Karstarma umbra, a new species of cavernicolous crab (Brachyura: Sesarmidae) from Vanuatu, with a key to the genus

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A new species of cavernicolous sesarmid crab is described from Vanuatu. Karstarma umbra sp. nov. belongs to a group of three species that has the chitinous distal part of the male first gonopod slender and elongate. The new species can easily be distinguished from K. balicum (from Bali, Indonesia) and K. loyalty (from Loyalty Islands) by its distinctly longer ambulatory legs and its relatively more slender male first gonopod. Karstarma umbra is morphologically closest to K. waigeo (from Indonesian Papua) but has proportionately longer legs and the outer margin of the male first gonopod is distinctly concave. The taxonomy of the genus is discussed and a revised key to the 18 species is provided.