Arthropods horseshoe crabs

By far the most definitive study on an arthropod hemocyanin has been that by Volbeda and Hoi (1989b), a crystallographic tour de force. Crystals of subunit b can be formed from solutions of native hemocyanin which contain three types of subunits (a, b, and c). Two subunits (a and b) are nearly identical (3% difference in sequence), whereas subunit c differs more. Subunits a and b are glycosylated at a single residue (Asn-167). While the Panuliris form has been shown to be deoxy (Volbeda et al., 1989), unpublished observations indicate that the horseshoe crab structure Limulus) is in the oxygenated state (K. Magnus, personal communication, 1988, cited by Volbeda and Hoi, 1988). 

The hairpin-like structure has been conserved in the course of evolution, since it was found in many peptides isolated from various classes of arthropods, such as the primitive horseshoe crabs (tachyplesins [110,111] and polyphemusins [112,113]), arachnids (androctonin in scorpion [114] and gomesin in spider [115]) insect (thanatin [116]), in two classes of vertebrates, mammalian (protegrin [117,118], lactoferricin B [119] and hepcidins [120]) and fish (hepcidins [121]), and in plants (76-AMPl[122]). 

Arthropods make up an extremely large group of animals in the intertidal zone. Their terrestrial cousins are the familiar, ever-present insects and spiders. Along the coast, typical marine arthropods include crabs, shrimp, lobsters, and horseshoe crabs. 

It is assumed that TTX is involved in a defense mechanism to protect the horseshoe crab eggs from predators (Jeon et al., 1984 Sheumack et al., 1984). Horseshoe crab mainly feeds on mollusks, arthropods, and detritus (Chatterji et al, 1992). Some bacteria inhabiting the marine sediments might be the primary origin of TTX for this crab. 

The shells of shellfish consist not only of chitin. In the edible crab Cancer pagurus and the lobster Homarus americanus) the calcified, rigid parts of the exoskeleton also contain a series of proteins that are also found in the noncalcified, flexible regions. Most of these proteins have a molar mass below 20 kDa some of them seem specific to crustaceans but others are also present in some insects (Andersen, 1999). The cuticular proteins of the horseshoe crab [Limulus polyphemus) show similarities with those of other arthropods, in particular those of the spider Araneus diadematus, but their molar masses are lower (7-16 kDa) Dietzel, Andersen, and Hojrup, 2003). 

The immune response of arthropods is provided by six types of blood cell, but for primitive arthropods such as horseshoe crabs there is only one category of cell responsible for this function, the granular amoebocyte. On stimulation, granular amoebocytes produce arachidonic acid derivatives, particularly 8-hydroxyeicosatetraenoic acid (8-HETE), whose biosynthesis has been studied. This acid is also produced by vertebrates but, curiously, in the marine environment it is mainly the 8J -enantiomer that is formed, whereas in mammals the 8S enantiomer is the main product (MacPherson, Pavlovitch, and Jacobs, 1996). 

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