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Of the four extant species of horseshoe crabs in the world (Xiphosura Limulidae), Indonesia have three species, i.e., Tachypleus tridentatus (Leach, 1819), T. gigas (O.F. Müller, 1785) and Carcinoscorpius rotundicauda (Latreille, 1802). The species identification based on morphology of especially the females of the two Tachypleus species can be difficult. This study aimed at evaluating morphological characters used in species determination of T. tridentatus and T. gigas. Two morphological characters for separating the two species are the number and position of immovable spines and the spinnerets on the opisthosomatic carapace. The study was conducted in Balikpapan coastal area in Indonesia. Two morphotypes of T. tridentatus were found. Of the 120 collected specimens, 13.33% had one immovable spine, while 86.67% had three immovable spines on the opisthosomatic carapace. The two morphotypes were found in both males and females. DNA barcoding of both males and females with two morphotypes confirmed that the specimens were T. tridentatus. Thus, three immovable spines at opisthosoma can still be used as a morphological characters for T. tridentatus identification, but another morphotype with only one spine is present. This makes the distinction between the females of the two Tachypleus species more difficult. However, spinnerets on the dorsal opisthosoma can still be used to separate them.

biodiversity journal

Two morphotypes of tri-spine horseshoe crab, Tachypleus tridentatus (Leach, 1819) (Xiphosura Limulidae) in Indonesia and implications for species identification