﻿Thailandorchestiarhizophila sp. nov., a new genus and species of driftwood hopper (Crustacea, Amphipoda, Protorchestiidae) from Thailand

﻿Abstract During a scientific survey, a new genus of driftwood hopper was found in mangrove roots in Ko Kut District, Trat Province, Thailand. We placed this new genus, Thailandorchestiagen. nov., within the family Protorchestiidae. The new genus can be distinguished from the remaining genera by uropod 1 outer ramus with robust setae, uropod 2 outer ramus without robust setae, and pereopod 7 basis without a posterodistal lobe. The type species of Thailandorchestiagen. nov., Thailandorchestiarhizophilasp. nov., is described herein, and an updated key to the genera of the family Protorchestiidae is provided.

Herein, we describe a 4-dentate noncuspidactylate palustral amphipod with basis of pereopod 7 without a posterodistal lobe as a new genus and species of the family Protorchestiidae. The new species was discovered in mangrove roots (Rhizophora sp.) and rotting logs in Ko Kut District, Trat Province, Thailand.

Materials and methods
Amphipods were collected from driftwood, rotting logs and mangrove roots (Rhizophora sp.) in a mangrove forest near Ao Phrao, Ko Kut District, Trat Province, Thailand (11°35'40.2"N, 102°33'52.6"E) (Fig. 1). The mangrove forest is located near a small creek 50 meters from the beach. Twelve rotting logs were broken apart and 15-30 amphipod individuals were found inside each log (see Suppl. material 1). The amphipod specimens were sorted and fixed in 70% ethanol. The specimens were transferred from ethanol onto a glycerol slide for morphological study in the laboratory. Drawings were made using a drawing tube attached to an Olympus CH30 light microscope. The pencil drawings were scanned and digitally inked using a WACOM bamboo CTH-970 graphics board in Adobe Illustrator CC 2017, following the method described in Coleman (2003). Setae and mouthparts were following Zimmer et al. (2009). Abbreviations used in the text are as follows: A, antenna; G, gnathopod; UL, labrum; LL, labium; MD, mandible; MX, maxilla; MP, maxilliped; P, pereopod; p, palp; pl, pleopod; T, telson; U, uropod; L, left; R, right. Institutional abbreviations: THNHM, Thailand Natural History Museum, Bangkok, Thailand.
Etymology. The generic name, Thailandorchestia gen. nov., is derived from "Thailand" in combination with the Orchestia stem.
Ecological type. Driftwood hoppers (virtually confined to rotting driftwood where they live in galleries, consuming rotting driftwood and reproducing with relatively small broods).
The new genus is closely related to Microrchestia in having: 1) left mandible larcinia mobilis 4-dentate; 2) carpus of pereopod 3 subequal to those of pereopod 4; and 3) pereopods 6 and 7 sexually dimorphic. However, the current genus differs from Microrchestia from Australia by having: 1) maxilliped palp article 2 distomedial lobe absent (vs. well developed); 2) pereopod 7 posterodistal lobe absent (vs. present), and 3) U1 outer ramus with marginal robust setae (vs. without marginal robust setae) ( Table 1). Only one protochestiid amphipod had been previously reported from Thailand. Bussarawich (1985) studied the diversity of amphipods in the mangrove forest and reported Microchestia sp., a member of the family Protorchestiidae. Later, Lowry and Springthorpe (2015) revised the genus Cochinorchestia Lowry & Peart, 2010. Although the Microrchestia sp. from Thailand was also mentioned as a Cochinorchestia sp. based on the illustration of the previous publication, some details such as the maxilliped and gnathopods 1 and 2 remain unclear. The specimens from the report of Bussarawich (1985) presumed lost, which makes the Cochinorchestia sp. in this report still tentative.
The new genus is identifiable using the following key to genera of Protorchestiidae.
Ecology. Driftwood hoppers, living inside rotten logs and mangrove roots in the softest part under the bark. The mangrove forest is located near a small creek 50 meters from the beach. The sediment in the forest is muddy sand mixed with leaf litter.
Etymology. The specific epithet refers to the habitat of this amphipod, which is also found inside mangrove roots.
Habitat. Mangrove wood, inside roots and rotting logs. Distribution. Thailand, Ko Kut District, Inner Gulf of Thailand.

Discussion
Most species of Protorchestiidae are known to be semiaquatic marsh hoppers that inhabit salt marshes and mangrove swamps (Myers and Lowry 2020), except for Neorchestia, which has adapted to life on land (Friend 1987). Protochestiid amphipods were previously reported to live in hard substrates (rock) and soft substrates (sand, mangrove debris, and wet forest soil) (Myers and Lowry 2020). Surprisingly, Thailandorchestia gen. nov. specimens live in galleries inside the mangrove roots, where gravid females are also found, implying that these amphipods reproduce inside the roots. According to this ecology, these amphipods should be classified as driftwood hoppers. This is the second genus reported as a driftwood hopper; a previous driftwood hopper report is of the genus Macarorchestia in the northeast Atlantic and Mediterranean coastal regions (Wildish 2014). Based on these observations, the adaptations observed in Thailandorchestia rhizophila sp. nov. are akin to those in Macarorchestia in having: 1) reduced pleopod and oostegites; 2) fewer ova per brood (5-6 individuals); 3) small eyes; and 4) lack of dorsal pigment (Wildish 2017). Another behavior found in the present study was negative phototaxis, whereby T. rhizophila sp. nov. specimens escaped deeper inside the wood upon its splitting. According to the recent checklist of the amphipods of Southeast Asia (Azman et al. 2022), a total of 25 species of Talitroidea amphipods have been reported, with four species (16%) occurring in Thailand. From that, Thailandorchestia rhizophila sp. nov. is the only one species has been reported from mangrove forest while consider the area of mangrove forest in Thailand covers 2,300 square kilometres (Pumijumnong 2014). Further intensive study of mangrove amphipods, especially in the marsh hopper group, is required.