Pseudempleurosoma haywardi (Figs. 2–4)

Host: S. aeolus and S. sihama (Figs. 5A, B)

Locality: Samut Sakhon Province, the upper Gulf of Thailand (13°19′23.9″N 100°23′23.2″E) and Surat Thani Province, the middle Gulf of Thailand (9°22′55.2″N 99°42′32.5″E) Site of infection: Stomach

Original records: Oesophagus/proximal stomach of Nibea soldado (type host), Johnius amblycephalus and Otolithes ruber from Cilacap, South Java coast and Kedonganan, South Bali coast, Indonesia (Theisen et al., 2017; Theisen, 2019). Specimen deposition: No. ZMKU-PM-002001 at Zoological Museum, Kasetsart University: ZMKU, Bangkok, Thailand .

Infection rates of the monogenean in sillaginid fishes.

Of 629 sillaginid fishes examined, 16 individuals (10 of S. aeolus and 6 of S. sihama) (Fig. 5) were infected with the monogenean P. haywardi, representing 2.5% and 1.3 monogeneans/infected fish in terms of overall prevalence and mean intensity of the infection, respectively (Table 1). Prevalence and mean intensity of the infection in S. aeolus were 3.4% and 1.0, respectively; in the case of S. sihama they were 3.6% and 1.7 respectively (Table 1). The remaining four fish species were not infected.

Morphological description. The morphological features of the P. haywardi (Figs. 2, 3) observed were considered based on both descriptive and measurement characteristics. All measurement parameters (Table 3; Fig. 1) were represented as minimum and maximum values (range) followed by the mean value within parentheses; most of these parameters were shown as dimensions of length × width. All values are in micrometres (µm). The body shape is slender and unspined (Figs. 2A, 3A), 1000–2112 (1468) × 221–362 (294). A single pair of head glands extends to the pharyngeal region (Figs. 2A, 3A) and two pairs of eye spots were observed (Figs. 2A, 3A). The oral aperture is present ventrally (Figs. 2A, 3A), and the pharynx is subspherical (Figs. 2A, 3A), 58–79 (73) × 54–88 (75). Bifurcated intestinal ceca reached the testis posteriorly, coexisting with vitelline follicles and with lateral diverticula lacking hematin pigment (Fig. 3A). The opisthaptor is not distinctly set off from the body (Figs. 2A–C, 3A), 139–167 (147) × 80–103 (91). Two pairs of dissimilar hamuli and marginal hooks were observed. Dorsal anchors are present, 57–68 (61) in length, connecting with a single dorsal bar (Figs. 2B, C, 3E). Ventral anchors (Figs. 1H, 2B, C, 3C), 15–17 (16) in length, each one with two bars: one with an attached ventral anchor (Figs. 1H, 2C, 3C), 14–18 (16) in length, and another one with a detached ventral anchor (Figs. 2C – 3G) that is a free irregular bar, 20–22 (21) in length (Fig. 1I). Dorsal bar is slightly concave (Figs. 1G, 2B, C, 3H), 17–22 (19) × 10–21 (14). Fourteen marginal hooks, 11–18 (14) in length (Figs. 1J, 2B, C, 3D). MCO is sclerotized, slender and tubular, 53–57 (55), initial part of MCO resembles a cup and funnel (Figs. 3B, 4). Sclerotized accessory piece is irregularly shaped and located in the middle part of MCO (Figs. 1A, 3B, 4), length 20–23 (22). Testis sub-elongated and oval in shape, size 58–100 (73) × 34–57 (40), ovary heart-shaped, size 60–99 (86) × 56–71 (61) (Figs. 1D, 3A). Vaginal opening simple, surrounded with muscular genital atrium, 34–41 (37) × 30–32 (31) (Figs. 1C, 3A). Vitelline follicles form numerous transversely elongated lobes and longitudinally arranged along lateral fields of the body (Figs. 2A, 3A), and vitelline ducts are connected in a position between the MCO and ovary (Fig. 3A). Egg, 57–97 (71) × 51–84 (64), without polar filament (Figs. 1B, 2D, E, 3F).

Phylogenetic relationship and molecular identification.

The overall topology of the phylogram (Fig. 6) based on 28S rRNA revealed a monophyletic group of species members of dactylogyrid lineage, using diplectanids as an outgroup. Two distinct and major clades were resolved (clades A and B in Fig. 6). In clade A, the high support value of the two novel sequences (ON969400 and ON969401) from our study strongly indicated that the monogenean species found in this study was conspecific to P. haywardi (MF115714), with genetic divergences ranging between 0.0–0.2% among these three sequences (Supplementary Table S1). A sequence of Paradiplectanotrema klimpeli (MG763101) was arranged as a sister taxon of P. haywardi . All endoparasitic monogenean sequences investigated here formed a monophyletic cluster. Species members of both Ancyrocephalidae and Dactylogyridae (following the NCBI database classification) are present in the two major clades A and B (see Fig. 6; considered as subfamilies Dactylogyrinae and Ancyrocephalinae, respectively, in Kmentová et al., 2022).