Description of four new species from the Haploniscus unicornis Menzies, 1956 complex (Isopoda: Asellota: Haploniscidae)

Based on the adult males the four species Haploniscus bihastatus n. sp., H. monoceros n. sp., H. machairis n. sp. and H. angolensis n. sp. are described from the deep sea of the Southeast Atlantic. These belong to a species complex, which also includes H. unicornis Menzies from the Puerto Rico Trench and H. aduncus Lincoln from the Iceland Basin and shares several characters with species from the genus Mastigoniscus Lincoln. While males of the species complex can be reasonably well distinguished by their habitus and pleopods 1 and 2, the discrimination of female specimens is hampered by the pronounced sexual dimorphism. The Haploniscus unicornis complex has a wide distribution in the Atlantic Ocean, but appears to be absent from basins south of the Walvis Ridge and also from the southwest Atlantic basins.