Chelicerata
Table of Contents
Chelicerata are among the most important subdivisions of arthropods – a curious set of about 70,000 known and identified species and up to 500,000 unknown species, out of which just a small percentage live in the ocean. Consisting mainly of invertebrates such as sea spiders and horseshoe crabs, the marine species belonging to this subphylum are still relatively diverse, both when it comes to their overall structure and development and to their behavior or the habitat they are normally found in.
Chelicerata subdivisions are generally well-established, and include marine dwelling species such as Xiphosura and Pycnogonida (sea spiders). Unfortunately, even though chelicerates originated as marine creatures, most of the diverse aquatic species are now extinct, including the varied (but extinct) order called Eurypderida (sea scorpions). The surviving marine chelicerates add up to around 1,300 documented species, but there may be more we still don’t know about.
As in the case of most arthropods, all the species of chelicerates feature a segmented body and jointed appendages present on each segment. They also have an exoskeleton that protects them from harm and has a few intricate characteristics that differ from other arthropods. One of the most interesting characteristics of chelicerates’ bodies, however, is that they are divided into a prosoma – which is made up of a presegmental acron and eight distinct segments – and a posterior area known as the opisthosoma. This latter area of the body is made of twelve similar segments and a telson that stands out. The unique part of the chelicerates’ bodies is known as the chelicerae, a unique set of pointed appendages that the invertebrate uses to get hold of their food before consuming or digesting it.
Arthropods view the world in a very different way from most types of animals, and most species of chelicerates are no exception to that – in fact, they may well be much more unusual than at least some of the mainstream types of arthropods. While the cuticles are designed to block out much of the outside information, chelicerates feature many special sensors and connectors going from the cuticle directly to the invertebrate’s nervous system to provide information about strong and weak air currents, taste, touch and a number of other aspects that are vital to chelicerates’ survival.
Most marine chelicerates reproduce through external fertilization. Horseshoe crabs, for instance, migrate to coastal waters during the mating season, where the male chooses a female and literally clings to her back. The female will dig a hole in the sand and lay her eggs there, while the male will fertilize them. The eggs will hatch in about two weeks after fertilization, yielding larvae, but many of them, unfortunately, are eaten by shore birds before hatching.
Most chelicerata species are known to have separate sexes, but there is one pygnogonid that is hermaphroditic (meaning that an individual has both female and male reproductive organs).
Atlantic Horseshoe Crab
Limulus Polyphemus, commonly known as Atlantic horseshoe crab, is an arthropod that is closely related to spiders, scorpions and ticks and can be found most often in the Gulf of Mexico and along the North American Atlantic coast. There are several other horseshoe crab varieties belonging to the family Limulidae, including the tri-spine horseshoe crab and the mangrove horseshoe crab; however, the Atlantic crab is the only one that is prevalent in North Atlantic regions.
This species of horseshoe crab is particularly well-known for its uncommon biological traits, which made researchers interested in them since their first discovery. The crabs can grow to 24 inches in length, and their common colors are green and dark brown. There is little distinction between the sexes, except for the fact that females are about 25% larger than male specimens. The name of the horseshoe crab comes from the arthropods’ smooth horseshoe-shaped shell. Some of the more uncommon abilities and traits of these crabs also include their monochromatic vision, six pairs of appendages, jawless mouth, a small chemoreceptor organ unique to the species, and their rare ability to regrow lost limbs, just like a starfish.
The Atlantic horseshoe crab is popular in scientific circles for its unique vision, whose special qualities have been known since 1967, when the Nobel Prize in Medicine was awarded for research conducted on the horseshoe crab’s eye. The crab is monochromatic, and its large compound eye is complemented by two additional eyes on the underside and five simple eyes located on the carapace. The compound eyes are made from 1000 complex receptors known as ommatilda, and the large nerves and additional light-sensing organs on the tail make it possible for the crab to sense variations in brightness that far surpass what most other species are capable of detecting.
Although it isn’t easy to assess the longevity of these horseshoe crabs, it is believed that their average lifespan is between 20 and 40 years. When hatching from their eggs, the crabs are protected by a transparent membrane secreted by the embryo, which shapes the organism into a larva. This form is maintained for a period of 5 days, after which the crabs settle and start the first molt. The Atlantic horseshoe crab sheds its shell up to 17 times before becoming sexually mature, at age 9; depending on their size, females can produce up to 64,000 eggs.
Sea Spider
The sea spider is a unique creature that, despite being classified as a chelicerata, is actually more closely related to spiders than many other well-known groups of arthropods, such as insects and crustaceans. Its spider-like appearance gives it its obvious name. However, Pantopoda – as it is commonly known in scientific circles – is much larger and more unusual in appearance than most spiders. Some species even have 10 or 12 legs, and their thin legs and small bodies place them among the most curious creatures that marine biologists have studied.
Although most species of sea spiders don’t grow to unusually impressive sizes – with some species being as tiny as 1 mm in diameter – a few unique specimens were discovered close to the Arctic and Antarctic Oceans that can grow to immense sizes of up to 90 cm in length. This is comparable to the size of the largest spider in the world, and researchers still don’t know what is causing the phenomenon. Some hypothesis point to the rich oxygen content of the waters close to the polar regions, while others consider that the diminished temperatures also play a key role. This phenomenon is known as polar gigantism, and it affects many other types of marine creatures, including copepods, mollusks and echinoderms.
Sea spiders’ small bodies make a respiratory system obsolete, which means the organism manipulates gases through the process of diffusion. This is considered an important detail in solving the mystery of polar gigantism and its impact on giant sea spiders. Some of the spiders are so tiny that their small muscles each consists of a single cell. Many species in this group have either four or six pairs of legs, with some also featuring appendages that resemble legs.
These animals dwell in many different areas of the world, including the United States, Australia, New Zealand, the Mediterranean and the Caribbean Sea. Most commonly you can spot them in shallow waters, although some species can be found as deep as 7,000 meters below the surface. Most sea spider species will be spotted on the seabed using their large legs to move around and feed on small organisms such as cnidarians, bryozoans or sea sponges.