Xiphosura

20200813 Lunataspis aurora

Naturkundemuseum Berlin - Mesolimulus walchi - Solnhofen

Xiphosura is a group of arthropods known as horseshoe crabs. Despite their common name, horseshoe crabs are more closely related to arachnids (a group that includes spiders, scorpions, and ticks) than to true crabs. Xiphosura is an ancient group of marine animals, with a fossil record that stretches back to at least the late Ordovician period, approximately 445 million years ago. This makes them one of the oldest surviving groups of arthropods, often referred to as "living fossils" due to their minimal evolutionary change over millions of years.

Description

Xiphosurans are distinguished by their hard, horseshoe-shaped carapace that covers the front part of their bodies. This carapace is an exoskeleton, made of chitin and proteins, which provides protection against predators and physical damage. Behind the carapace, the body extends into a long, spine-tipped tail known as a telson, which is used for steering while swimming. Horseshoe crabs have multiple eyes, including compound lateral eyes and simple median eyes that can detect both visible light and ultraviolet light.

Ecology and Behavior

Horseshoe crabs are primarily found in shallow ocean waters on soft sandy or muddy bottoms. They are benthic feeders, meaning they feed on the ocean floor, consuming worms, mollusks, and other small invertebrates. Their feeding habits play a crucial role in the marine ecosystem, as they help to recycle nutrients and aerate the soil.

During spawning season, horseshoe crabs migrate to beaches, where females lay thousands of eggs in the sand. These eggs are a vital food source for many species of birds, especially during migration. The survival of certain bird species, such as the Red Knot, is closely linked to the availability of horseshoe crab eggs, highlighting the ecological importance of Xiphosura.

Conservation

Despite their long history, horseshoe crabs face threats from habitat destruction, pollution, and overharvesting. They are harvested for bait in the fishing industry, and their blue blood is collected for the production of Limulus Amebocyte Lysate (LAL), a substance used for detecting bacterial endotoxins in medical applications. Conservation efforts are underway to ensure the sustainable management of horseshoe crab populations, including regulations on harvesting and habitat protection measures.

Taxonomy

The class Xiphosura includes four living species, which are divided into two families: Limulidae, which contains the American horseshoe crab (Limulus polyphemus) and three species of Asian horseshoe crabs (Tachypleus tridentatus, Tachypleus gigas, and Carcinoscorpius rotundicauda). The evolutionary history of Xiphosura is well-documented by a rich fossil record, which includes numerous extinct species that provide insights into the ancient marine environments and the evolutionary processes that have shaped current biodiversity.

In Culture

Horseshoe crabs have also found a place in human culture and history, with their unique blue blood being of particular interest to science and medicine. Additionally, their presence on beaches has made them a familiar sight to people living in coastal areas, where they are often seen during their spawning season.