Innate Immunity in the Horseshoe Crab

Abstract

Arthropod horseshoe crabs belong to the class Merostomata and are phylogenetically more related to Arachnoidea than Crustacea. Many fossils of horseshoe crabs can be found, primarily in deposits from the Paleozoic era to the Cenozoic era of North America and Europe. However, the distributions of the four extant species, including Limulus polyphemus, Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda, are different from those of fossil species. L. polyphemus is distributed along the east coast of North America, and the other three species are found in Southeast Asia. In Japan, T. tridentatus is distributed along the limited coastal areas of the Inland Sea and the northern part of Kyushu Island. The divergence between L. polyphemus and T. tridentatus has been calculated to have occurred 135 million years ago during the late Jurassic period of the Mesozoic era, based on mutation distances in a clottable protein, coagulogen: 52.5 million years between T. gigas and the two Asian species and 36.3 million years between T. tridentatus and C. rotundicauda (1).