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Research Article
Thailandorchestia rhizophila sp. nov., a new genus and species of driftwood hopper (Crustacea, Amphipoda, Protorchestiidae) from Thailand
expand article infoKoraon Wongkamhaeng, Pongrat Dumrongrojwattana§, Ratchaneewarn Sumitrakij|, Tosaphol Saetung Keetapithchayakul
‡ Kasetsart University, Bangkok, Thailand
§ Burapha University, Bangsaen, Thailand
| National science museum, Pathum Thani, Thailand
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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, Thailandorchestia gen. 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 Thailandorchestia gen. nov., Thailandorchestia rhizophila sp. nov., is described herein, and an updated key to the genera of the family Protorchestiidae is provided.

Keywords

Description, Ko Kut District, marsh hopper, Talitroidea, Thailandorchestia gen. nov.

Introduction

The family Protorchestiidae is a mascupod family established by Myers and Lowry (2020) and contains 24 species belonging to six genera, namely Cochinorchestia Lowry & Peart, 2010, Eorchestia Bousfield, 1984, Microrchestia Bousfield, 1984, Neorchestia Friend, 1987, and Protorchestia Bousfield, 1982. All of them are classified as a post-Gondwanaland group (Myers and Lowry 2020). Each genus is distributed in different areas of the world, with Cochinorchestia located in southern India and Mozambique on the western coast of Africa (Lowry and Peart 2010; Lowry and Springthorpe 2015), Eorchestia in South Africa (Richardson 1993), and Microchestia, Neochestia, and Protochestia in Australia (Bousfield 1984; Friend 1987; Richardson 1996). All members of this group are marsh hoppers who occupy mangrove forests, except Neorchestia, which are forest hoppers. They all have some primitive characteristics, including: 1) maxilliped palp article 2 without a distomedial lobe; 2) article 4 small, distinct and gnathopod 2 subchelate; 3) pereopods 3–7 simplidactylate; and 4) pereopod 4 dactylus basidactylate.

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.

Figure 1. 

Thailandorchestia rhizophila sp. nov. a holotype, male, 8.04 mm, THNHM-Iv- 18760 b allotype, female, 7.80 mm, THNHM-IV- 18961 c rotting mangrove log, habitat of Thailandorchestia rhizophila sp. nov.

Results

Systematics

Order Amphipoda Latreille, 1816

Suborder Senticaudata Lowry & Myers, 2013

Family Protorchestiidae Myers & Lowry, 2020

Thailandorchestia gen. nov.

Type species

Thailandorchestia rhizophila sp. nov., here designated.

Diagnosis

Protorchestiidae with maxilliped palp article 2 distomedial lobe absent. Mandible left lacinia mobilis 4-dentate. Gnathopod 2 coxal gill simple. Pereopod 4 carpus significantly shorter than carpus of pereopod 3. Pereopods 6–7 sexually dimorphic (male merus and carpus incrassate). Pereopod 7 posterodistal lobe absent. Uropod 1 peduncle distolateral robust setae present, very large (1/3–1/2 length of outer ramus); inner ramus linear, not modified; outer ramus with marginal robust setae. Uropod 2 outer ramus without marginal robust setae. Uropod 3 peduncle with 2 robust setae; ramus shorter than peduncle, linear (narrowing). Telson apically incised, with 2 robust setae per lobe.

Etymology

The generic name, Thailandorchestia gen. nov., is derived from “Thailand” in combination with the Orchestia stem.

Type locality

Mangrove forest near Ban Ao Prao Beach (11°35'40.2"N, 102°33'52.6"E), Trat Province, Thailand.

Ecological type

Driftwood hoppers (virtually confined to rotting driftwood where they live in galleries, consuming rotting driftwood and reproducing with relatively small broods).

Remarks

The new genus clearly belongs to Protorchestiidae due to the presence of: 1) maxilliped palp article 2 without distomedial lobe; 2) article 4 small, distinct; 3) gnathopod 2 subchelate; 4) pereopods 3–7 simplidactylate; 5) pereopod 4 dactylus basidactylate; and 6) telson with apical robust setae only or with apical and marginal robust setae, with 1–6 robust setae per lobe.

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).

Table 1.

Comparison of diagnostic characteristics in different protorchestiid genera.

Genus MP palp article 2 distomedial lobe LMD lacinia mobilis G1 sexual dimorphism Carpi of P4:P3 P6–7 sexual dimorphism P7 postero- distal lobe U1 outer ramus U2 outer ramus marginal setae U3 robust setae on peduncle U3 ramus Number of setae per telsonic lobe
Carpentaria well developed 4-dentate absent, palm obtuse subequal absent present linear without marginal setae present 1–4 bud-like 3–6
Cochinorchestia present 4-dentate absent, palm transverse longer unknown present spoon-shape with marginal setae absent 1 linear 2
Eorchestia absent 4-dentate absent, palm transverse longer absent present linear without marginal setae absent 3 linear 1–2
Microrchestia well developed 4-dentate present, palm transverse longer present present linear without marginal setae absent 2 linear 2
Neorchestia absent 5-dentate absent, palm transverse longer unknown present linear without marginal setae absent 2 linear 1
Protorchestia absent 5-dentate absent, palm transverse subequal absent present linear without marginal setae absent 3 linear 2
Thailandorchestia gen. nov. absent 4-dentate present, palm transverse longer present absent linear with marginal setae absent 2 linear 2

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 similar to Cochinorchestia from China in having: 1) left mandible larcinia mobilis 4-dentate; 2) carpus of pereopod 3 longer than that of pereopod 4; and 3) uropod 1 outer ramus with marginal setae. However, the current genus differs from Cochinorchestia in having: 1) pereopod 7 without a posterodistal lobe (vs. pereopod 7 with a posterodistal lobe); 2) uropod 1 outer ramus linear (vs. spoon-shaped) and uropod 3 peduncle with 2 robust setae (vs. with 3 robust setae); and 3) uropod 2 outer ramus without robust setae (vs. with marginal robust setae in 1 row).

The new genus is identifiable using the following key to genera of Protorchestiidae.

Key to genera of Protorchestiidae

1 Uropod 3 peduncle with 4 robust setae Carpentaria
Uropod 3 peduncle with less than 4 robust setae 2
2 Uropod 3 peduncle with 3 robust setae 3
Uropod 3 peduncle with less than 3 robust setae 4
3 Maxilliped palp article 2 distomedial lobe absent; mandible left lacinia mobilis 4-dentate; pereopod 4 carpus shorter than carpus of pereopod 3 Eorchestia
Maxilliped palp article 2 distomedial lobe present; mandible left lacinia mobilis 5-dentate; pereopod 4 carpus subequal to carpus of pereopod 3 Protorchestia
4 Uropod 3 peduncle with 1 robust seta Cochinorchestia
Uropod 3 peduncle with 2 robust setae 5
5 Mandible left lacinia mobilis 5-dentate Neorchestia
Mandible left lacinia mobilis 4-dentate 6
6 Uropod 1 outer ramus without marginal robust setae; basis of pereopod 7 with a posterodistal lobe Microrchestia
Uropod 1 outer ramus with marginal robust setae; basis of pereopod 7 without a posterodistal lobe Thailandorchestia gen. nov.

Thailandorchestia rhizophila sp. nov.

Diagnosis

As for the genus unless otherwise stated. Antenna 1 long, reaching from midpoint to end of article 5 of antenna 2 peduncle. Eye medium (1/5–1/3 of head length). Gnathopod 1 not sexually dimorphic, palm transverse, dactylus shorter than palm. Gnathopod 2 sexually dimorphic (male subchelate, female mitten-shaped). Pleopod 1 outer ramus subequal in length to peduncle. Pleopod 3 outer ramus longer than peduncle.

Material examined

Holotype , male, 8.04 mm, THNHM-Iv- 18760; allotype, female, 7.80 mm, THNHM-IV- 18961; Paratypes, 2 males, 1 non-gravid female, and 2 gravid females, THNHM- Iv 18761. All collected from the type locality on 4 May 2019, KW and PD leg.

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.

Type locality

Mangrove forest near Ban Ao Prao Beach (11°35'40.2"N, 102°33'52.6"E), Ko Kut District, Trat Province, Thailand.

Etymology

The specific epithet refers to the habitat of this amphipod, which is also found inside mangrove roots.

Description of male holotype

(THNHM-Iv- 18760, Figs 25).

Head. Eye medium (1/5–1/3 head length). Antenna 1 (Fig. 2A1) long, reaching from midpoint to end of article 5 of antenna 2 peduncle. Antenna 2 (Fig. 2A2) peduncular articles slender, article 5 longer than article 4. Upper lip (Fig. 3UL) without robust setae. Mandible (Fig. 3LMD) left lacinia mobilis 4-dentate. Maxilla 1 (Fig. 3MX1) with small palp, 1-articulate. Maxilliped (Fig. 3MP) palp article 2 distomedial lobe absent; article 4 small, well defined.

Figure 2. 

Thailandorchestia rhizophila sp. nov. holotype, male, 8.04 mm, THNHM-Iv- 18760. Scale bars: 1 mm.

Figure 3. 

Thailandorchestia rhizophila sp. nov. holotype, male, 8.04 mm, THNHM-Iv- 18760. Scale bars: 0.2 mm.

Pereon. Gnathopod 1 (Fig. 2G1) sexually dimorphic; subchelate; coxa 1 smaller than coxa 2; posterior margins of merus, carpus, and propodus each with lobe covered in palmate setae, palmate lobes present; propodus shorter than carpus, subrectangular; palm transverse. Gnathopod 2 (Fig. 2G2) sexually dimorphic; subchelate; coxal gill simple (or slightly lobate); basis slender; carpus triangular, reduced (enclosed by the merus and propodus), posterior lobe absent, not projecting between merus and propodus; 1.8× as long as wide; palm acute, weakly toothed, with a subquadrate protuberance near dactylar ringe, lined with robust setae, posterodistal corner with socket; dactylus subequal in length to palm. Pereopod 3–4 (Fig. 4P3–P4) coxae wider than deep. Pereopods 3–7 (Fig. 4P3–P7) simplidactylate. Pereopod 4 (Fig. 4P4) subequal or slightly shorter than pereopod 3; carpus similar in length to pereopod 3 carpus; dactylus similar to pereopod 3 dactylus. Pereopod 5 propodus distinctly longer than carpus. Pereopod 6 (Fig. 4P6) slightly sexually dimorphic; shorter than pereopod 7; coxa posterior lobe inner view posteroventral corner rounded, posterior margin oblique with respect to ventral margin, posterior lobe without a ridge, posterior lobe without marginal setae; coxal gill lobate. Pereopod 7 (Fig. 4P7) sexually dimorphic (merus and carpus broadly incrassate); basis lateral sulcus absent, posterodistal lobe absent; distal articles (merus and carpus) expanded; merus posterior margin expanded distally, subtriangular.

Figure 4. 

Thailandorchestia rhizophila sp. nov. holotype, male, 8.04 mm, THNHM-Iv- 18760. Scale bars: 1 mm.

Pleon. Pleopods all well developed. Pleopod 1 (Fig. 5PL1) peduncle without marginal setae; biramous, outer ramus subequal in length to peduncle; inner ramus with 17 articles, outer ramus with 13 articles. Pleopod 2 (Fig. 5PL2) peduncle without marginal setae; biramous, outer ramus subequal in length to peduncle; inner ramus with 15 articles, outer ramus with 14 articles. Pleopod 3 (Fig. 5PL3) peduncle without marginal setae; biramous, outer ramus subequal in length to peduncle; inner ramus with 15 articles, outer ramus with 13 articles. Uropod 1 (Fig. 5U1) peduncle with 4 robust setae, distolateral robust seta present, large (1/4 length of outer ramus), with simple tip; inner ramus subequal in length to outer ramus, inner ramus with marginal robust setae; outer ramus with 3 marginal robust setae. Uropod 2 (Fig. 5U2) inner ramus subequal in length to outer ramus, with marginal robust setae, with 3 lateral robust setae; outer ramus without marginal robust setae. Uropod 3 (Fig. 5U2) peduncle with 2 robust setae; ramus shorter than peduncle, ramus triangular, with 2 apical setae. Telson (Fig. 5T) longer than broad, apically incised, dorsal midline vestigial or absent, with apical robust setae only and 2 robust setae per lobe.

Figure 5. 

Thailandorchestia rhizophila sp. nov. holotype, male, 8.04 mm, THNHM-Iv- 18760. Scale bars (U1–U3, T): 0.1 mm; (PL1–PL3): 0.5 mm.

Description of female allotype

(THNHM-Iv- 18761, Figs 67)

Pereon. Gnathopod 1 (Fig. 6G1) propodus narrower than that of male; dactylus subequal to palm. Gnathopod 2 (Fig. 6G2) mitten-shaped; basis slightly expanded; posterior margins of merus, carpus, and propodus each with lobe covered in palmate setae; carpus well developed (not enclosed by merus and propodus), posterior lobe present, projecting between merus and propodus; propodus length twice as long as wide; palm obtuse, smooth, without a protuberance or shelf near dactylar hinge, posterodistal corner naked; dactylus shorter than palm; gill lobate. Pereopod 5 (Fig. 7P5) propodus shorter than carpus. Distal articles (merus and carpus) slender. Preopods 6–7 (Fig. 7P6–7) sexually dimorphic (merus and carpus not broadly incrassate). Oostegites long (length greater than 2× width), longer than wide, weakly setose, setae with simple, smooth tips.

Figure 6. 

Thailandorchestia rhizophila sp. nov. allotype, female, 7.80 mm, THNHM-Iv- 18761. Scale bars: 1 mm.

Figure 7. 

Thailandorchestia rhizophila sp. nov. allotype, female, 7.80 mm, THNHM-Iv- 18761. Scale bars: 1 mm.

Figure 8. 

Map showing distribution of genera in the family Protorchestiidae.

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.

Acknowledgements

This study is a part of the research project “Diversity of amphipods along coastal area of Koh Kut, Trat Province” under the Plant Genetic Conservation Project under the Royal initiative of Her Royal Highness Princess Maha Chakri Sirindhorn. This work was financially supported by the Office of the Ministry of Higher Education, Science, Research and Innovation of Thailand and the Thailand Science Research and Innovation, Higher Education, Science, Research and Innovation of Thailand through the Kasetsart University Reinventing University Program 2021 and grants from the Kasetsart University Research and Development Institute (KURDI), Kasetsart University, Bangkok, Thailand. In addition, we are grateful to the Department of Zoology, Faculty of Science, Kasetsart University for the laboratory facilities.

References

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Supplementary material

Supplementary material 1 

Video of living Thailandorchestia rhizophila

Koraon Wongkamhaeng, Pongrat Dumrongrojwattana, Ratchaneewarn Sumitrakij, Tosaphol Saetung Keetapithchayakul

Data type: Mp4 file.

Explanation note: Video of living Thailandorchestia rhizophila inside the rotting branch and their food item.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
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