CHELICERIFORMES: PYCNOGONIDA AND MEROSTOMATA MODULE 3
MODULE 3 CHELICERIFORMES: PYCNOGONIDA AND MEROSTOMATA
Unit 1
Class Pycnogonida (G. pyknos, thick or knobby + gony, knees) These are a small group of marine animals called sea spiders. They live in shallow waters crawling over seaweeds and hydroids upon which they feed. Other adult sea spiders live as external parasites. They are found in oceans throughout the world. Pycnogonids have a very small compact body composed of a few segments with very long legs. They do not have a well- Figure 1: Sea spider defined head and the abdomen is reduced; therefore, they consist entirely of cephalothorax. Sea spiders usually have four, or less commonly five or six pairs of legs. Male sea spiders carry eggs on their legs (the ovigers, which are anterior to the walking legs and posterior to the pedipalps) until they hatch, thus providing a measure of parental care. There are no special excretory or respiratory systems. They appear to carry out these functions by direct diffusion. They have a dorsal ostiate heart, supraoesophageal and suboesophageal ganglia, and ventral chain of ganglia. Examples include Pycnogonum, Colossendeis and Pentapycnon.
Unit 2 Class Merostomata These are the eurypterids and xiphosurans. The eurypterids are wholly extinct while there are extant representatives of the xiphosurans, the most familiar of which is the ‘King Crab’ or ‘Horseshoe crab’. Examples include Carcinoscorpius, Limulus and Tachypleus Limulus polyphemus is common along the east coast of North America. It is marine. Limulus lives in comparatively shallow waters along the sandy and muddy shores of sheltered bays and
Figure 2: A group of Limulus species on a estuaries. They are sluggish creatures and spend sandy beach. most of their time burrowing in sand or mud for worms, soft molluscs and small animals on which they feed.
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CHELICERIFORMES: PYCNOGONIDA AND MEROSTOMATA MODULE 3
The body consists of an anterior prosoma and a posterior opisthosoma. The prosoma and opisthosoma are covered by fused, hardened carapaces hinged (movably articulated) on one another. The opisthosoma terminates in a long, strong spine called telson. The prosomatic carapace is unsegmented, semicircular and horse- shoe shaped. It is convex above with sloping sides Figure 3: Horseshoe crab and bears three longitudinal ridges; one median (a) Dorsal view (b) Ventral view and two lateral. The dorsal surface of the prosomatic carapace bears anterior median ocelli (a pair of simple eyes) and a pair of large, lateral rudimentary compound eyes. The prosoma bears six pairs of segmental appendages on the ventral surface. The most anterior is the pre-oral, three segmented chelicerae (used to manipulate food), which is followed by four pairs of chelate legs and a last pair of non-chelate legs. The coxae of the four pairs of chelate legs carry coarse spines and form gnathobases, which are used to grind up the soft bodied invertebrates on which the animal feeds. The last pair of walking limbs does not form a gnathobase, but bears coxal process (epipodite) used in cleaning the gills. Its penultimate segment bears four spatulate processes used in locomotion and digging. The opisthosomatic carapace is hexagonal. It is composed of the fusion of an anterior large six-segmented mesosoma and a posterior small, three segmented metasoma. Dorsally, the mesosoma bears a single median row of three prominent spines and two lateral rows of six small pits. Each lateral side of the mesosoma is serrated, produced into six short immovable spines. The metasoma is much reduced and bears the mid-dorsal anus. The opisthosoma bears six pairs of appendages (overlapping broad, flattened plates; the two halves of each uniting in midline). The first pair, which is the most anterior forms the genital operculum, bears the genital openings on its posterior surface and partially covers the posterior gills. The remaining five pairs carry book lungs or gill books (respiratory appendages for gas exchange). These appendages (with numerous thin-walled lamellae) are moved backwards and forwards, creating a respiratory and swimming current and at the same time pumping blood into and out of the internal space of the lamellae. Excretion is by coxal or brick-red glands. The large caudal spine is considered to be an extension of the non-segmental telson. Individuals of Limulus may grow up to 60cm long. They mature in 9 to 12 years and have a life-span of 14 to 20 years. They
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CHELICERIFORMES: PYCNOGONIDA AND MEROSTOMATA MODULE 3
emerge from sea to mate on moonlit nights when the tide is high. They swim on their backs by moving their abdominal plates. They can also walk on their four pairs of legs. Horseshoe crabs serves as food and bait; and can be processed into fertilizers. The study of horseshoe crab has also advanced human health, for example it has led to therapies for human eye disorders, the chitin is used for production of contact lenses, surgical sutures and skin lotion; the chitin also forms chemicals used to remove metals and toxin from water; and fat and cholesterol from human body. The blood of horseshoe crabs forms a substance, Limulus Amobocyte Lysate (LAL) used to identify gram negative bacteria (through detection of bacterial endotoxins) in medical fluids, drugs and on surgical gloves. This is because horseshoe crabs have strong immune systems and are very helpful in diagnosing human bacterial diseases, checking for the presence of bacterial endotoxins in drugs and intravenous solutions; and in finding remedies for diseases that have built immunities against penicillin and other drugs. Recently, new innovations have resulted in a synthetic substitute for the Figure 4: Extraction of the blood of horseshoe crabs blue blood of horseshoe crabs.
Conclusion Subphylum Cheliceriformes comprise the Pycnogonida (sea spiders) and Chelicerata (horse-shoe crabs and arachnids). Sea spiders are marine and benthic while horseshoe crabs live in shallow sandy or muddy shores of sheltered bays and estuaries. Blood cells of horse- shoe crabs aggregate at the sides of a wound and release granules containing coagulable protein, forming a clot that stops the bleeding. This same system responds to minute amounts of bacterial endotoxins and serves to contain invading bacteria. Extracts of horse-shoe crab blood cells are used in diagnosing human bacterial diseases and in checking for the presence of bacterial endotoxins in drugs and intravenous solutions.
References/Further Reading Davies M (2019): Horseshoe crabs are drained for their blue blood. The practice will soon be over. https://bigthink.com/surprising-science/horseshoe-crab-blue-blood (published March, 18, 2013; accesses on March 19, 2020).
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CHELICERIFORMES: PYCNOGONIDA AND MEROSTOMATA MODULE 3
Encyclopedia.com (2020). Sea Spiders: Pycnogonida. https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and- maps/sea-spiders-pycnogonida (updated March 9, 2020). Encyclopedia.com (2020). Merostomata (Horseshoe crabs). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and- maps/merostomata-horseshoe-crabs (updated March 12, 2020). Kumar V, Roy S, Sahoo AK, Behera BK, Sharma AP (2015). Horseshoe crab and its medicinal values. International Journal of Current Microbiology and Applied Sciences 4(2): 956-964. https://www.ijcmas.com/vol-4-2/Vikash%20Kumar,%20et%20al.pdf .
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