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SYSTEMATICS AND HYBRIDIZATION IN THE FOUR LIVING SPECIES OF HORSESHOE CRABS

The horseshoe crabs, or Xiphosura, are the striking "living fossils" in the present sea. These living species are very similar to a fossil specimen, Mesolimulus walchi, which was found in the Jurassic lithographic shale of Germany. Therefore, it is considered that the horseshoe crabs have undergone little morphological evolution during their 200-million-year history. The fossil Xiphosura is known from all geological systems from the Cambrian onward (St0rmer, 1952). The recent horseshoe crabs are divided into two subfamilies by their distribution and morphological characteristics (Pocock, 1902). One is Limulinae, including Limulus polyphemus which is confined to the waters along the east coast of North America, and the other is Tachypleinae, including Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda, which are confined to the waters along the southeast and east coasts of Asia. As for the Moluccan horseshoe crab, T. hoeveni, recorded by Pocock (1902), Waterman (1958) compared it with T. gigas and concluded that only a single species should be recognized. Thus, the four species mentioned above are the only members of living Xiphosura. In order to make clear the relationships of the four species, some embryological, morphological, ecological, and biochemical studies have been carried out in our laboratory (Sekiguchi, 1973; Sekiguchi et al., 1976, 1977, 1978; Shishikura and Sekiguchi, 1978). In the present paper, the systematic relationships of horseshoe crabs are studied