Tropodiaptomus longiprocessus sp. nov.

Saetang, Thanida & Maiphae, Supiyanit, 2023, Diversity of the genus Tropodiaptomus Kiefer, 1932 (Crustacea, Copepoda, Calanoida, Diaptomidae) in Thailand, with the description of two new species, Zoosystematics and Evolution 99 (2), pp. 399-422 : 399

Tropodiaptomus longiprocessus sp. nov.

Type locality.

Nong Ping swamp (NP2), Thong Phaphum district, Kanchanaburi province, western Thailand (14°38'49.1"N, 98°33'48.8"E). Temporary habitat without macrophytes.

Other localities.

Nong Ping swamp (NP3) Thong Phaphum District, Kanchanaburi Province, Western Thailand (14°39'00.4"N, 98°34'33.7"E). Temporary habitat without macrophytes.

Material examined.

Holotype. Adult male, dissected and mounted onto one slide, Thong Phaphum District, Kanchanaburi province, western Thailand (14°38'49.1"N, 98°33'48.8"E), 22 June 2019, Thanida Saetang and Supiyanit Maiphae; PSUZC-PK2009-01 . Allotype. One adult female, collected with holotype; PSUZC-PK2009-02. Paratype. One adult male, collected with holotype; PSUZC-PK2009-03.

Description of the adult male.

Body (Fig. 9A, B ). Total body length about 1,500 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Prosome length about 2.4 times as long as urosome (including caudal rami). Fourth and fifth pedigers separated by distinct septum. Fifth pediger produced into small symmetrical posterolateral wings, each distal end with spine. Urosome 5-segmented. Genital somite with dorsolateral sensillum on right side, fourth somite with expanded right corner. Anal somite with deep cleft, length as long as wide. Caudal rami parallel, symmetrical, length about 1.9 times as long as wide, with setules on inner margin. Each ramus with six setae.

Rostrum (Fig. 9C ). Two rostral elements on anterior margin with suture in the middle.

A1 (Figs 10A-C , 11A, B , 16H ). Asymmetrical. Left A1 non-geniculate, 25-segmented, reaching beyond the end of caudal rami. Armature formula of each segment as follows: 1+ae, 3+ae, 1+ae, 1, 1+ae, 1, 1+ae, 1+s, 2+ae, 1, 1, 1+ae+s, 1, 1+ae, 1, 1+ae, 1, 1, 1+ae, 1, 1, 2, 2, 2, 5+ae. Right A1 transformed and geniculate, 22-segmented. Strongly dilated between segment 13 and segment 18. Spinous process on segment 20 (antepenultimate) straight and bent at distal end, reaching longer than next segment, and with longitudinal hyaline membrane on outer margin (Fig. 10D ). Armature formula of each segment as follows: 1+ae, 3+ae, 1+ae, 1, 1+ae, 1, 1+ae, 1 +s, 2+ae, 1+sp, 1+sp, 1+ae+s, 1+ae+sp, 2+ae, 2+ae+sp, 2+ae+sp, 1+s, s, 1+3s, 4+sp, 2, 5+ae.

A2 (Fig. 9D ). Coxa with one inner seta on distal corner. Basis with two inner setae on distal corner. Exopod 7-segmented, exp-1-6 with 1, 3, 1, 1, 1, and 1 inner setae, respectively, and exp-7 with one inner and three apical setae. Endopod 2-segmented, enp-1 with two inner setae and one longitudinal row of outer spinules, enp-2 with nine inner and seven apical setae; and one group of outer spinules.

Mandible (Fig. 12A ). Coxa with eight strongly chitinised teeth and one seta on gnathobase. Basis with four inner setae. Exopod 4-segmented with 1, 1, 1, and 3 setae, respectively. Endopod 2-segmented, enp-1 with four inner setae, enp-2 with nine apical setae and three horizontal rows of outer spinules.

Maxillule (Fig. 12B-E ). Precoxal arthrite with three plumose setae and twelve bipinnate spines. Coxal endite with four plumose setae, and coxal epipodite with seven plumose setae and two bipinnate spines. Basis with two endites; the proximal with four plumose setae and the distal with eight plumose setae, and basal exite with one bipinnate spine. Exopod 1-segmented with six plumose setae, one longitudinal row of inner setules. Endopod 1-segmented with four plumose setae and one horizontal row of outer setules.

Maxilla (Fig. 12F ). Proximal praecoxal endite with six setae, distal praecoxal endite with three setae. Proximal and distal coxal endites with three setae each. Allobasis protruding into endite with four setae. Endopod reduced to two segments, each with three setae.

Maxilliped (Fig. 12G ). Praecoxa completely fused to coxa; endites with 1, 2, 3, and 3 setae, respectively. Distal corner of coxa produced into rounded lobe with spinules on inner margin. Basis with three setae in distal third, one row of setules and one row of spinules in proximal half. Endopod 6-segmented; with 2, 3, 2, 2, 1+1, and 4 setae, respectively.

P1-P4 (Figs 13A-D , 16I ). Biramous. Intercoxal sclerite naked. Coxa with one inner seta. Basis without seta except P4 with one seta on outer distal margin. P1 with 3-segmented exopod and 2-segmented endopod, P2-P4 with 3-segmented exopod and endopod, endopod reaching proximal of exp-3. P1 (Figs 13A , 16I ). Basis with lateral setules close to outer margin. Exp-2, exp-3, enp-1 and enp-2 with one longitudinal row of outer setules. Exp-3 and enp-2 with one row of spinules close to distal end. P2-P4 (Fig. 13B-D ). Exp-1 with one longitudinal row of inner setules. Exp-2 and exp-3 with one longitudinal row of outer and inner setules in P2 and P4, and one longitudinal row of inner setules in P3. Enp-1 with one longitudinal row of outer setules in P3 and P4. Enp-2 and enp-3 with one longitudinal row of outer setules in P2, one longitudinal row of outer and inner setules in P3 and P4, enp-2 of P2 with Schmeil’s organ. Exp-3 and enp-3 with one and two rows of outer spinules close to distal end in P2-P4, respectively. Armature formula of P1-P4 as same as Tropodiaptomus pedecrassum sp. nov. (Table 1 ).

P5 (Figs 14A, E , 16A, E ). Asymmetrical. Left leg, reaching beyond middle of exp-2 of right P5. Coxa as long as wide, with spine inserted on outer lobe. Basis cylindrical, about 1.9 times as long as wide, with one distal outer smooth seta. Exopod flattened, about 2.2 times as long as wide, inner margin two lobes with uniform serration (Figs 14A , 16A, E ). Apex of exopod with usual ‘finger-and-thumb’ combination, ‘finger’ slim, and set with radiant, hair-like ‘thumb’ sphere, ventral surface of exopod with two hairy pads. Endopod 2-segmented (Fig. 16G ), conical, reaching beyond middle of exp-1, rounded distally with two parallel rows of setules. Right leg, coxa as long as wide, with spine inserted on outer lobe. Basis cylindrical, about 1.7 times as long as wide, three structures occurring on dorsal surface: (i) one triangular process in proximal third close to inner margin, (ii) one longitudinal hyaline lamella inserted near inner margin in middle, and (iii) one laterodistal smooth seta. Exopod 2-segmented. Exp-1 small, about 0.5 times as long as wide, with triangular lobe on inner margin, laterodistal corner produced into acute spinous process. Exp-2 rectangular, about 1.5 times as long as wide, dorsal surface with two longitudinal hyaline lamellae in middle and distal inner margin and one large and long process in middle near outer margin. Lateral spine nearly straight, acutely pointed, about 1.3 times as long as exp-2, inserted in laterodistal corner of exp-2, with spinules on its inner margin. End claw curved and gradually tapering to acuminate tip, about 2.5 times as long as exp-2, inner margin with spinules distally. Endopod 1-segmented, conical, reaching beyond proximal margin of exp-2, distal end with two rows of setules.

Description of the adult female.

Body (Figs 15A-C , 16J, K ). Total body length about 1,640 μm (1.6 mm) (measured from anterior margin of rostrum to posterior margin of caudal rami). Prosome length about 3.1 times as long as urosome (including caudal rami). Fourth and fifth pedigers completely fused, fusion being indicated by indentation on each side. Fifth pediger produced into asymmetrical posterolateral wings (right wing shorter than left wing), each distal end with posterior spine, and each wing with one dorsal spine on inner margin (right spine smaller than left spine) (Fig. 16K ). Dorsal surface of fifth pediger without prominence (Fig. 16J ). Urosome 2-segmented. Genital double-somite asymmetrical, about twice times as long as wide, right sides of anterior part swollen (Figs 15B (arrowhead), 16K), with two unequal dorsolateral spines (right spine smaller than left spine) in anterior third (Fig. 15B ). Right distal corner of genital double-somite without lobe (Figs 15B , 16K (arrowhead)). Genital area on ventral surface shows opercular pad protecting gonopores, characterised by rectangular and semicircular expansions (Fig. 15C ). Anal somite about 1.2 times as long as wide. Caudal rami parallel, symmetrical, about 1.5 times as long as wide, with setules on outer and inner margins (Fig. 15B ). Each ramus with six setae.

A1, A2, mandible, maxillule, maxilla, maxilliped, P1-P4 (not shown) and rostrum (Fig. 15D ) same as male.

P5 (Figs 14F , 16L ). Symmetrical. Coxa spine on posterior lobe on caudal surface. Basis with one smooth outer seta on distolateral margin. Exopod 3-segmented. Exp-1 cylindrical, length about 1.9 time as long as wide. Exp-2 tapering into long claw, each side with one row of spinules starting in middle of segment. Exp-3 fused with exp-2 (Figs 14F , 16L (arrowhead)), with two unequal spines, inner spine about 4.9 times as long as outer spine, and with short spine laterally. Endopod 1-segmented, cylindrical, length about 0.7 time as long as exp-1, two strong smooth spiniform setae distally (Figs 14F , 16L (arrowhead)), outer seta longer than inner seta, two parallel rows of spinules on distal end.

Variability.

Morphological variability has been observed in: (i) the total body length (except of caudal setae) which ranged from 1,490 -1,545 µm (mean 1,510 µm, n = 4) in the adult males and 1,570-1,700 µm (mean 1,646 µm, n = 6) in the adult females; and (ii) shape and length of large and long process on exp-2 of the adult male right P5 (Figs 14B-D , 16B-D ). Moreover, it needs to be noted that the inner margin of exopod of the adult male left P5 appeared single lobe in positions other than dorsal which can lead to misidentification (Fig. 16F ).

Etymology.

The specific name ' longiprocessus ' is derived from the presence of a long process on exp-2 of the right P5 in the adult male.

Co-occurring species.

There were no other diaptomid copepods in these samples.

Distribution and ecology.

Tropodiaptomus longiprocessus sp. nov. was found only in its type locality. It is rare because it was found only in two of 471 samples collected from 206 freshwater habitats throughout Thailand between September 2017 and July 2019.Water temperature 26.6-28.0 °C, conductivity 623.3-672.7 µs cm-1, salinity 0.3 ppt, total dissolved solids 385-424 mg L-1, dissolved oxygen 2.5-6.0 mg L-1, pH 7.2-7.3, and water depth 0.2-0.3 m, substrate with mud. However, in order to understand more in their habitat preference, the whole year samples are needed to be examined.

Differential diagnosis.

Tropodiaptomus longiprocessus sp. nov. differs from the congeneric species by the following respects: (i) antepenultimate segment of the male right antennule with straight spinous process reaching beyond the distal margin of next segment; (ii) inner margin of exopodal segment of the male left P5 with two lobes, and with uniform serration; (iii) basis of the male right P5 with one triangular process and one longitudinal hyaline lamella; (iv) exp-1 of the male right P5 with triangular lobe on inner margin, distal outer corner produced into acute spinous process; and (v) exp-2 of the male right P5 rectangular, dorsal surface with two longitudinal hyaline lamellae in middle and distal inner margin and one large and long process in middle in lateral third.

Tropodiaptomus longiprocessus sp. nov. is the most similar to T. lanaonus , but it differs distinctively in the following characters: (i) the dorsal surface of exp-2 of the adult male right P5 has two longitudinal hyaline lamellae and one large and long process; (ii) lateral wings of the fifth pediger of adult female are asymmetrical; (iii) anterior part of the genital double-somite of the adult female is dilated on the right side; (iv) right posterior corner of the genital double-somite of the adult female does not have a round lobe; and (v) distal end of the endopodal segment of the adult female P5 has smooth spiniform setae (Table 2 ).

The diversity of Tropodiaptomus in Thailand is presented as a pictorial key in Fig. 17 to facilitate easy and quick identification.

Figures

Figure 9. Tropodiaptomus longiprocessus sp. nov., male holotype. A. habitus, dorsal view; B. pediger 5 and urosome, dorsal view; C. rostrum; D. antenna. Scale bars: 100 µm.

Figure 10. Tropodiaptomus longiprocessus sp. nov., male holotype. A. Segment 1 - 11 of right antennule; B. Segment 12 - 19 of right antennule (gray color indicates spine); C. Segment 20 - 22 of right antennule; D. Segment 20 and 21 of right antennule of the adult male specimen from the same population of holotype. Scale bar: 100 µm.

Figure 11. Tropodiaptomus longiprocessus sp. nov., male holotype. A. Left antennule, segment 1 - 15; B. Left antennule, segment 16 - 25 (gray color indicates spine). Scale bar: 100 µm.

Figure 12. Tropodiaptomus longiprocessus sp. nov., male holotype. A. Mandible; B. Maxillule; C-E. Maxillule; F. Maxilla; G. Maxilliped. Scale bar: 100 µm.

Figure 13. Tropodiaptomus longiprocessus sp. nov., male holotype. A. P 1 (arrowhead indicates lateral setules close to outer margin); B. P 2; C. P 3; D. P 4. Scale bar: 100 µm.

Figure 14. Tropodiaptomus longiprocessus sp. nov., male holotype. A. P 5, dorsal view; B-D. Shape variation of hyaline lamella on exp- 2 of right P 5 in adult male, dorsal surface; E. Exp- 2 of right P 5 of the adult male specimen from the same population of holotype, lateral view (arrowhead indicates hyaline lamella); Tropodiaptomus longiprocessus sp. nov., female allotype: F. P 5 (arrowhead indicates exp- 3). Scale bar: 100 µm.

Figure 15. Tropodiaptomus longiprocessus sp. nov., female allotype. A. Habitus, dorsal view; B. Pediger 5 and urosome, dorsal view; C. Genital double-somite, ventral view; D. Rostrum. Scale bars: 100 µm.

Figure 16. Tropodiaptomus longiprocessus sp. nov; male. A. P 5, dorsal view; B, C. Hyaline lamella on exp- 2 of right P 5, dorsal surface; D. Hyaline lamella on exp- 2 of right P 5, lateral surface; E, F. Exopodal segment of the male left antennule, dorsal surface; G. Exopodal segment of the male left antennule, ventral surface; H. Antepenultimate segment of right antennule; I. Lateral part of P 1 basis; female: J. Pediger 5 and urosome, lateral view; K. Pediger 5 and urosome, dorsal view; L. P 5. Arrowheads show the characters discussed in the description.

Figure 17. Pictorial key of the adult male P 5 of Tropodiaptomus species found in Thailand. Right exp- 2 of P 5 of Tropodiaptomus lanaonus and T. hebereri were modified from Kiefer (1982).