyphemus from harm.

Sea water saturated with microbes and environmental pressures led to the development of this physiological adaptation as a method to protect itself.

An American Horseshoe Crab being ‘milked’ for its unique blood.

It is interesting to note that this blood-clotting effect when exposed to bacteria works so well, that it has been used for decades by the pharmaceutical and medical industries, including NASA. One quart of its “blue blood” yields an estimated $15,000 U.S.


With its coastal environment welcoming the ocean waves, L. polyphemus may be occasionally turned over by a strong one. The telson is a structural adaptation to the environment, extremely helpful to turn it over to its correct position. While the telson is also proven to be effective in maintaining the balance of L. polyphemus on land, it’s also suggested that the telson may also be used for mobility when swimming in its environment.

Tolerance to Habitat

A significant physiological adaptation of L. polyphemus, is the ability to tolerate salinity levels in its environment. This has allowed them to survive variations in salinity (though low salinities less–than four parts per thousand–become lethal) and move to safety.

L. polyphemus can also tolerate a wide range of oxygen levels. Physiological adaptations in the blood enabled them to survive hypoxic condition when spending winter on the continental shelf or when partially buried during spawning. They can also survive hyperoxic conditions when exposed to air. These physiological adaptations allowed L. polyphemus to cope with periods of oceanic deoxygenation – periods that were fatal to much of marine life.

Laying Eggs on Land

The behavioral adaptation of laying eggs on land instead of the ocean is an important one; it has allowed L. polyphemus to survive eras through the current day. In the open ocean, exposed eggs are prone to predators and would take longer to develop.

Environmental pressures have caused L. polyphemus to evolutionary develop through long periods of time until it could walk on land.  On land, the eggs develop quicker because of higher temperatures and have a higher chance of successful hatching due to eggs being protected by sand when burrowed. It is noted that another pressure might have been the climate. Climate would begin to decline as Earth’s tectonic plates moved northward. To ensure survival of the eggs, they would begin to lay them on warmer land.


127701sLimulus polyphemus seems to be an indestructible organism. It has survived three extinction events, marched through the age of dinosaurs, the dawn of humans, and continue to thrive today. Wearing its prosoma like a warrior’s helmet, it plows through sediments, using the adapted distal leaf-like ends to push through, searching for food with an army of receptors on their claws. Navigating and mate finding with its ten eyes when heading for shore, it suddenly overturns by rouge wave. It wields its harmless telson sword and turns itself over. Its own blood acts like a liquid warrior, fighting the slightest foreign organism – one of the many adaptations that allows the species to thrive in its environment. Its journey through time is a testament. To understand that its blood is being used by the medical industry to save lives, employed by NASA to test spacecraft surfaces for bacteria–even preparing the blood for use in the search for life in other planets–is quite fascinating. This humbling warrior may yet have something profound to offer humanity.



Barlow, Robert B.  1981. Ultraviolet Responses of the Limulus Median Ocellus. Biological Bulletin 161: 352-353

Berkson, Jim, Smith, Stephen A. and Walls, Elizabeth A. The Horseshoe Crab, Limulus polyphemus: 200 Million Years of Existence, 100 Years of Study. 2002. Review in Fisheries Science 10:39-73.

Department of Natural Resources, Maryland, United States. 2010. Horseshoe Crabs: A Living Fossil. Retrieved October 24, 2012, from

Eldredge, N. 1970. Observations on the Burrowing Behavior in Limulus polyphemus, with Implications on the Functional Morphology of Trilobites. American Museum Novitates. 2436: 17 pp.

Fahrenbach, Wolf H. The Morphology of the Horseshoe Crab (Limulus polyphemus) Visual System. 1981. Cell and Tissue Research 216: 655-659

Fredericks, Anthony D. 2012. Horseshoe Crab: Biography of a Survivor. Ruka Press, Washington DC.

Shuster, Carl N., Barlow, Robert B. and Brockmann, Jane H. 2003. The American Horseshoe Crab. Harvard University Press, Cambridge.

Waters, John F. 1970. The Crab From Yesterday: The Life-Cycle of a Horseshoe Crab. Frederick Warne and Company, New York.