The horseshoe crab has been on Earth for 350 million years. An ancient and complex anatomy hides within its domed shell. From its 10 eyes to its tube-like heart, the horseshoe crab’s unique physique may surprise you.

SHELL

When you first look at a horseshoe crab, chances are the first thing that grabs your attention is its large, hard carapace, or shell. Like all invertebrates, the horseshoe crab lacks an internal skeleton. Instead, this external shell acts as an exoskeleton, providing structure from the outside and protection to the animal against predators or other threats. Made of the cellulose-like material called chitin, the shell is so hard that only sharks or sea turtles can penetrate it. The crab will shed its shell continually throughout its lifetime, as many as 17 times, including four times while still inside the egg.

The carapace of the horseshoe crab is made up of three sections: the cephalothorax, abdomen and tail. The largest section of the animal, the cephalothorax, houses parts of the intestinal tract, nervous system and circulatory system. The size of the cephalothorax differs greatly between males and females. A female’s cephalothorax can reach almost twice the size of a male’s. Attached by a hinge to the cephalothorax, the abdomen contains the musculature for the operation of the book gills and the tail. The tail is attached to the abdomen at the terminal base. Misunderstood as a stinger, the tail is not at all poisonous. It acts as the horseshoe’s rudder, helping it steer and right itself if it gets flipped on its back by the surf.

MOUTH & LEGS

Flip the animal over (gently) and you’ll easily see its six pairs of feeding and walking appendages. Starting from the front of the crab, the first pair of appendages is called the chelicerae. These are feeding appendages used to place food into the animal’s mouth. Going down the body, the next pair of appendages is the pedipalps. These are the first walking legs and they enable the horseshoe crab to move along the rugged seafloor. Each pedipalp has a small claw at the tip except the last pair, the pusher legs. This pair of legs is used for locomotion but also has been equipped with a leaf-like structure that is used for pushing and clearing away sediment as the crab burrows into the sea floor.

NERVOUS SYSTEM

Don’t let its hard exterior fool you. The horseshoe crab is actually quite a sensitive creature. This invertebrate uses a system of specialized nerves that extend from the brain throughout the body. Several large nerves supply the crab with information about its surroundings, including two optic nerves and eight pairs of hemal nerves that are spread throughout the body.

An interesting feature of the pusher leg is the flabellum, an organ that tests the composition of the water passing to the gill chamber. There are approximately one million sensory cells in this organ alone.

The horseshoe crabs uses its tail as a rudder, and to help it turn over when it gets flipped upside-down.

EYES

A total of 10 eyes help the horseshoe crab get around. These eyes are distributed around the body including on top of its shell, on the tail and near the mouth to help orient the animal when swimming.

Two compound eyes are easily seen on each side of the animal’s shell. The main function of this set of eyes is to find mates during the spawning season.

LUNGS AND HEART

On the horseshoe crab’s underside is a series of six page-like structures called book gills. These organs absorb oxygen from the water while keeping the water out. Each gill contains approximately 150 large flap-like membranes called lamellae that look like pages in a book.

The book gills are versatile organs used not only to breathe but also for swimming. Swimming is an alternative mode of transportation used in emergencies, mainly to escape from predators or if the animal finds itself in rough surf. The gills also function as paddles to propel juvenile horseshoe crabs through the water.

The horseshoe crab’s heart is a long tube that runs down the middle of the cephalathorax and abdomen, extending almost the entire length of its body. On average, the heart rate of the horseshoe crab is about 32 beats per minute.

The real dragons of today’s world stomp around like dinosaurs in the remote equatorial hills of their namesake island in Indonesia, Komodo. While these giant lizards may not fly, or breathe fire (although their bacteria-laden saliva is deadly), they are capable of a feat equally miraculous: virgin births.

In 2006, Flora and Sungai, two female Komodo dragons housed at the Chester and London Zoos, respectively, were the first discovered cases of virgin birth in the world’s largest lizard. They are examples of a process called parthenogenesis, the scientific term for single-parent reproduction. This process is almost never seen in animals as complex as the Komodo, and has only been documented in 0.1 percent of vertebrates, according to a report in the December 2006 issue of the journal Nature.

After the discovery, Richard Gibson, a curator of herpetology at the London Zoo, said in a National Geographic article that virgin birth is “considered a very rare phenomenon, but the fact that we’ve got these two lizards suggests it’s not as rare as we thought. We recorded it in two unrelated females within the space of a year in two different collections.”

Virgin births, by the process of parthenogenesis, happen when an unfertilized egg develops into an embryo using its two sets of maternal chromosomes. Scientists are more familiar with parthenogenesis in smaller invertebrates, such as aphids and zooplankton. In evolutionary terms, single-parent reproduction is not the best way to make babies. The gene pool of an isolated population dependent upon parthenogenesis becomes smaller, making it more vulnerable to disease and less adaptable to altered climate conditions or new predators.

There is, however, a unique advantage to the female Komodo’s self-sufficiency. As a denizen of a chain of desert islands in Indonesia, a female dragon might very easily be swept away from her island to another island where no other dragons live. Yet she still can reproduce parthenogenetically and keep the species going. Gibson explained that because of the genetics involved in lizard self-fertilization, the offspring are always male. As soon as those baby boy lizards grow up, they can then reproduce with their mother. Eventually, a male dragon from another island might wash up on shore and diversify the gene pool.

That’s not all that makes the Komodo remarkable. It also shares a mythical dragon’s predatory prowess. A member of the goanna family, with ancestors that date back more than 100 million years, the Komodo shares the feeding and dental characteristics of extinct dinosaurs, sharks, and sabre-toothed cats, according to a study released in the April 2008 issue of the Journal of Anatomy.

Given their ability to attack and butcher very large animal prey, one might assume the Komodo dragon would possess a steely, snapping jaw like an alligator’s and a dense, sturdy skull. The opposite is true. The Komodo’s physiology is distinguished by what scientists call a “space-frame” skull, made of a light, rigid structure with interlocking struts that can handle big loads. What’s key is the shape of the bones and the way bones of different strengths are arranged. Instead of clamping down like an alligator, the dragon rapidly yanks off chunks of meat, and its powerful neck muscles and space-frame skull support the forces involved. Sixty razor-sharp, serrated teeth help, too.

According to one of the scientists behind the report, Stephen Wroe of the University of New South Wales, the Komodo could do serious damage to even buffalo-sized prey. Quoted in an article in ScienceDaily, Wroe said, “The Komodo displays a unique hold and pull-feeding technique. Its delicate skull differs greatly from most living terrestrial large prey specialists, but it’s a precision instrument, beautifully optimized to make the most of its natural cranial and dental properties.”

The Komodo was discovered by Western scientists in 1910, and its total population in the wild is estimated at 4,000 to 5,000 individuals. It can grow up to 10 feet long (3 meters) and weigh more than 300 pounds (136 kilograms). Its tremendous size is a result of island gigantism, as Komodos are the apex predators, dominating the ecosystems in which they live.

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