Diplectanum timorcanthus n. sp. (Fig. 1; Fig. 2; Fig. 3A, C, E)

Type host: Protonibea diacanthus (Lacep`ede, 1802) (Sciaenidae) ‘Black Jewfish’ or ‘Black-spotted croaker’.

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Type locality: Caution Point, Beagle Gulf, Northern Territory, Australia. Other localities include Van Diemen Gulf, Timor Sea, Northern Territory, Australia.

Site in host: Gill filaments.

Type-material: Holotype: MAGNT D001920; Paratypes: MAGNT D001921, QM G240632, WAM V11009.

GenBank accession: 28S (OQ846930-OQ846931), ITS1 (OQ846935- OQ846937).

Etymology: The specific name resembles a combination of the geographical location, and the host species, from which the type specimen of Diplectanum timorcanthus n. sp. was collected.

3.1.1. Description

Based on 39 specimens. Body elongate, fusiform, with smooth tegument; total length 923 (546–1,830); greatest width 145 (75-214), near level of testis. Four cephalic lobes moderately defined. Two pairs of head organs, 13 (8-18) long, 12 (8-21) wide, moderately developed as cephalic lobes, adjacent cephalic areas. Cephalic glands poorly defined, extending to pharyngeal region. Eye spots 2 when visible, poorly developed, or represented by scattered granules, dorsal, anterior to pharynx. Mouth subterminal; pharynx bulbous, length 56 (38-79), width 46 (34-60). Oesophagus short to absent. Intestinal caeca bifurcates short distance posterior to pharynx, crura blind posteriorly. Peduncle slightly tapered posteriorly, 133 (57-212) long, 75 (35-147) wide, peduncular spines absent. Haptor laterally expanded (Figs. 3C), 64 (37-94) long, 226 (191-291) wide. Squamodiscs ovate, ventral and dorsal (Fig. 3C and E); each formed with approximately 25 rows of sclerotized dumbbell-shaped rods (Fig. 2B); squamodisc 118 (64-164) long, 105 (34-156) wide. Anchors 2 pairs, ventral and dorsal (Fig. 2E). Ventral anchor length 61 (56-67), anchor base width 8 (2-15); anchor with elongate deep root longer than superficial root, knob-like superficial root, curved shaft, short point (Fig. 2E). Dorsal anchor length 62 (52-69), anchor base width 9 (6-12); anchor with wide tapered deep root, incipient superficial root, straight shaft, short point (Fig. 2E). Hooks not observed. Ventral bar with tapering ends, constricted midregion, ventral longitudinal groove (Figs. 2C), 118 (98-133) long, greatest width 21 (9-26). Paired dorsal bar, 97 (64-108) long, greatest width 23 (14-28); not overlapping each other; medial end expanding wider than lateral end, medial end overlapping ventral bar extremities (Figs. 1 and 2D). Male copulatory organ (MCO) sclerotized, elongate (Figs. 2A), 74 (42-96) long, greatest width 10 (5-15); composed of two nested tubes which appear to be joined at the proximal end. Slightly curved at distal end, inner tube tapers slightly, outer tube does not. Sclerotized piece can be seen extended from distal end of outer tube (Fig. 2A). Accessory piece absent. Vitellarium extensive throughout body, extended from level of pharynx to posterior level of intestinal caeca (Figs. 1 and 3A). Most reproductive and other internal structures throughout middle body not defined through vitellarium.

3.1.2. Remarks

Based on body shape and the comparative morphology of the haptoral structures, squamodisc and male copulatory organ, Diplectanum timorcanthus n. sp. most closely resembles D. oliveri and D. gladulosum, both parasitising the mulloway Argyrosomus hololepidotus (Lacep`ede, 1802) (Sciaenidae) from the Swan River Estuary and Cockburn Sound, Perth, Western Australia (Williams 1989) (Table 2). Diplectanum timorcanthus n. sp. can be differentiated from D. oliveri by possessing a much shorter MCO, and with the central tube often erect from the curved distal margins of the outer tube. The distal end of the MCO from D. timorcanthus n. sp. also tapers slightly, while the MCO of D. oliveri does not and is rather spatulate at the distal end. Despite similarities in the MCO structure between D. timorcanthus n. sp. and D. glandulosum, the body length of D. timorcanthus n. sp. is much greater and the ventral bar groove less conspicuous, when compared with D. glandulosum.