A major subphylum under Arthropoda, the Chelicerata includes sea spiders (pycnogonids), horseshoe crabs, and arachnids, such as harvestmen, scorpions, spiders, ticks, and mites. The chelicerates derive their name from ‘chelicerae,’ the pincer-like appendages (found anterior to the mouth) that form an integral part of their feeding apparatus.
This subphylum is believed to have diverged from Mandibulata (another subphylum of arthropods) during the mid-Cambrian era. This division is supported by the presence of early chelicerate ancestors during that period, including those belonging to the order Habeliida and the extinct genus Mollisonia.
Like other arthropods, chelicerates have characteristically segmented bodies and jointed appendages covered with a thick, chitinous cuticle. They are one of the most diverse groups of animals, with over 77,000 described extant species, including around 50,000 species of spiders, 32,000 species of ticks and mites, and 1,400 scorpion species.
The body of a chelicerate is covered with a thick cuticle and divided into two tagmata – the anterior prosoma or cephalothorax and the posterior opisthosoma or abdomen. However, these two tagmata are not visibly partitioned in mites and ticks.
The prosoma is formed by the fusion of the ocular somite (formerly called the ‘acron’), which houses the eyes and labrum, with the next six post-ocular segments (somites 1 to 6). In most chelicerates, the opisthosoma may comprise up to thirteen segments; however, in a few taxa, like scorpions and eurypterids, the opisthosoma is divided into two parts: the front part, called the mesosoma, and the back part, the metasoma, with the abdominal appendages either missing or significantly altered for special purposes (spinnerets for silk production).
Though it was initially believed that chelicerates had lost their first antennae-bearing somite, scientists have confirmed that the pair of chelicerae corresponds to the antennae. The second somite is home to the pair of pedipalps (used for sensory purposes), while the subsequent four segments (somites 3 to 6) possess pairs of walking legs. Strategically positioned, the mouth lies between the chelicerae and the pedipalps, corresponding to the 1st and 2nd somites.
In most chelicerates, the chelicerae are usually divided into three distinct segments, with the third segment resembling a claw. However, spiders have only two segments; the second segment usually folds under the first when not in use. The sizes of chelicerae vary considerably, ranging from large claws in fossil eurypterids and modern harvestmen to tiny pincers in scorpions.
The pedipalps usually perform specialized sensory functions in most chelicerate groups; however, in advanced ones, like spiders and scorpions, they are modified to pincers for catching prey and may also contain bulbous tips for injecting sperm into the females’ gonopores.
However, the chelicerae and pedipalps differ in embryonic origin. This difference stems from their embryonic positioning and the associated neural innervation. While the chelicerae are deuterocerebral and originate from the first somite anterior to the mouth, pedipalps are tritocerebral and develop from the second somite posterior to the mouth.
The primary body cavity in chelicerates is the hemocoel, which spans most of the body’s length and serves as a channel through which blood circulates. This circulation is facilitated by a tubular heart that collects blood from the rear parts of the body and propels it forward. Like arthropods, chelicerates have an open circulatory system, with their arteries having open ends and do not connect directly to veins.
Modern terrestrial chelicerates generally possess both book lungs and tracheae for respiration. Book lungs facilitate the exchange of oxygen and removal of waste gasses through the bloodstream, while tracheae perform the same function independent of the blood transport system.
However, the aquatic horseshoe crabs respire through a horizontally placed series of thin, membranous plates called book gills. Scientists have long speculated that extinct eurypterids also possessed gills, but the fossil records were always inconclusive. Later, the discovery of the fossil of an extinct eurypterid, Onychopterella, belonging to the Late Ordovician period, confirmed the existence of four pairs of vertically oriented book gills in them.
Chelicerates are ammonotelic, expelling waste from the body in the form of ammonia, which is usually released through gills or eliminated through the anus as feces. They also use specialized nephridia, known as ‘little kidneys,’ to remove additional waste in the form of urine.
Since ammonia demands high quantities of water for dilution, most terrestrial chelicerates, which need to conserve water, often convert nitrogenous waste into other chemicals and excrete them as dry matter. This conversion involves nephridia and Malpighian tubules of their kidneys, which filter waste from the blood and deposit it into the hindgut as solids.
Chelicerates have a nervous system similar to all other arthropods, consisting of a pair of nerve cords with a ganglion per segment. These nerve cords converge, forming the brain with the fusion of ganglia located just behind the mouth with those in the front part. Scientists previously assumed that the first brain segment (usually contains antennae in other arthropods) was absent in chelicerates, and the brain includes only one pair of pre-oral ganglia instead of the usual two. However, further evidence negates this assumption and reveals that the first segment, which bears the chelicerae, is present.
In some chelicerates like horseshoe crabs, the brain integrates ganglia from the prosoma and the first two opisthosomal segments, while the remaining opisthosomal segments contain separate ganglia.
In most arachnids (except scorpions), all the ganglia fuse into a single mass in the prosoma, and the opisthosoma does not contain any such neural connection. However, in Mesothelae spiders, primitive ganglia retention occurs in the opisthosoma and rear prosoma.
Chelicerates possess either of the two types of eyes: compound eyes and pigment-cup ocelli (simple eyes). Compound eyes are found only in horseshoe crabs, while other chelicerates usually have reduced compound eyes, made up of clusters of no more than five pairs of ocelli. While compound eyes are found on both sides of the head, the pigment-cup ocelli are seated right in the middle. These median ocelli-type eyes are thought to be evolutionarily related to the nauplius eyes of crustaceans and the ocelli of insects.
There is considerable variability in the extent of visual perception in different species of chelicerates. The compound eyes in horseshoe crabs are sensitive enough to detect movement but fail to form images, whereas jumping spiders have a wide field of vision, color acuity, and UV light perception.
In chelicerates, the cuticle is profusely innervated by multiple sensors from the nervous system. Spiders and other arthropods have evolved intricate sensory arrays from their cuticles, including touch and vibration sensors, mainly ‘setae’ (bristles), which detect a range of forces from strong contact to gentle air currents.
The traditional family tree of Chelicerata showed that these arthropods are more distantly related to the other major groups, like crustaceans, hexapods, and myriapods, than the relatedness of these groups within themselves. However, recent advancements in molecular phylogenetics and embryological studies on the development of arthropod nervous systems reveal a revised phylogeny. These findings predict chelicerates and myriapods to be closely related, while the hexapods and crustaceans are reported to be the closest kin.
Scientists have reached a global consensus suggesting Chelicerata include the classes Arachnida and Merostomata (Xiphosura and the extinct Eurypterida), though the inclusion of Pycnogonida (sea spiders) under this subphylum remains debatable.
Based on the current classification, chelicerates are divided into 3 classes, 2 superorders, and 18 orders.
They inhabit a wide range of terrestrial, freshwater, and marine habitats worldwide.
Spiders and scorpions are terrestrial, inhabiting forests, grasslands, deserts, and urban environments, whereas water mites occupy freshwater habitats. In contrast, sea spiders and horseshoe crabs are completely marine, ranging from shallow waters to the deep sea.
Originally predators, chelicerates have diversified to use all the major feeding strategies, including predation, parasitism, herbivory, and scavenging.
Although most chelicerates are carnivores, feeding mainly on soft-bodied invertebrates, the spider Bagheera kiplingi, with a herbivorous diet, is an exception, while many others supplement their diets with nectar and pollen. Most ticks and mites are parasitic and suck on host blood, whereas harvestmen and a few other chelicerates are scavengers and detritivores feeding on solid food like detritus.
The reproductive strategies of chelicerates depend largely on the habitat in which they live.
Horseshoe crabs reproduce through external fertilization, where sperm and eggs unite outside the parents’ bodies. Despite living in water, they spawn on land in the intertidal zone of beaches. The female digs a depression in the wet sand to deposit her eggs, and then the male (often multiple males) releases sperm onto them. The larvae of horseshoe crabs resemble miniature adults with full sets of appendages and eyes. Initially, they bear two pairs of book gills, gaining three more pairs as they molt. Sea spiders also have a similar fertilization strategy in which sperm and eggs are released into the water to be fertilized.
Most arachnids (being air-breathing) employ indirect internal fertilization, though many mites have become secondarily aquatic. In one method, the male deposits a spermatophore (a packet of sperm) on the ground, which the female picks up. Alternatively, the male may store sperm in specialized appendages, such as the pedipalps in male spiders, and transfer them to the female during mating. Courtship rituals are common, especially in species at high risk of being eaten before mating. Although most arachnids lay eggs (oviparous), all scorpions and a few species of mites are viviparous, giving birth to young ones.
Their parental care varies from a short period to a prolonged one, depending on the chelicerate species. While scorpions carry the young ones on their backs till their first molt, a few spider species, like wolf spiders, have developed specialized, rough bristles to help their young cling to their backs. Some female spiders also regurgitate food in response to the begging of their young ones.
Chelicerates are preyed upon by natural predators like birds, mammals, amphibians, reptiles, and other arthropods.
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