(Figs. 126–131)

Type host and locality: Marr’s fusilier, Pterocaesio marri Schultz (Caesionidae): off Heron Island, Great Barrier Reef, Australia (23 o 27'S, 151 o 55'E), 15 July 2001.

Site of infection: Gills.

Specimens studied: Holotype, QM G230705; 19 paratypes, USNPC 101376, BMNH 2008.11.19.70-73, QM G230706-710.

Etymology: The root of the specific name (an adjective) is from Italian (isola = an island) appended to the suffix (- ens = belonging to) and refers to Heron Island, Australia, near which the species was collected.

Description: Greatest width of body usually in anterior trunk. Cephalic lobes poorly to well developed. Posterior eyespots with lenses. Pharynx ovate. Testis ovate; seminal vesicle not observed. MCO comprising small saucer-shaped base, delicate coiled tubular shaft appearing G-shaped; coil clockwise, with about 1 complete ring. Prostatic reservoir variable in size, lying to left and anterior to base of MCO. Germarium bacilliform to ovate. Vaginal pore simple in anterior half of trunk, frequently with a club-shaped spermatophore extruding from it; seminal vestibule absent, vaginal canal not observed; seminal receptacle small, subspherical, at anterior end of germarium. Ventral anchor with elongate superficial root, short knob-like deep root, slightly curved shaft, recurved point with delicate superficial grooves. Dorsal anchor with long superficial root, inconspicuous to absent deep root, slightly curved shaft, recurved point superficially grooved. Ventral bar broadly V shaped, with two submedial projections each with a lateral pocket; dorsal bar forming broad V, with pair of small delicate anteromedial membranes. Hook distribution typical.

Measurements: Body 427 (362–464; n = 8) long; greatest width 90 (74–107; n = 9). Haptor 91 (81–102; n = 8) long, 82 (78–88; n = 7) wide. Pharynx 27 (23–31; n = 8) wide. MCO 55 (49–62; n = 8) long. Ventral anchor 61 (56–64; n = 11) long; dorsal anchor 66 (61–71; n = 11) long. Ventral bar 33 (31–36; n = 7) long; dorsal bar 42 (40–47; n = 7) long. Hook 13 (12–14; n = 22) long. Germarium 39 (38–40; n = 4) long, 22 (21–24; n = 4) wide; testis 44 (40–48; n = 7) long, 33 (29–40; n = 7) wide.

Remarks: Haliotrematoides isolens most closely resembles H. caesionis, both species of which occur on caesionid fishes of the western Pacific. Haliotrematoides isolens differs from H. caesionis by having more delicate anchors (bases of anchors robust with superficial root of ventral anchor comparatively longer and heavier in H. caesionis) and by lacking a medial flap on the anterior margin of the ventral bar.