Tropodiaptomus pedecrassum 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

publication ID	https://dx.doi.org/10.3897/zse.99.105511
publication LSID	lsid:zoobank.org:pub:F8AA89A2-56BC-4A96-9835-C16AAE2744C1
persistent identifier	https://treatment.plazi.org/id/57CAE0BB-3278-5314-ABD9-28FF3F02ADD0
treatment provided by	Zoosystematics and Evolution by Pensoft
scientific name	Tropodiaptomus pedecrassum sp. nov.
status

Treatment

Type locality.

Swamp near rice field, Dok Kham Tai District, Phayao Province, northern Thailand (19°13'57.6"N, 100°02'56.5"E). Temporary habitat without macrophytes.

Material examined.

Holotype. One adult male, dissected and mounted onto five slides, Dok Kham Tai, Phayao province, northern Thailand (19°13'57.6"N, 100°02'56.5"E), 31 January 2018, Thanida Saetang and Supiyanit Maiphae; PSUZC-PK2008-01-PSUZC-PK2008-05 GoogleMaps . Allotype. One adult female, collected with holotype; PSUZC-PK2008-06-PSUZC-PK2008-08. Paratype. One adult male, collected with holotype; PSUZC-PK2008-09-PSUZC-PK2008-11.

Description of the adult male.

Body (Figs 1A View Figure 1 , 2A View Figure 2 ). Total body length about 1,145 µm (1.1 mm) (measured from anterior margin of rostrum to posterior margin of caudal rami). Prosome length about 2 times as long as urosome (including caudal rami). Fourth and fifth pedigers separated by distinct septum. Fifth pediger produced into small asymmetrical posterolateral wings (left wing shorter than right wing), each distal end with spine. Urosome 5-segmented. Genital somite with dorsolateral sensilla on right and left side, fourth somite with expanded right corner. Anal somite with deep cleft, length about 0.8 times as long as wide. Caudal rami parallel, symmetrical, length about 1.8 times as long as wide, with setules on inner margin. Each ramus with six setae.

Rostrum (Fig. 3A View Figure 3 ). Two rostral elements on anterior margin with suture in the middle.

A1 (Figs 1B, C View Figure 1 , 3B-D View Figure 3 ). 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 13and segment 18. Spinous process on segment 20 (antepenultimate) straight and bent at distal end, reaching 3/4 next segment, and with longitudinal hyaline membrane on outer margin (Fig. 7A-G View Figure 7 , arrowhead). 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. 4A View Figure 4 ). 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. 4B View Figure 4 ). 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 one longitudinal row of outer spinules.

Maxillule (Fig. 4C View Figure 4 ). Precoxal arthrite with four plumose setae and eleven bipinnate spines.

Coxal endite with three 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 spines. Exopod 1-segmented with six plumose setae, one longitudinal row of setules on inner margin of segment. Endopod 1-segmented with four plumose setae.

Maxilla (Fig. 4D View Figure 4 ). Proximal praecoxal endite with four setae, distal praecoxal endite with three setae. Proximal and distal coxal endites with three setae each. Allobasis protruding into endite with five setae. Endopod reduced to two segments, enp-1 with two setae and enp-2 with three setae.

Maxilliped (Fig. 4E View Figure 4 ). Praecoxal endite with one seta. Coxal endites with 2, 3, and 3 setae, respectively. Distal corner of coxa produced into rounded lobe with spinules on inner margin. Basis with three setae on distal third, and one row of setules and one row of spinules on inner margin. Endopod 6-segmented; with 2, 3, 2, 2, 1+1, and 4 setae, respectively.

P1-P4 (Fig. 5A-D View Figure 5 ). 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 (Fig. 5A View Figure 5 ). Exp-1 with one longitudinal row of inner setules. Exp-2 and exp-3 with one longitudinal row of outer setules. Enp-2 with one row of spinules close to distal end. P2-P4 (Fig. 5B-D View Figure 5 ). Exp-1 with one longitudinal row of inner setules. Exp-2 and exp-3 with one longitudinal row of outer and inner setules. Enp-1 with one longitudinal row of outer setules in P2 and P4. Enp-2 with one longitudinal row of outer setules in P2-P4, enp-2 of P2 with Schmeil’s organ. Enp-3 with one longitudinal row of outer setules in P2, one longitudinal row of outer and inner setules in P4. 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 in Table 1 View Table 1 .

P5 (Figs 6A View Figure 6 , 8A-D View Figure 8 ). Asymmetrical. Left leg (Figs 6A View Figure 6 , 8A-D View Figure 8 ), reaching slightly beyond proximal margin of exp-2 of right P5. Coxa as long as wide, with spine inserted on outer lobe. Basis cylindrical, about 1.4 times as long as wide, with one distal outer smooth seta. Exopod flattened, about 1.7 times as long as wide, inner margin one lobe, the serration gradually decreases in size from the proximal to distal end (Fig. 8C View Figure 8 , arrowhead). Apex of exopod with usual ‘finger-and-thumb’ combination, ‘finger’ slim, and set with radiant, hair-like ‘thumb’ sphere, ventral surface of exopod with 2 hairy pads. Endopod 2-segmented with incomplete separation of segments (Fig. 8D View Figure 8 , arrowhead), conical, reaching beyond middle of exp-1, rounded distally with two parallel rows of setules. Right leg (Figs 6A View Figure 6 , 8A, B View Figure 8 ), coxa as long as wide, with spine inserted on outer lobe. Basis cylindrical, about 1.3 times as long as wide, four structures occurring on dorsal surface: (i) one round process on proximal third close to inner margin, (ii) one triangular process in middle close to inner margin, (iii) one longitudinal hyaline lamella inserted near inner margin, and (iv) one distal outer smooth seta. Exopod 2-segmented. Exp-1 small, about 0.6 times as long as wide, with triangular lobe on inner margin, distal outer corner produced into acute spinous process, nearly as long as its segment. Exp-2 rhomboid, about 2.7 times as long as wide, dorsal surface with one semicircular hyaline knob on proximal outer margin, one semicircular hyaline lamella on distal inner margin, and one triangular process in middle of segment (Figs 6A View Figure 6 , 8A, B View Figure 8 (arrowhead)), and distal end of segment with one round hyaline prominence inserted between end claw and lateral spine (Fig. 8A, B View Figure 8 (arrowhead)). Lateral spine nearly straight, acutely pointed, about as long as exp-2, inserted on distal corner of exp-2, with spinules on its inner margin. End claw curved and gradually tapering to acuminate tip, about 2.6 times as long as exp-2, inner margin with spinules distally. Endopod 1-segmented, conical, reaching distal end of exp-1, distal end with two rows of setules.

Description of the adult female.

Body (Figs 1E View Figure 1 , 2B View Figure 2 , 6B View Figure 6 , 8E, F View Figure 8 ). Total body length about 1,626 µm (1.6 mm) (measured from anterior margin of rostrum to posterior margin of caudal rami). Prosome length about 2.6 times as long as urosome (including caudal rami). Fourth and fifth pedigers completely fused, fusion being indicated by indentation on each side (Figs 1E View Figure 1 , 2B View Figure 2 ). 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) (Figs 1E View Figure 1 , 2B View Figure 2 ). Dorsal surface of fifth pediger with prominence (Figs 2B View Figure 2 (grey ellipse), 8E (arrowhead)). Urosome2-segmented. Genital double-somite asymmetrical, about 1.8 times as long as wide, both sides of anterior part swollen (Fig. 2B View Figure 2 (arrowhead)), with two unequal dorsolateral spines (right spine smaller than left spine) in anterior third (Figs 1E View Figure 1 , 2B View Figure 2 , 8F View Figure 8 ). Right distal corner of genital double-somite one triangular-like lobe (Figs 2B View Figure 2 (arrowhead), 8F (arrowhead)). Genital area on ventral surface shows opercular pad protecting gonopores, characterised by rectangular and semicircular expansions (Fig. 6B View Figure 6 ). Anal somite about 1.6 times as long as wide (Figs 1E View Figure 1 , 2B View Figure 2 , 8F View Figure 8 ). Caudal rami parallel (Figs 1E View Figure 1 , 2B View Figure 2 , 8F View Figure 8 ), symmetrical, about 1.9 times as long as wide, with setules on outer and inner margins (Figs 1E View Figure 1 , 2B View Figure 2 , 8F View Figure 8 (arrowhead)). Each ramus with six setae.

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

P5 (Figs 6C View Figure 6 , 8G-I View Figure 8 ). Symmetrical. Coxal spine on posterior lobe on caudal surface. Basis with one smooth outer seta on distolateral margin. Exopod 3-segmented. Exp-1 cylindrical, length about 2.2 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 6C View Figure 6 (arrowhead), 8I (arrowhead)), with two unequal spines; inner spine about 3.6 times as long as outer spine, and with short spine laterally. Endopod 1-segmented, cylindrical, length about 0.6 time as long as exp-1, two unequal strong smooth spiniform setae distally (Figs 6C View Figure 6 , 8G, H View Figure 8 (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 1,145-1,380 µm (mean 1,283 µm, n = 6) in the adult males and 1,520-1,626 µm (mean 1,575 µm, n = 5) in the adult females; (ii) the length of the spinous process on antepenultimate segment of the adult male right A1 is 3/4 to equal of segment 21 (Fig. 7A-G View Figure 7 ; see Table 2 View Table 2 in Saetang et al. 2022); (iii) segment 13 of the adult male right antennule with one hyaline knob (Fig. 3E View Figure 3 (arrowhead)); and (iv) the number of setae on segment 13 of the adult male left A1 has one seta (n = 10) or two setae (n = 2) (Fig. 1D View Figure 1 (arrowhead); see Table 2 View Table 2 in Saetang et al. 2022)

Etymology.

The specific name ' pedecrassum ' is derived from the chubby shape of the adult male P5 that is clearly different from the more rectangular shape in other species of the genus.

Co-occurring species.

In our samples, the new taxon co-occurred with one other copepod species, Mongolodiaptomus botulifer (Kiefer, 1974).

Distribution and ecology.

Tropodiaptomus pedecrassum sp. nov. was found only in its type locality so far. It was recorded in two out of 471 samples collected from 206 freshwater habitats throughout Thailand between September 2017 and July 2019. Water temperature 19.6 °C, conductivity 620 µs cm-1, salinity 0.3 ppt, total dissolved solids 450 mg L-1, dissolved oxygen 3.3 mg L-1, pH 7.1, and water depth 30-40 cm, substrate with mud.

Differential diagnosis.

Tropodiaptomus pedecrassum sp. nov. is confirmed to belong to the genus Tropodiaptomus based on the combination of characteristics mentioned by Lai and Fernando (1979a, 1979b), Sanoamuang (2002), Saetang et al. (2020): (i) the process on the antepenultimate segment of the adult male right antennule is always smooth; (ii) the exopod of the adult male left P5 fused into a single flattened piece and its inner margin is denticulate or serrate; (iii) the inner margin on the basis of the adult male right P5 has a hyaline lobe; (iv) the urosome of the adult female comprises two somites; and (v) the endopod segment of the adult female P5 has slender setae at the distal end.

This species differs from the congeneric species by the following characters: (i) antepenultimate segment of the male right antennule with straight spinous process reaching 3/4 or equal of next segment; (ii) inner margin of exopodal segment of the male left P5 with single lobe and the serration gradually decreases in size from the proximal to distal end; (iii) basis of the male right P5 with two processes 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, length about as long as its segment; and (v) exp-2 of the male right P5 with rhomboid shape, dorsal surface with one semicircular hyaline knob on proximal outer margin, one semicircular hyaline lamella on distal inner margin, and one triangular process in middle of segment.

According to the identification key given by Saetang et al. (2020), Tropodiaptomus pedecrassum sp. nov. is most similar to T. hebereri . However, it distinctly differs from T. hebereri in the following characters: (i) inner margin of the basis of the adult male right P5 has one round process, one semicircular process, and one longitudinal hyaline lamella; (ii) length of spinous process of the outer corner of exp-1 of the adult male right P5 is as long as exp-1; (iii) inner margin of the exopod of the adult male left P5 has one lobe, and the serration gradually decreases in size from the proximal to the distal end; (iv) dorsal surface of exp-2 of the adult male right P5 has one semicircular hyaline knob, one semi-circular hyaline lamella, and one triangular process; and (v) caudal rami of the adult female have outer and inner setules (Table 2 View Table 2 ).