Two new species of Xouthous Thomson, 1883 (Copepoda, Harpacticoida, Pseudotachidiidae) from the Caribbean

During a coastal biodiversity survey of meiofauna taxa in southwest Puerto Rico, specimens of the genus Xouthous Thomson, 1883 were recovered. Morphological and genetic studies were ensued and two new species, Xouthous spinifurca sp. nov. and Xouthous dichroma sp. nov. are herein described. The new species were recovered from epiphytic algae growing on red mangrove (Rhizophora mangle) roots. Xouthous spinifurca sp. nov. is closely related to X. aemula (Thompson & Scott, 1903), X. wellsi Huys, 2016, and X. yeoghooni Song, Lee, Lee & Khim, 2020 sharing with them the palisade-like spines on the female baseoendopod of the fifth pereopod and the seven-segmented female antennule. It differs from the first two species by the presence of longer baseoendopodal spines of the fifth pereopod, as long or longer than the baseoendopod, the baseoendopod reaching the middle of the longer exopod. It is closely related to X. yeoghooni, sharing the same female baseoendopod/exopod ratio of the fifth pereopod, a rectangular endopod-2 of the first pereopod, and a strong outer process in the endopod-2 of the second pereopod. However, in the new species, the baseoendopod spines are longer, the endopod-2 of the first pereopod has three inner setae instead of two present in X. yeoghooni, and the outer process on the endopod-2 of the second pereopod is stronger in X. spinifurca sp. nov. Xouthous dichroma sp. nov. does not belong to the palisade group and appears to be closely related to X. simulans (Brady, 1910), and X. naroensis Karanovic, 2023, sharing the presence of two inner setae on the endopod-2 of the second pereopod. It differs from X. simulans by the presence of a shorter baseoendopodal seta 1, even shorter than setae 3-5, not ending in a long flagellate portion, and the presence of a shorter female exopod of the fifth pereopod exp, not longer than twice its width (about three times as long as wide in X. simulans), and with five setae (six in X. simulans). Xouhous naroensis and X. dichroma share the same ratio and armature of the female exopod/baseoendopod of the fifth pereopod, differing, however, in the presence of serrate outer spines in the third exopod of the second to fourth pereopods. A gap between setae four and five of the female baseoendopod of the fifth pereopod occur in both X. dichroma and X. naroensis. A genetic analysis of available cytochrome oxidase subunit I (COI) Pseudotachidiidae sequences indicate that the two new Xouthous species share a closer genetic distance than to other available species of Pseudotachidiidae in GenBank. The 20% sequence divergence in COI between the two Xouthous species may be indicative of the phylogenetic separation of the species belonging to the palisade group (e.g., X. spinifurca sp. nov.) and those that do not (e.g., X. dichroma sp. nov.). Within species divergence in two different species of Pseudotachidiidae ranges from 0.7 to 1% and between species belonging to different genera can be as high as 36-37%.