Tropodiaptomus lannaensis, Koompoot & Sanoamuang, 2025

Tropodiaptomus lannaensis sp. nov.

Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 View Figure 6 , 7 View Figure 7 , 8 View Figure 8 , 9 View Figure 9

Tropodiaptomus sp.: Sanoamuang and Dabseepai 2021: 5, 8, 16, 18, 23.

Material examined.

Holotype: Thailand • one ♂ (adult), northern Thailand, Mae Hong Son Province, Mae La Noi District , 18°33'00.28"N, 97°91'01.42"E; 8 December 2012; water temperature 23.1 ° C, pH 8.75, conductivity 386 µS cm - 1, and dissolved oxygen 8.84 mg L - 1. P. Dabseepai and K. Koompoot leg.; a temporary pond with aquatic plants near the road No. 108 (Fig. 1 View Figure 1 ); accession number: THNHM -lv-18787 ; dissected, mounted on one slide in glycerol, covered with a coverslip, and sealed with nail polish . Paratypes: Thailand • three ♂ (adult) and two ♀ (adult); date and collectors as for holotype; accession number: THNHM -lv-18789 , undissected and preserved in 70 % ethanol .

Description of adult male.

Total body length, measured from anterior margin of rostrum to posterior margin of caudal rami, 1.16–1.21 mm (mean = 1.15 mm, n = 3) (Fig. 2 B View Figure 2 ). Body smaller and slender than in female. Prosome ~ 2.2 × as long as urosome (Figs 2 B View Figure 2 , 3 A View Figure 3 ). Rostrum (Fig. 2 A View Figure 2 ) well-developed, with two spiniform processes. Pdg 4 and Pdg 5 separated by distinct septum dorso-laterally. Lateral wings of Pdg 5 small and slightly asymmetrical; right wing larger than left one; both wings with one tiny spine at distal corner and one inner sensillum-like spine (Fig. 3 B View Figure 3 ).

Urosome (Figs 2 C View Figure 2 , 3 A – C View Figure 3 ) with five somites, unornamented. Genital somite slightly dilated postero-laterally on right side, shorter than wide. Urosomite 2 longer than wide, urosomite 3 approximately as long as wide, urosomite 4 with expanded right dorso-posterior corner (Fig. 3 A View Figure 3 ). Urosomites 2 and 3 without ventral hairs (Figs 2 C View Figure 2 , 3 C View Figure 3 ). Urosomites 3 and 4, anal somite, and caudal rami bent to right side. Anal somite asymmetrical, shorter than preceding urosomites. Caudal rami symmetrical, each ramus ~ 2.3 × as long as wide, inner margin hairy (Fig. 3 C View Figure 3 ). Ventral surface of right caudal ramus without any prominence structures. Each ramus armed with six setae, subequal in length and size, plumose: dorsal seta bare and thinner than others.

Antennules asymmetrical, extending to approximately posterior end of genital somite. Left antennule (Fig. 4 A View Figure 4 ): 25 - segmented. Armature formulae as in Table 1 View Table 1 . Right antennule geniculated (Fig. 3 D – F View Figure 3 ), consisting of 22 segments, strongly dilated between segment XIII and segment XVIII (Fig. 3 E View Figure 3 ). Segment XIII with largest strong spinous process. Antepenultimate segment (segment XX) longer than next segment. Spinous process on antepenultimate segment straight and slightly bent at distal end, reaching 2 / 3 of next segment (Fig. 3 F View Figure 3 ). Armature formulae as in Table 2 View Table 2 .

Antenna (Fig. 4 B View Figure 4 ) biramous. Coxa and basis with one and two bare setae on inner distal corner, respectively. Enp two-segmented; Enp - 1 with two setae along inner margin and spinules on distal part of outer margin; Enp - 2 with nine setae along inner margin, seven setae apically; all setae bare. Exp seven-segmented: Exp - 1–6 with 1, 3, 1, 1, 1, 1, setae along inner margin; Exp - 7 with one seta on inner margin and three setae apically; all setae bare.

Mandible (Fig. 4 C View Figure 4 ): ~ 6 cuspidate teeth dorsally and one seta on coxal gnathobase dorsally. Basis with two serrate setae and two bare setae along inner margin. Enp - 1 with four setae on inner distal corner. Enp - 2 with eight setae apically. Exp four-segmented. Exp - 1–3 each with one seta on inner margin; Exp - 4 with three setae apically; all setae bare.

Maxillule (Fig. 4 D View Figure 4 ): praecoxal arthrite with nine strong setae laterally and four slender submarginal setae. Coxal endite with four setae; coxal epipodite with nine setae. Basal endites fused to segment bearing them: proximal and distal endites, each with four setae apically; basal exite with one short seta. Enp - 1 and Enp - 2 each with four setae apically, proximal segment fused to basis. Exp unsegmented with six setae apically and hairy inner margin.

Maxilla (Fig. 4 E View Figure 4 ): praecoxa fused to coxa. Proximal and distal endites on praecoxa, each with three setae apically. Two coxal endites, each with three setae apically. Allobasis with four setae apically. Enp two-segmented, each with three setae.

Maxilliped (Fig. 4 F View Figure 4 ): four medial lobes on syncoxa: setal formulae 1, 2, 3, 3, respectively; subdistal inner margin produced into a spherical lobe with a patch of tiny spinules. Basis with three setae along distal inner margin, with a row of tiny spinules proximately. Enp six-segmented, with 2, 3, 2, 2, 2, and 4 setae, respectively.

Swimming legs (P 1 – P 4) (Fig. 5 A – D View Figure 5 ) biramous, with three-segmented rami, except for two-segmented Enp on P 1. Exp longer than Enp. Each coxa on P 1 – P 4 with one pinnate seta at innermost distal corner. P 1 – P 3 bases without setae. P 4 basis with one outer seta (Fig. 5 D View Figure 5 ). P 2 Enp - 2 with Schmeil’s organ on posterior surface of (Fig. 5 B View Figure 5 ). Armature formulae of P 1 – P 4 as in Table 3 View Table 3 .

P 5 (Figs 2 D – H View Figure 2 , 6 A, B View Figure 6 ): intercoxal sclerite fused to coxa, produced into a triangular lobe distally (Fig. 6 A View Figure 6 ). Coxae asymmetrical, right part more prominent, each bears a stout spine on posterior surface; right spine larger than left spine. Right P 5 (Figs 2 D View Figure 2 , 6 A, B View Figure 6 ), basis rectangular, ~ 1.5 × as long as wide; ornamented with a small semicircular knob at proximal inner margin and a larger, triangular hyaline knob occurring close to sub-proximal inner margin (Figs 2 G View Figure 2 , 6 A View Figure 6 ); subdistal outer margin with a thin seta. Exp - 1 shorter than wide, with a hyaline lobe on inner margin; outer distal corner with an acute tip (Fig. 6 A, B View Figure 6 ). Exp - 2 somewhat cylindrical, ~ 1.5 × as long as wide, carrying accessory slender spine located at proximal 1 / 3 of outer margin and ~ 3 / 4 as long as outer distal spine (Figs 2 D View Figure 2 , 6 A, B View Figure 6 ); outer distal spine moderately strong, slightly curved and acutely pointed, ornamented with spinules along inner margin, ~ 0.8 length of Exp - 2, and located at anterior 2 / 3 of segment (Fig. 6 A, B View Figure 6 ). Distal accessory element minute, situated close to insertion of end claw (Figs 2 D View Figure 2 , 6 A View Figure 6 ). Enp one-segmented, conical shaped, reaching ~ 1 / 3 of Exp - 2, bearing a row of apical spinules.

Left P 5 (Figs 2 D – F, H View Figure 2 , 6 A, B View Figure 6 ) slightly bent inwards, reaching to mid-length of right Exp - 2. Coxa with relatively short spine inserted on ~ mid-length posterior lobe at outer margin. Basis rectangular, elongated, ~ 1.8 × as long as wide, bearing a small smooth seta at outer distal margin (Figs 2 D View Figure 2 , 6 A View Figure 6 ); inner margin almost straight, without hyaline outgrowths. Exp somewhat semicircular-shaped, flattened and ~ 1.6 × as long as wide; outer margin curved, inner margin two-lobed, both lobes with large denticles, small denticles near distal end only (Fig. 2 E, H View Figure 2 ); apex of Exp with typical digitiform appendix and spinulate seta, while anterior surface displays two elongate, hairy pads (Fig. 2 E, H View Figure 2 ). Enp one-segmented and dilated proximally, extending to mid-length of Exp; apex rounded with a row of subapical spinules (Fig. 6 A, B View Figure 6 ).

Description of adult female.

Total body length, measured from anterior margin of rostrum to posterior margin of caudal rami, 1.27–1.41 mm (mean 1.35 mm, n = 3) (Figs 7 A View Figure 7 , 8 A View Figure 8 ). Prosome: urosome ratio ~ 2.1: 1. Rostrum (Fig. 7 B View Figure 7 ) symmetrical, with moderately long point and ornamented with a pair of sensilla. Pdg 4 and Pdg 5 completely fused. Last pedigerous somite with asymmetrical posterolateral wings (Figs 7 D View Figure 7 , 8 A, B View Figure 8 ); left wing larger and longer than right wing. Each wing with one hyaline spine (Figs 7 D View Figure 7 , 8 B, D View Figure 8 ).

Urosome (Figs 7 D, E View Figure 7 , 8 A – D View Figure 8 ) two-segmented. Genital double-somite (Figs 7 D View Figure 7 , 8 A, B View Figure 8 ) asymmetrical, longer than urosomite 2, anal somite, and caudal rami combined (Fig. 8 A, B View Figure 8 ); right proximal region slightly expanded, right distal corner produced into a small lobe; left margin slightly curved, with a small spine at proximal 1 / 3 of somite (Fig. 8 B View Figure 8 ). A pair of gonopores and copulatory pores located centrally at ~ 1 / 3 length of genital double-somite (Figs 7 E View Figure 7 , 8 C View Figure 8 ). Urosomite 2 symmetrical, shorter than wide. Anal somite symmetrical, as long as length of caudal rami (Fig. 8 B View Figure 8 ). Caudal rami parallel, symmetrical; each ramus ~ 1.5 × as long as wide and with hairy outer and inner margins (Fig. 8 A, B View Figure 8 ). All principal caudal setae slightly dilated anteriorly.

Left antennule, antenna, mouthparts, and P 1 – P 4 similar to those of the male. Antennules symmetrical.

P 5 (Figs 7 F, G View Figure 7 , 9 A, B View Figure 9 ) asymmetrical. Intercoxal sclerite forming a narrow, elongate triangle. Coxa massive, with broadly strong process situated anterolaterally on distal outer margin; coxal spine on right side larger and longer than left side (Figs 7 F View Figure 7 , 9 B View Figure 9 ). Basis with a bare, minute sensory seta on outer margin (Figs 7 F View Figure 7 , 9 B View Figure 9 ). Exp (Figs 7 F, G View Figure 7 , 9 A, B View Figure 9 ) three-segmented. Exp - 1 cylindrical, ~ 1.9 × as long as wide, convex outer margin and almost straight inner margin; Exp - 2 sub-triangular, with a row of strong spinules along both margins; Exp - 3 fused into small prominence on Exp - 2, armed with one short spine and one long seta apically. Enp one-segmented (Figs 7 F View Figure 7 , 9 A, B View Figure 9 ), ~ 0.7 × as long as Exp - 1, armed with two strong, unequal spiniform setae; outer seta longest, with a row of apical spinules.

Distribution and ecology.

Tropodiaptomus lannaensis sp. nov. has so far been found exclusively in its type locality, a temporary pond in Mae Hong Son Province, northern Thailand. At the time of the new species’ collection, no other calanoid copepods were present at the same site. Representatives of the new species were recorded solely once among more than 5,000 sampled locations in Thailand. This species is rare and currently endemic to Thailand.

Etymology.

The specific epithet lannaensis is derived from Lanna, the name of the area in present-day northern Thailand from the 13 th to 18 th centuries in which the type locality is located. The name is an adjective in the nominative singular, feminine gender. The suffix - ensis originates from Latin and indicates the specific origin.