(Figs. 110–115)

Type host and locality: Yellow snapper, Lutjanus argentiventris (Peters) (Lutjanidae): Perlas Archipelago, Panama (8 o 22'N, 79 o 01'W), 24 May 2007.

Other record: Lutjanus argentiventris (Lutjanidae): off Isla Tabugilla, Panama (8 o 47.869'N, 79 o 31.446'W), no date available.

Site of infection: Gills.

Specimens studied: Holotype, USNPC 101372; 8 paratypes, USNPC 101373, 101374, BMNH 2008.11.19.63-64, CNHE 6467.

Etymology: The specific name (a noun) is from Greek (mastix = a whip) appended to the diminutive ending (-ion) and refers to the male copulatory organ.

Description: Body elongate; greatest width usually at level of anterior end of germarium. Cephalic lobes well developed. One anterior eyespot frequently missing; posterior eyespots with lenses, lenses usually obscured by chromatic granules. Pharynx spherical. Testis ovate; seminal vesicle not observed. MCO comprising vase-shaped base, tapered tubular shaft with distal counterclockwise loop forming 1–2 rings; prostatic reservoir not observed. Germarium bacilliform. Vaginal pore simple within small tegumental indentation about midlength of trunk, unsclerotized; seminal vestibule absent; vaginal canal an unsclerotized tube extending diagonally to body midline where it bends back to the right to unite with small seminal receptacle. Ventral anchor with subtriangular superficial root, large deep root, straight shaft with proximal reduction in diameter, recurved point having delicate superficial grooves. Dorsal anchor with long superficial root, inconspicuous deep root, straight shaft with reduction of diameter near midlength, recurved point superficially grooved. Ventral bar rod shaped, with anteromedial protuberance having complex bilateral pockets; dorsal bar a V-shaped rod. Hook distribution typical.

Measurements: Body 425 (394–479; n = 3) long; greatest width 75 (67–82; n = 3). Haptor 77 (72–84; n = 3) long, 64 (60–72; n = 3) wide. Pharynx 20 (18–21; n = 3) wide. MCO 46 (43–49; n = 6) long. Ventral anchor 47 (45–49; n = 6) long; dorsal anchor 62 (60–65; n = 6) long. Ventral bar 31 (29–33; n = 4) long; dorsal bar 32 (31–34; n = 5) long. Hook 12 (11–13; n = 24) long. Germarium 38 (34–42; n = 3) long, 24 (22–26; n = 3) wide; testis 59 (55–63; n = 3) long, 30 (25–34; n = 3) wide.

Remarks: Haliotrematoides mastigion resembles H. gracilihamus in the general morphology of the anchors and copulatory complex. It differs from H. gracilihamus by having fewer rings in the coil of the MCO (1–2 rings in H. mastigion; between 2 & 3 rings in H. gracilihamus) and by having abrupt reductions in the diameters of the shafts of the dorsal and ventral anchors (shafts uniform or slightly tapered in H. gracilihamus).