Holocentricola exilis, Cutmore & Cribb, 2021

Cutmore, Scott C. & Cribb, Thomas H., 2021, A new order of fishes as hosts of blood flukes (Aporocotylidae); description of a new genus and three new species infecting squirrelfishes (Holocentriformes, Holocentridae) on the Great Barrier Reef, Parasite (Paris, France) 28 (76), pp. 1-17 : 9-10

Holocentricola exilis n. sp. ( Figs. 2B , 3B )

urn:lsid:zoobank.org:act:5DAB8EDD-EF49-45DC-95ED-F9437F91B4F0

Type host: Neoniphon sammara (Forsskål) , Slender squirrelfish ( Holocentriformes : Holocentridae ).

Type locality: Off Lizard Island (14 ° 40 0 S, 145 ° 27 0 E), northern Great Barrier Reef, Australia .

Site in host: Heart, branchial arteries, vessels of liver, wash of head split.

Prevalence: 16 of 30 Lizard Island (adult worms in 13); 0 of 1 Heron Island.

Intensity: 1–4 worms per fish (mean 1.53), when adult worms were detected.

Type material: Holotype ( QM G239110 ) and 14 paratypes ( QM G239111–24 ).

Representative DNA sequences: Partial cox 1 mtDNA, three identical sequences (one submitted to GenBank, OK421321); ITS2 rDNA, one sequence (submitted to GenBank, OK422499); partial 28S rDNA, one sequence (submitted to GenBank, OK422503).

Etymology: This species is named from the Latin exilis (slender or thin) for the type and only host, the Slender squirrelfish.

Description [based on 15 specimens]: Body lanceolate, ventrally concave, broadest at level of testis or caeca, 961–1232 × 123–190 (1055 × 152), 5.8–7.8 (7.0) times longer than wide; notch usually noticeable at level of male genital pore, sometimes indistinct; distinct terminal notch at posterior end; distinct bulge sometimes present at level of uterus. Tegumental spines arranged in ventro-marginal transverse rows for entire body length, 8–9 long, straight for most of body length, those in final 5–10 rows slightly curved with small hook on tip, 10–11 long. First spine row with 3–4 spines, increasing in number prior to dorsal nerve commissure, 7 spines per row for most of body length, decreasing in number in posterior third of body, 3 spines in final row; spine rows 10–14 (12) wide, spaced 3–4 apart. Dorsal nerve commissure 33–53 (40) across, 89–126 (111) from anterior extremity. Nerve cords well- defined, 6–9 (7) in diameter, run length of body, 24–38 (29) from body margin at midbody. Oral sucker poorly delineated, weakly muscularised, 16–19 × 19–25 (17 × 22), bearing 5 concentric rows of fine spines. Mouth a simple pore, ventrally subterminal, 4–11 (8) from anterior extremity. Oesophagus almost straight to gently sinuous, thick-walled, 359–420 (396) long. Oesophageal glands enveloping oesophagus posterior to dorsal nerve commissure, thickening and forming distinct glandular bulb immediately anterior to anterior caeca. Caeca form X-shape; intestinal bifurcation in middle third of body, 356–424 (397), or 34.4–41.1% of total body length, from anterior extremity. Anterior caeca equal to subequal in length, much shorter than posterior caeca; left anterior caecum 31–57 (40); right anterior caecum 23–64 (43); longer anterior caecum occupying 3.3–6.2% of total body length. Posterior caeca equal to subequal in length, 3.4–8.5 (5.3) times longer than anterior; left posterior caecum 189–259 (210); right posterior caecum 157–281 (217); longer posterior caecum occupying 19.3– 24.5% of total body length. Total caecal length 224–295 (260), occupying 22.3–28.6% of body length.

Testis single, roughly rectangular, with margins irregularly lobed, immediately posterior to posterior ends of posterior caeca, usually extends laterally beyond lateral nerve cords and posteriorly to anterior margin of ovary, 98–209 × 61–150 (140 × 103), occupying 10.1–17.0% of total body length; post-testicular space 258–367 (299), or 25.7–30.1% of body length. Vas deferens originates medially from posterior margin of testis, passing ovary and uterus ventrally, widening posteriorly in some specimens, entering cirrus-sac dorso-anteriorly. External seminal vesicle absent. Cirrus-sac retort-shaped, rounded anteriorly, dramatically narrowed posteriorly; anterior rounded portion 45–66 × 29–58 (56 × 42), contains seminal vesicle and pars prostatica; posterior narrow portion 65–93 (73) long, notably thickened at marginal genital pore, contains ejaculatory duct (un-everted cirrus; everted cirrus not observed), 4–7 (5) wide at midpoint, 6–10 (8) wide at marginal thickening. Seminal vesicle roughly round, 25–46 × 23–49 (35 × 35), restricted to anterior, rounded portion of cirrus-sac, joining coiled pars prostatica; prostatic cells not observed. Ejaculatory duct long. Male genital pore on sinistral margin at distinct to indistinct marginal notch, 89–124 (101), or 8.9–10.6% of body length, from posterior extremity.

Ovary oblong to roughly rectangular, medial, with margins irregularly lobed, immediately posterior to testis, usually extending laterally beyond lateral nerve cords, 43–73 × 65–138 (53 × 103); post-ovarian space 216–306 (249), or 21.1–25.4% of total body length. Oviduct originates from posterior margin of ovary, passes posteriorly dorso-lateral to vitelline duct and dextro-lateral to ascending portion of uterus, posteriorly curving sinistrally to meet oötype, usually heavily distended with sperm. Oötype posterior to rest of genitalia, medial, surrounded by Mehlis’ gland, 94–129 (109) from posterior extremity. Uterus weakly convoluted, passing anteriorly between oviduct and dextral side of cirrus-sac, ventrally overlapping posterior portion of ovary, then passing posteriorly, sinistral to cirrus-sac, to female genital pore; distal portion of uterus often forming egg reservoir, creating distinct marginal bulge. Female genital pore dorsal, sinistro-submedial, separate from and anterior to male pore, at level of constriction dividing anterior and posterior portions of cirrus-sac, 16–37 (27) from sinistral margin, 145–205 (167) from posterior extremity. Eggs in utero ovoid to subspherical, very thin-shelled, anoperculate, 19–34 × 12–19 (24 × 16). Vitellarium follicular, distributed from just posterior to dorsal nerve commissure to posterior half of testis, rarely to level of ovary, laterally exceeding nerve cords, largely confluent anterior to testis, sometimes interrupted partially by ends of caeca and oesophageal gland, interrupted partially by testis ventrally, completely or partially interrupted by testis dorsally. Vitelline duct passes ovary ventrally, passing posterio-dextrally to oötype, ventrally overlaps oviduct, posteriorly curving sinistrally to meet oötype.

Excretory vesicle small, saccular; paired collecting ducts not traceable. Excretory pore at apex of terminal notch.

Remarks

Holocentricola exilis was found in four of the five body sites examined (heart, branchial arteries, vessels of liver, wash of head split), but was most commonly found in the wash of the head split and the branchial arteries. The intensity of infection of H. exilis was notably lower than that found for H. rufus (1–4 worms per fish, mean 1.53 vs 1–12 worms per fish, mean 4.27).

Figures

Figure 2. Species of Holocentricola from Great Barrier Reef Holocentridae, whole worms, ventral views. (A) Holocentricola rufus n. sp. ex Sargocentron rubrum from off Heron Island (holotype, QM G239429); (B) Holocentricola exilis n. sp. ex Neoniphon sammara from off Lizard Island (paratype, QM G239111); (C) Holocentricola coronatus n. sp. ex Sargocentron diadema from off Lizard Island (holotype, QM G239125). Scale-bars: A–C, 200 µm.

Figure 3. Species of Holocentricola from Great Barrier Reef Holocentridae, terminal genitalia, dorsal views; spines illustrated are ventral. (A) Holocentricola rufus n. sp. ex Sargocentron rubrum from off Heron Island (paratype, QM G239440); (B) Holocentricola exilis n. sp. ex Neoniphon sammara from off Lizard Island (paratype, QM G239119); (C) Holocentricola coronatus n. sp. ex Sargocentron diadema from off Lizard Island (paratype, QM G239126). Abbreviations: CS, cirrus-sac; ER, egg reservoir; FGP, female genital pore; MGP, male genital pore; Od, oviduct; Oö, oötype; Ov, ovary; PP, pars prostatica; SV, seminal vesicle; Ut, uterus; VD, vas deferens; VitD, vitelline duct. Scale-bars: A–C, 100 µm.