Pallisentis rexus Wongkham & Whitfield 1999

Pallisentis rexus Wongkham & Whitfield, 1999 Figs 4, 5 Host. Striped snakehead Channa striata (Bloch) (Anabantiformes, Channidae). Locality. Nong Samrong Lake, Nong Samrong Town, Udon Thani province, Thailand (17°27.065'N, 102°45.791'E) and a fish farm in Kong Nang village, Tha Bo Town, Nong Khai province, Thailand (17°54.190'N, 102°35.211'E). Site of infection. Intestine. Infection rates. Prevalence 84.9 %, intensity 1–35. Molecular data. The nuclear 18 S rRNA (1,735 bp), 28 S rRNA (1,085 bp), ITS (774 bp) and the mitochondrial COI (623 bp) genes sequences of Pallisentis rexus were deposited in the GenBank database (Acc. nos. PQ 636379 – PQ 636381, PQ 636386 – PQ 636388, PQ 636390, PQ 636391, PQ 631042 – PQ 631044). Morphology (based on 5 males, 7 females). Medium-sized, white acanthocephalans. Anterior part of trunk with two fields of spines in rings (Figs 4 A, 5 A). Distance between anterior and posterior fields 105–151. Proboscis length less than its width (Fig. 4 A). Proboscis with four rows of hooks, 12 hooks in each row. Hooks size decreases from apical to basal row. Proboscis receptacle with single-layer muscular wall, with cephalic ganglion in middle part. Neck conical. Lemnisci longer than proboscis receptacle. Gonopore terminal in both sexes. Male. Trunk 4.03–7.3 mm long, 389–446 wide. Tegumental spines of anterior field with 14 rings, 14–18 spines in each. Length of spines 27–31 (Fig. 4 C). Somatic spines of posterior field in 20–32 rings, anterior rings complete, ten spines in each, posterior rings incomplete, 2–4 in each. Length of spines 27–38 (Fig. 4 D). Proboscis 164–214 × 253–289 (Figs 4 A, 5 B). Length of hook blades of anterior row 69–81, second 59–69, third 38–42, basal 25–33. Length of hook roots of anterior row 47–58, second 39–44, third 30–49, basal 19–25. Proboscis receptacle 419–594 × 98–160. Neck 164–186 long. Lemnisci 921–1,359 × 70–98. Organs of reproductive systems in posterior half of trunk (Fig. 4 B). Testes I 575–695 × 172–200, II 636–654 × 125–198. Single cement gland 1,079 × 115–183, cement reservoir 121–432 × 153–200. Saefftigen’s pouch club-shaped, 307–478 × 115–125. Female. Trunk 10.00– 12.55 mm long, 264–405 wide. Tegumental spines of anterior field with 12–15 rings, anterior rings complete, 14–20 spines in each, posterior rings incomplete dorsally, 6–14 in each. Length of spines 25–34. Somatic spines of posterior field with 40–47 rings, ten spines in anterior rings, 3–11 in posterior rings. Length of spines 39–48. Proboscis 110–216 × 206–319. Length of hook blades of anterior row 74–85, second 63–74, third 45–52, basal 31–39. Length of hook roots of anterior row 48–67, second 43–65, third 35–46, basal 27–29. Proboscis receptacle 520–760 × 105–166. Neck 184–275 long. Lemnisci 1,094 –1,115 × 55–72. Reproductive tract 398–531. Egg fusiform, elongate, no polar prolongation of fertilization membrane, 92–102 × 42–53 (Fig. 4 E). Remarks. Pallisentis rexus (Eoacanthocephala, Quadrigyridae) was described from the striped snakehead Channa striata (Bloch) in the Chiang Mai Basin in Thailand (Wongkham; Whitfield, 1999). Later, immature specimens of this species were found in the Asian swamp eel, Monopterus albus (Zieuw) from Bangkok, Thailand (Amin; Taraschewski 2003). Adult P. rexus were also found in Channa sр. from a river in northern Taiwan (Lisitsyna et al. 2023).