Hidden Hong Kong: A look into Hong Kong’s horseshoe crabs

Classification and physiology

Although it bears the name of a “crab,” the horseshoe crab is more closely related to spiders and scorpions instead of crustaceans. Around the world, there exist four kinds of horseshoe crabs—Limulus polyphemus, Tapinauchenius gigas, Tachypleus tridentatus, and Carcinoscorpius rotundicauda—and the latter two can be found in our brackish waters.

Unlike humans, who possess a skeletal structure underneath skin and flesh, the horseshoe crab maintains its shape with an outer shell called the endoskeletal structure. Each horseshoe crab has six pairs of legs; the primary pair, closest to the mouth, functions like pincers to clamp food to its mouth. When used by males, the second pair firmly “grasps” the females during mating. It is the last pair of legs—also called the “pusher legs”— that generate the significant pushing force required to carry the horseshoe crab forward in its movements.

Crawling on the sand, horseshoe crabs take worms and molluscs as their food. Usually, they make their appearances during the summer and stay inside their burrows during the winter, all the while tracking the changing of the seasons through the water temperature.

Unusual homecoming rites

Horseshoe crabs are particularly “nostalgic.” No matter how far they travel out to sea, during the summertime, all adult horseshoe crabs unanimously embark on a homecoming trip back to their birthplaces to mate. A male horseshoe crab will hop on a female’s back, with the female laying mature eggs in the burrow. When the waves hit, the free-swimming sperms released by males are brought into the egg sites, completing the fertilisation process. After two to four weeks, the larvae will hatch and can then hunt for food in the nearby ocean.

But it is never easy for a horseshoe crab to become an adult. On its arduous journey to maturity, a horseshoe crab will shed its shell at least 16 or 17 times during the warmer months, with each rite of passage becoming progressively more difficult as its exoskeleton grows bigger and harder. A horseshoe crab takes 10 years to fully reach adulthood and can grow up to two feet in length. Its greyish-green shell gives it a tank-like appearance.

When horseshoe crabs mature into adults, they usually enter into deep waters several kilometres away from their nursery grounds. When it’s time to procreate, they revisit the same nursery grounds, just like their parents and generations of ancestors have done.

Remarkable longevity

On your first encounter with a horseshoe crab, the plain-looking shell and grotesque pairs of legs may bear an uncanny resemblance to prehistoric beasts, but these unsettling feelings are well-justified as horseshoe crabs are, in fact, millions of years old.

Our oldest fossil record of a horseshoe crab dates back to the Late Ordovician period, around 445 million years ago, and there has not been much change between then and now. If this is true, then horseshoe crabs are one of the only living creatures that have retained their primitive, prehistoric features while bearing witness to the extinction of the dinosaurs, trilobites, and eurypterids, which are thought to be sister animals to the horseshoe crab.

In addition to a family lineage spanning millions of years, horseshoe crabs are fairly long-lived. Horseshoe crabs can live up to 20 years, and it is hard to tell the age unless through indirect means, such as tagging or ageing the symbiotic organisms plastered on their shells.

Scientific groundbreakers

Despite their ancient lineage, the horseshoe crab’s eye structure is actually quite similar to that of humans. In fact, the fundamental understanding of our visual system is greatly indebted to the horseshoe crab research done by scientist Haldan Keffer Hartline.

In one experiment, Hartline isolated a light-sensitive cell unit from a horseshoe crab’s eye and detected an electrical current. Probing through the cell, he discovered depolarisation along the cell membrane, which coincided with the inner workings of a nerve impulse. It established the mechanism with which all animals can “see,” and that the visual stimuli are converted into nerve impulses, which the brain then processes and generates into images.

In another experiment, Hartline studied the eye of a horseshoe crab and found that the optic nerve response decreased in the presence of ambient light. He realised that activating one photoreceptor region would inhibit the optic responses nearby. Called “lateral inhibition,” this unique natural mechanism is designed to enhance the contrast between light and dark areas and increase the definition of an image. Haldan Keffer Hartline, along with his research centred around horseshoe crabs, was awarded the Nobel Prize in Physiology or Medicine in 1967, honouring his work of furthering modern understanding of the optic nerve network.

Sacrifice for humans

In the past, Gram-negative bacteria (GNB) used to be a major headache in the medical and pharmaceutical realms. Gram-negative bacteria are so small but ubiquitous that over one million of them are swimming in just a millilitre of seawater! Also, they produce a lethal substance called endotoxins, which can cause fevers, intravascular coagulation, or organ failure if they enter the bloodstream via medical procedures.

But horseshoe crabs have lived with this bacteria long enough to develop a specific defence mechanism. Known for its distinctive baby-blue colour, the blood of the horseshoe crab contains amebocyte cells that form gel clots when exposed to endotoxins, even in one part per trillion. Using horseshoe crab blood, this unique characteristic was used to develop the limulus amebocyte lysate (LAL) test and tachypleus amebocyte lysate (TAL) test, two methods which are now the industry’s standard for detecting bacterial contamination in injectables, surgical implants, drugs, and more. Each year, more than 400,000 horseshoe crabs are bled for this purpose, protecting human lives at a “bloody” cost.

On the verge of extinction

For all their crucial contributions to humanity, horseshoe crabs still face the death knell of extinction. In Hong Kong, local horseshoe crabs have been drastically dwindling in number. In 2009, authorities recorded a 90 percent plummet in the density of juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda, compared to data from 2002. In the most recent findings, experts estimate that only around 2,000 juvenile Tachypleus tridentatus and 2,400 juvenile Carcinoscorpius rotundicauda still call the city home.

Several threats are locally present, such as the loss of spawning grounds and habitat, environmental pollution, and overharvesting for culinary value or biomedical use. Other extenuating factors include the extremely low survival rate of horseshoe crabs bred in the wild (around 0.001 percent) and the surrounding shorebirds feeding on their eggs. One local species, Tachypleus tridentatus, has already been flagged as endangered, with several organisations working to conserve the horseshoe crab breed before it slips away forever.

Although Hong Kong is known for its populated urban sprawl, local animals like the horseshoe crab deserve to be protected and treasured. In addition to housing millions of people, the city’s capacity as a nature reserve is equally important. Without our efforts to try and conserve the local horseshoe crab, it may well fade from our shores.