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).