Horseshoe Crab Basics
The Horseshoe Crab is an ancient animal that belongs to the Arthropoda family, which has been around for around 400 million years. This puts them ahead of the dinosaurs in age! Horseshoe crabs are neither genuine crabs nor crustaceans, despite their popular name.
Horseshoe crabs belong to the Xiphosura order and are more closely connected to scorpions and spiders than they are to ancient crabs. They may be found in both marine and brackish waters. Crustaceans, mollusks, and marine worms are all eaten by horseshoe crabs, which are carnivores.
The Western horseshoe crabs, American horseshoe crabs (Limulus polyphemus), and Eastern horseshoe crabs, tri-spine horseshoe crabs (Tachypleus tridentatus), coastal horseshoe crabs (Tachypleus gigas), and mangrove horseshoe crabs are the four species of horseshoe crabs (Carcinoscorpius rotundicauda).
Only North America, Central America, and Southeast Asia are home to horseshoe crabs. Limulus polyphemus is a species of limulus that may be found in the Gulf of Mexico and the Atlantic Ocean. The three Eastern horseshoe crab species have a large range that includes Japan, China, Java, Sumatra, the Philippines, and India’s coasts.
The tri-spine horseshoe crab is designated as endangered, whereas the American horseshoe crab is classed as vulnerable. The IUCN’s red list will shortly include the other two species.
Horseshoe Crab Anatomy
The horseshoe crab receives its name from the rounded form of its head, which resembles that of a horseshoe. The brain, heart, neurological system, mouth, and many glands are all found in the prosoma, which is the biggest section of its body and houses most of the nerve systems and critical biological organs. The exoskeleton’s hard plate serves as a protective covering for the organs.
The next portion of its anatomy is the abdomen, also known as the opisthosoma. It has spines on the sides and a ridge in the middle, and is triangular in form. The horseshoe crab’s spines may move and help defend it. The gills, which are used for breathing, are located on the underside of the belly.
The telson, or tail, is the last portion of a horseshoe crab’s body. The telson is long and pointed, and although it seems to be deadly, it is not. Horsehoe crabs mostly use their tails to help them correct themselves if they fall down. The 10 legs of the horseshoe crab allow it to traverse the ocean bottom. From head to tail, females are about a third bigger than males.
Horseshoe Crab Life-History
Horseshoe crabs spawn in the late spring and early summer, when the adults migrate from deep ocean water to the shore. Males arrive first and wait for females to arrive, who produce pheromones (natural chemicals) when they arrive at the beach. These pheromones alert males that they have arrived and that it is time to mate. Before the males fertilise the eggs, the ladies dig little nests and lay their eggs. During high tides, this is normally done at night. The breeding couple may repeat the procedure several times, laying tens of thousands of eggs each time.
The eggs are a food source for various birds, fish, and reptiles, so most horseshoe crabs will not make it to the larval stage. If a horseshoe crab egg is fortunate enough to escape being eaten, it will hatch in about two weeks. With the exception of the lack of a tail, the larva resembles the adult horseshoe crab. The larva goes into the ocean after hatching and spends about a year on the sandy bottom of a tidal flat. They move to deeper water as they mature, where they eat the same food as adult crabs.
It takes a juvenile horseshoe crab around 10 years to mature into an adult. The horseshoe crab will moult 16 or 17 times throughout this period. They lose their little exoskeletons and build bigger shells as part of this process. In the spring, when they reach maturity, they return to the beaches to reproduce. Horseshoe crabs have a lifespan of almost 20 years.
Fun Facts about the Horseshoe Crab!
Horseshoe crabs are ancient creatures that have lived on the planet for millions of years. These creatures have been extensively researched and have a number of biological adaptations that have enabled them to stay on the planet for such a long time. Let’s have a look at it more closely!
Horseshoe crabs are significant to the biomedical sector because their blue blood includes a substance called Limulus amebocyte lysate (LAL), which is used to prevent germs from contaminating biomedical equipment and vaccinations. This is so vital to the business that the biomedical sector in the United States gathers blood from around half a million horseshoe crabs each year for use in medications.
After the crabs have been captured, up to 30% of their blood is extracted before they are released to the ocean bottom. While the horseshoe crabs are returned to the water alive and healthy, studies have indicated that blood collection has a short-term effect on their behaviour. The scientists show that the crabs get disoriented throughout the harvesting procedure, which may have a detrimental influence on their reproduction.
Horseshoe Crabs Have Nine Eyes
Horseshoe crabs have nine eyes, and their eyes have been the subject of a lot of scientific study. On the prosoma, there are two huge compound eyes, one on each side. These eyes have monochromatic vision and are used mostly for mating.
On the carapace, they have five basic eyes: two median eyes, one endoparietal eye, and two rudimentary lateral eyes. Even unhatched embryos appear to be able to detect light levels from within their eggs, indicating that these eyes are likely important during embryonic and larval development. Visible and UV light irritate the median eyes.
The photoreceptors in the primitive lateral eyes begin functioning soon before the egg hatches. The crab’s remaining two eyes may be located on the underside. These two ventral eyes, which are located near the animal’s mouth, are supposed to aid in orienting the animal when swimming. The horseshoe crab possesses a multitude of light-sensing organs down the length of its tail, in addition to its nine eyes.
The eyes of horseshoe crabs have been examined so thoroughly that one researcher, Dr. Hartline, was awarded the Nobel Prize in Physiology or Medicine in 1967 for his discovery of the optical nerve network using horseshoe crabs as a model.
Horseshoe Crab Living Fossils
Horseshoe crabs are referred to as “living fossils.” They’ve been around since before dinosaurs existed! Horseshoe crabs have survived catastrophic extinctions and stayed basically unaltered for over 400 million years.
Scientists use the phrase “living fossil” to describe creatures that have been extinct for millions of years and have few or no surviving descendants. Most of them have odd characteristics that make them seem to be from another planet, and they provide a rare peek into how life on Earth used to be. The komodo dragon, hagfish, koala, and pig-nosed turtle are some of the other species that are considered living fossils.