Abstract
Background: Bathynomusparacelensis sp. nov., a medium-sized supergiant Bathynomus, is described from specimens obtained at Zhengbin fishing port in Keelung, Taiwan and had been caught in the water near Paracel Islands, South China Sea. Due to its similar shape to B.jamesi, this species has often been mistaken for juveniles or immatures of B.jamesi by fishermen working in this area. Species of Bathynomus can be distinguished morphologically and genetically. The differences from B.jamesi are in the shorter body, clypeus shape, uropod endopod and gene sequence. The difference from B.vaderi is in the body shape, clypeus shape, hook number of maxilliped endite and spines number of maxilulla. Based on the morphological and genetic data results, the specimen is a hitherto undescribed species. The samples were collected as a bycatch species in the deep-sea bottom trawl fishery. The distribution area and depth of this new species and population size are still unclear. New information: B.paracelensis sp. nov. is the third supergiant Bathynomus discovered in the South China Sea after B.jamesi and B.vaderi. Its remarkable feature is its short body length and sub-parallel shape. In addition, it is different from B.jamesi and B.vaderi in features such as clypeus shape, number of maxillula keratinised spine and pleotelson spine almost straight. Phylogenetic and barcoding gap analyses confirm that B.paracelensis sp. nov. is not the same species as B.jamsei. Many morphological differences also indicate that it should be a different species from B.vaderi. B.paracelensis sp. nov. may be an intermediate species between giant and supergiant, possessing characteristics of both categories, which can increase researchers' understanding of Bathynomus biodiversity.