Horseshoe crabs have been known as “living fossils” because their fossil forms date as far back to 500 million years ago in the Paleozoic (Sekiguchi, 1988). There are four species (one American and three Asian species) in the world:
During a visit to the Folklore and Natural History Museum in Jeju Special Self-governing Province, a specimen of
Order Xiphosurida R. & E. Richter, 1924
Family Limulidae Leach, 1819
Material examined. 1♀ (total length 49.5 cm, prosoma length 14.3 cm and width 23.0 cm, telson length 25.1 cm, middle part of telson width 7.5 mm and height 8.0 mm), Udo Island (Jeju), 17 Nov 1997, Yang KC, on coarse sandy beach, 33°30′18.457′′N, 126°56′29.808′′E.
Description. Dorsal surface of prosoma smooth, convex, with posterior lateral angles prolonged backward approximately 1/2 length of opisthosoma; small median eye located anterior to each side of median ridge, compound eye located laterally to each lateral ridge (Figs. 1A, B, 2A). Prosoma concave ventrally; anterior dorsal surface reflected ventrally, forming triangular apex; 7 pairs of appendages present (Figs. 1C, 3A); first pair 3-segmented chelicerae (Figs. 1C, 3A, B); second pair pedipalps with chelate tips (Figs. 1C, 3A, C); third to sixth pairs 4 walking legs with chelate tips; last walking leg with movable spur distally on segment 4, with flat and strongly compressed segment 5, which longer than segment 6, with 4 leaf-like processes for pushing and sweeping away mud and silt during burrowing; seventh pair chilaria for food manipulation (Figs. 1C, 3A, 4A).
[Fig. 4.] Tachypleus tridentatus (Leach, 1819), female (total length 49.5 cm). A, Ventral view of last walking leg (right); B, Ventral view of spines on lateral margin of opisthosoma (right); C, Ventral view of opisthosoma.
Opisthosoma unsegmented dorsally, hexagonal, with 6 small pits along 2 median furrows; lateral margin with 3 movable spines (Fig. 1A): first spine slender and straight, second spine longest, third spine shortest and curved slightly (Figs. 1A, C, 4B); 3 small spines present on posterior margin (Figs. 1A, 2A). Opisthosoma with 6 pairs of appendages ventrally; first pair biramous genital operculum covering gonophores, inner ramous not reaching to tip of distal segment of outer ramous; last 5 pairs of appendages modified as biramous flap-like appendage (book gills) (Figs. 1C, 4C).
Telson long, triangulate in cross section of middle part, crested above, slightly concave below (Fig. 1)
Color. Prosoma, opisthosoma, and telson glossy greenish brown. Lateral spines of opisthosoma pale brown. Eyes black.
Habitat. This species hibernates on sandy to muddy bottoms in a depth of approximately 20 m in winter and migrates to the spawning beaches at night in the spring or summer (Sekiguchi, 1988; Nishimura, 1995).
Remarks. Three Asian horseshoe crabs (