Bio Facts: Crab, Horseshoe
Atlantic Horseshoe Crab
East coast of North America from Nova Scotia (Canada) south to the Yucatan Peninsula (Mexico)
Shallow ocean waters on soft sandy or muddy bottoms
The shell is a greenish-brown, horseshoe-shaped convex carapace; the tail (or telson) is spike-like. The shell may reach a length of over 24 in.; on the underside around the mouth are five pairs of dark brown legs. In front of the mouth is a pair of pinchers. Six spines stick out from each side of the part of the carapace over the abdomen. Sexes are similar in appearance, but females are typically 25 to 30 percent larger than males.
20 – 40 years
8 – 10 years of age
In the wild, they feed on mollusks, annelid worms, other benthic invertebrates, and bits of fish; in the Zoo, they are fed a scientifically developed, commercially available diet, shrimp and fish.
IUCN – Near Threatened
Horseshoe crabs spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn. Females release a pheromone to attract males. When a male (smaller than the female) finds a female, he grabs on to the back of a female with a “boxing glove” like structure on his front claws, often holding on for months at a time. Often several males will hold on to a single female. Females reach the beach at high tide. After the female has laid a batch of eggs in a nest at a depth of 6 – 8 inches (15–20 cm) in the sand, the male or males fertilize them with their sperm. Egg quantity is dependent on the female’s body size and ranges from 15,000 - 64,000 eggs per female.
“Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule” (Sturtevant). The larvae form and then swim for about five to seven days. After swimming they settle, and begin the first molt. This occurs approximately twenty days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. Before becoming sexually mature around age 9, they have to shed their shells some 17 times. Longevity is difficult to assess, but the average lifespan is thought to be 20–40 years.
Horseshoe crabs molt, as do all of the crustaceans. The shell splits open and the crab emerges, now larger by a fourth than before. These empty “molts” may be found on the beach.
Newly laid horseshoe crab eggs are opaque, pastel-green in color, and about 1.5 mm (1/16 inch) in diameter. After fertilization, the eggs begin to develop into trilobite larvae. By day five, miniature legs are visible inside the translucent egg.
On day six, the larvae molt for the first time. If you look closely with the aid of a microscope, you won’t find a tail, but you will see each larva surrounding a small sac. This is the yolk — the only source of food available before the larva hatches.
On day seven, the outer membrane of the egg ruptures and the inner membrane swells to replace it. By the end of the second week, larvae have molted twice again in preparation for hatching. Thus, in total, the horseshoe crab undergoes four molts within the egg.
Ideally, the moisture supplied by the tides and the warmth of the sun allow the eggs to mature and hatch in the two-week period between spring tides (the higher-than-normal tides that occur at the new and full moons). In reality, however, it probably takes three or four weeks or even months for the eggs to hatch.
Upon hatching, the trilobite larvae dig their way out of the sand. They are approximately 3 mm (1/8 inch) across and look just like miniature adults, but lack a movable tail and functional compound eyes. Their digestive system is also not yet functional, and the baby crabs swim around for about a week absorbing the yolk sac as their digestive systems mature.
Around day 21, the larvae settle from the water column onto the soft sediments below. As they shed their shells, their bodies expand, a telson grows, and chitin hardens the new carapace. The juvenile horseshoe crabs now look like adults, but they are less than a quarter of an inch wide!
Horseshoe crabs initially molt an average of three or four times a year. Sub-adults (horseshoe crabs that are five to seven years old) appear to molt annually. Males are sexually mature at their sixteenth molt, which is usually their eighth or ninth year. During their final molt, they develop specialized clasping claws for holding the female during reproduction. Females need at least 17 molts, or one more than the males, so they mature in their tenth year or even later and are, on the average, 30% larger than the males. A small percentage of horseshoe crabs continue to molt after reaching sexual maturity.
Lacking jaws, horseshoe crabs grind their food with bristles on their legs and a gizzard that contains sand and gravel.
There is a large compound eye on each side of the prosoma with monochromatic vision, five simple eyes on the carapace, and two simple eyes on the underside, just in front of the mouth, making a total of nine eyes. The simple eyes are probably important during the embryonic or larval stages of the organism, and even unhatched embryos seem to be able to sense light levels from within their buried eggs. The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood. In addition, the tail bears a series of light-sensing organs along its length.
The body of a horseshoe crab is divided into three parts: the prosoma, opisthosoma and telson, or tail. The prosoma is the front, semicircular part of the horseshoe crab and combines the head and thorax under a hard shell or exoskeleton. The opisthosoma is attached to the prosoma with a hinge. The shell protects the gills and two genital pores located under the horseshoe crab.
The top or dorsal surface of the shell has ridges and depressions. These are locations where muscles are attached to the inside of the shell. Two large compound eyes are located on the prosoma, with other light receptors scattered all over the body.
Limulus means “askew” and polyphemus refers to the giant in Greek mythology. Polyphemus (Greek: Πολύφημος, Polyphēmos) is the gigantic one-eyed son of Poseidon, and his name means “everywhere famous”. The Atlantic horseshoe crab’s species name is based on the misleading idea that the animal had a single eye.
It is the tail that earns this order its name Xiphosura, which derives from the Greek for ‘sword tail’.
Despite their common name, they are not crabs but are related to arachnids (spiders, scorpions, ticks and mites), and are presumably the closest living relatives of the now extinct trilobites.
The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper-containing protein hemocyanin at concentrations of about 50 g per liter. These creatures do not have hemoglobin (iron-containing protein), which is the basis of oxygen transport in vertebrates. Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow, and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed. Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in vertebrates by hemoglobin in red blood cells.
The blood of horseshoe crabs contains one type of blood cell, the amebocytes. These play an important role in the defense against pathogens. Amebocytes contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial endotoxin is encountered. The resulting coagulation is thought to contain bacterial infections in the animal’s semi-closed circulatory system.
Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. The above-mentioned clotting reaction of the animal’s blood is used in the widely used Limulus Amebocyte Lysate (LAL) test to detect bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases.
Enzymes from horseshoe crab blood are used by astronauts in the International Space Station to test surfaces for unwanted bacteria and fungi.
A protein from horseshoe crab blood is also under investigation as a novel antibiotic.
LAL is obtained from the animals’ blood. Horseshoe crabs are returned to the ocean after bleeding, although some 3% die during the process. Studies show that blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound.
The eyes of the horseshoe crab are used in the medical study of human optic nerve structure.
Chitin from the “shell” of the horseshoe crab is non-toxic and biodegradable. When combined with chitosan, it can be used in cosmetics, hair spray, and contact lenses. Chitin can be used to purify drinking water and waste water. The substance can also be made into surgical suture and wound dressings.
At one time horseshoe crabs were so abundant that farmers used them as fertilizer.
Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years. Forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, 400 million years ago. However, the Atlantic horseshoe crab has no fossil record at all, and the genus Limulus “ranges back only some 20 million years, not 200 million.”
A horseshoe crab is virtually a “walking hotel,” with any number of creatures living attached to its shell — barnacles, blue mussels, slipper shells, bryozoans, sponges, flatworms, diatoms, fungi, and bacteria. While most of these hitchhikers have little or no effect on the day-to-day life of the horseshoe crab, certain fungi and bacteria can degrade the shell over time. Although the crab’s armored shell appears indestructible, daily wear and tear can leave it with small cuts or scratches that allow fungi and chitinase bacteria to gain a foothold. Specifically, fungi and chitinase bacteria attack these abrasions and gradually “eat” through the shell, exposing the horseshoe crab to additional microbes, which are eventually fatal.
In addition, several species of flatworm glide around the bottom of the horseshoe crab, eating scraps of food that the crab misses in its haphazard method of feeding. The worm cements its eggs to the horseshoe crab’s gills. As a result, small portions of the gill that surround the eggs become rigid and stiff, causing the formation of hairline cracks that allow deadly bacteria to invade the horseshoe crab’s system.
Limulus polyphemus is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for this animal’s future. Since the 1970s, the horseshoe crab population has been decreasing in some areas, due to several factors, including the use of the crab by fishermen to bait eel, whelk and conch traps.
Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on the horseshoe crabs’ eggs for food during their spring migration. Precipitous declines in the population of the red knots have been observed in recent years. Predators of horseshoe crabs, such as the currently threatened Atlantic loggerhead turtle, have also suffered as crab populations diminish.
Every year, about 10% of the horseshoe crab breeding population dies when rough surf flips the creatures onto their backs, a position from which they often cannot right themselves. In response, the Ecological Research and Development Group (ERDG) launched a “Just Flip ‘Em” campaign, in the hopes that beachgoers will simply turn the crabs back over.
A large-scale project to tag and count horseshoe crabs along the North American coast was underway in the spring and summer of 2008, termed http://www.projectlimulus.org. Due to the lack of information and knowledge regarding horseshoe crab populations, the management policies lack any abundance of rules and regulations. To implement management policies for the species, more population information needs to be attained.
Jacksonville Zoo History:
Horseshoe crabs first arrived in the Jacksonville Zoo collection in 2010.