A new species of Gammarus Fabricius, 1775 (Crustacea: Amphipoda: Gammaridae) from extreme high‐altitude Lake Tong Tso, Qinghai‐Tibetan Plateau

A new species of the genus Gammarus Fabricius, 1775 is described from Lake Tong Tso, an extreme high-altitude lake in the Qinghai-Tibetan Plateau. Gammarus pontual sp. nov. is considered part of the Gammarus lacustris G.O. Sars, 1863 group, which is characterized by having the inner ramus of uropod 3 longer than half the length of the outer ramus, and by having both rami covered with plumose setae. The new species stands apart from their congeners, mainly, by the following combination of characters: head with cephalic lateral lobe truncated; gnathopods 1-2 propodus weakly armed, with 1 stout seta defining palm; palm longer than posterior margin of propodus, with 1 main stout seta medially; pereopods 5-7 basis slender, posterior margin crenulate; epimeral plate  1 posterior margin produced into a short spine, facial ridge present; epimeral plates 2-3 posteroventral corner sharply pointed; uropod 3 rami covered with plumose setae, inner ramus about 80% the length of the outer ramus; telson apex with 2 stout setae on each side.


INTRODUCTION
Gammaridae Leach, 1814, is a monophyletic family whose members are often found inhabiting springs, rivers, streams, lakes, lagoons, and marine littoral zones (Hou & Sket, 2015). It consists of 42 genera, of which Gammarus Fabricius, 1775, is the most speciose and diverse. The genus is widely spread in the Holoarctic realm, consisting of 268 species whose distribution centered in Europe, but ranges as far as North America to the west and China to the east (Väinölä et al., 2008;Horton et al., 2021 onwards).
Currently, 15 Gammarus species in four species groups are known from the Tibetan Plateau, of which 14 are considered endemic (Hou & Li, 2018). In this paper, we describe a new species of Gammarus of the G. lacustris species group from the Qinghai-Tibetan Plateau.

MATERIAL AND METHODS
Five specimens were manually collected from shallow depths (10 to 20 cm) in the extreme high-altitude saltwater lake Tong Tso, Qinghai-Tibetan Plateau, 4,399 meters above sea level. All specimens are preserved in 70% ethanol and deposited at the Museu de Zoologia, Universidade de São Paulo (MZUSP).
For the taxonomic study, the mouthparts and appendages were dissected and mounted in glycerin gel slides. The drawings were made under an optical microscope with camera lucida, Leica DM 5000 B, and digitized with CorelDRAW 2018. The setal/spine classification adopted in this paper follows Garm & Watling (2013) and the nomenclature of gnathopod palms is based on Poore & Lowry (1997). Measurements refer to the total body length. Diagnosis: Head with deep antennal sinus. Maxilla 2 inner plate with a very dense facial oblique row of setae. Gnathopod 1 propodus weakly armed, lateral margin with 1 stout seta defining palm, palm longer than posterior margin of propodus, with a truncated projection bearing 1 stout seta medially. Gnathopod 2 propodus weakly armed, palm lateral margin with a proximal large subrounded projection bearing 1 main stout seta. Pereopods 5-7 basis slender, posterior margin crenulate. Epimeral plate 1 posterior margin produced into a short spine, facial ridge present. Epimeral plates 2-3 posteroventral corner sharply pointed. Uropod 3 rami covered with plumose setae. Telson apex with 2 stout setae on each side.
Etymology: This species is named after conservation biologist Francisco Pontual, in recognition for his efforts directed towards habitat protection. The specific epithet, pontual, is to be used as a name in apposition.