Acanthoplacatus shieldsi, Ernst & Jones & Whittington, 2001

Ernst, Ingo, Jones, Malcolm K. & Whittington, Ian D., 2001, A new genus of viviparous gyrodactylid (Monogenea) from the Great Barrier Reef, Australia with descriptions of seven new species, Journal of Natural History 35 (3), pp. 313-340 : 331-335

Acanthoplacatus shieldsi sp. nov.

( Figures 3 , 9A ±D)

Type host. Siganus lineatus (Valenciennes, 1835) (Siganidae) .

Type locality. Heron Island, Great Barrier Reef, Queensland, Australia.

Other localities. Lizard Island, Great Barrier Reef, Queensland, Australia.

Site on host. This species is found most frequently on the dorsal ®n (see table 1).

Holotype. QM 215988.

Paratypes. QM 215989 ±90.

Etymology. This species is named in honour of Dr JeOErey Shields (Virginia Institute of Marine Science).

Infection details. At Heron Island, prevalence 100% (n 5 4 ®sh, LCF 274 mm (250±294 mm )). At Lizard Island, prevalence 33% (n 5 3 ®sh, LCF 271 mm (248± 285 mm )).

Description. Sclerite measurements taken from 32 worms mounted in Malmberg’s ®xative. Measurements of other organs taken from 13 worms stained with Ehrlich’s haematoxylin. Numerous live worms examined for the structure of soft body organs and the excretory system. Total length 267 (222±292, n 5 13), maximum width 62 (42±85, n 5 13) at level of uterus when embryo contained within or testis for worms with small embryo (®gure 9A). Hamulus length 38 (35±41, n 5 32); hamulus point length 19.5 (16.5±21, n 5 31); hamulus root length 20 (16±22, n 5 32); hamulus shaft length 27 (25.5±29.5, n 5 32). Hamulus shafts with distinct longitudinal depression anteroventrally. Dorsal root tissue cap length 8 (6.5±9, n 5 31), width 3.5 (2.5±4.5, n 5 30) (®gure 9D). Ventral bar simple, variable in shape (®gures 3, 9D). Ventral bar 5.5 (4±7, n 5 30) long, 13 (12±14.5, n 5 26) wide. Ventral bar membrane 13 (11±15, n 5 18) long. Total length of ventral bar and membrane 18 (16±20, n 5 18). Transition between ventral bar and ventral bar membrane not distinct. Marginal hooks small with hook point extending to, or only slightly past toe; total length of marginal hook 31 (28.5±34, n 5 28); length of sickle 3.5 (3±4, n 5 29); length of handle 28 (25.5±31.5, n 5 25) (®gure 9B).

Gland cells, posterolateral to pharynx range from minimum diameter of 12.5 (7±16, n 5 12) to maximum diameter of 15.5 (13±21, n 5 12). Anterior lobe of pharynx 20.5 (19±23, n 5 13) wide, posterior lobe 27.5 (25±31, n 5 13) wide. Gut caeca extend posteriorly lateral to uterus, terminating adjacent to testis. Male copulatory organ with one large, two moderate and four small spines (11 21 4) (®gure 9C). Male copulatory organ 11.5 (10±13, n 5 3) wide and long. Developing sclerites of F2 embryos may be visible within large F1 embryos; no sclerites of F3 embryos visible. Oocyte 15.5 (8±24, n 5 12) long, 18 (12±28, n 5 12) wide, contained within ECFR. Testis 16.5 (7±27, n 5 11) long, 16 (10±27, n 5 11) wide. Testis overlaps ECFR ventrally. Five cells surround posterior portion of each gut caecum and single cell lies medially near posterior terminations; cells range from minimum diameter of 11 (7±16, n 5 12) to maximum diameter of 16.5 (14±18, n 5 12). Two anteriormost cell pairs and posterolateral cell pair, lie ventral to gut; remaining two cell pairs and single medial cell lie dorsal to gut (®gure 9A).

Sclerite size and seasonality. For this species at Heron Island, 16 specimens were collected in summer (January 1998) and ten specimens in winter (July 1996). Six specimens were also collected from Lizard Island in autumn (May 1997). Signi®cant diOEerences were found between at least two sample groups for hamulus shaft length, hamulus root length, marginal hook sickle length, total length of marginal hook, length of marginal hook handle and dorsal root tissue cap length. Hamulus shaft length was signi®cantly larger in winter (28.5, n 5 10) than both summer (26.5, n 5 16; p <0.001) and Lizard Island samples (27, n 5 6; p <0.01). Hamulus root length of the Lizard Island sample (18, n 5 6) was signi®cantly shorter than winter (20.5, n 5 10; p <0.05) and summer (20.5, n 5 16; p <0.01) samples. Marginal hook sickle length, total marginal hook length and marginal hook handle length were signi®cantly larger (p <0.001) in winter (4, n 5 9; 30, n 5 8 and 33, n 5 8) than in summer (3.5, n 5 16; 27, n 5 15 and 30, n 5 16). Dorsal root tissue cap length was signi®cantly smaller in summer (7.5, n 5 16) than both winter (8.5, n 5 9; p <0.001) and Lizard Island samples (8, n 5 6; p <0.05).

Comments. Acanthoplacatus shieldsi sp. nov. has similar haptoral sclerite size and shape to A. adlardi sp. nov., A. puelli sp. nov. and A. sigani sp. nov. Acanthoplacatu s shieldsi sp. nov. can be discriminated from these worms as described above for each species.

Examination for spike sensilla

Several techniques were used to determine whether spike sensilla are present in species of Acanthoplacatus gen. nov., including observations on live worms and whole mounts, ultra-thin serial sections for TEM through the cephalic lobes (®gure 10A) and SEM of the cephalic region (®gure 10B, C). No evidence of spike sensilla was found although another type of sensillum was observed. These sensilla diOEer from spike sensilla because they are smaller (approximately 1 m m compared to 10 m m for spike sensilla), they are not bound by an epidermal sheath, and they are not comprised of 10 cilia, as described for species of Gyrodactylus by Lyons (1969).

Observations on the excretory system

Despite small diOEerences in the position of ¯ame cells and their associated ducts, the protonephridial system appears to be uniform across species of the genus. Observations were made on all species (74 individual worms) but complete protonephridial systems were determined only for A. parvihamus sp. nov. and A. puelli sp. nov. (®gure 11 A, B). The excretory systems of species of Acanthoplacatus gen. nov., as for other gyrodactylids (see Malmberg, 1970), is bilaterally symmetrical and composed of two longitudinal canals on each side of the body. Each canal connects an excretory bladder and anterior and posterior groups of ¯ame cells (®gure 11). Excretory bladders open through ducts to the dorsal surface of the worm (®gure 10B). Three ¯ame cells occur in each of the lateral anterior and posterior groups (®gure 11). Acanthoplacatus gen. nov. has no connections between the two lateral systems.

Observations on the haptor and attachment

Temporary attachment of Acanthoplacatus species is facilitated by two areas of duct openings located anteroventrally (®gure 10C) but worms are normally attached by the haptor. The structure of the haptor in this genus is unlike that in any other gyrodactylid. The ventral lobe of the haptor (hamuli, ventral bar) is located anteriorly on the dorsal lobe rather than being located centrally and surrounded by radiating marginal hooks. The dorsal lobe is in the form of a muscular tube with marginal hook sickles around the posterior margin and marginal hook handles extending posteriorly parallel to the haptor wall (®gure 10D). Species of Acanthoplacatus gen. nov. were observed in situ on their hosts. They attach only by their marginal hooks and on no occasion were hamuli involved. With this mode of attachment, only the posterior margin of the haptor is in contact with the host when the parasite is attached to ¯at surfaces (®gure 12). Attempts to dislodge parasites with a ®ne needle or by strong jets of water from a Pasteur pipette failed to induce use of hamuli for attachment.

Figures

FIG. 3. Variation of ventral bar and ventral bar membrane within and between species of Acanthoplacatus gen. nov. Scale bar 5 20 mm.