Minyclupeoides dentibranchialus, Roberts, 2008

Roberts, Tyson R., 2008, Minyclupeoides Dentibranchialus, A New Genus And Species Of River Herring From The Lower Mekong Basin Of Cambodia (Teleostei: Clupeidae: Pellonulinae), Raffles Bulletin of Zoology 56 (1), pp. 125-127 : 126-127

Minyclupeoides dentibranchialus , new species

( Figs. 1–2 )

Holotype. – ZRC 50699, 21.3 mm, Cambodia, Mekong Basin, Takeo-Angkor Borei flood plain near Takeo during rainy season, 20–22 Sep.2000, Tyson R. Roberts.

24); and apparently in having much more numerous teeth on the maxillary bone, 25–27 (a drawing of teeth on the maxillary bone of C. borneensis shows only 7 teeth; Whitehead, 1985: 175).

Further comparisons. – Only four species of Clupeoides are recognized by Wongratana, 1980 ( Whitehead, 1985: 174). All of them are fully scaled. Minyclupeoides dentibranchialus agrees with C. borneensis but differs from the other species of Clupeoides in having teeth on the premaxillary bone. It agrees with C. borneensis and C. hypselosoma in having the supramaxillary bone with a slender anterior process that is absent or truncated in C. papuensis and C. venulosus .

Description. – Maxillary teeth 25–27. Premaxillary teeth 7– 8. Dentary teeth 4–5. Gill rakers on first gill arch 7/1/13. Pseudobranchial filaments 8. Dorsal fin rays iii 101 / 2 – 111 / 2. Anal fin rays 161 / 2 – 171 / 2. Pectoral fin rays 10–11. Pelvic fin rays 7. Caudal fin rays 10–11,10/9,8. Abdominal scutes 9– 10+4–6=13–16. Predorsal bones 11. Vertebrae 18+18=36 (vertebral counts of other Mekong Pellonulinae unknown).

Colouration. – The types and only known specimens were collected after dark and immediately preserved. Thus no observations are available on the coloration in life. Preserved specimens fixed in formalin and stored in 75% ethyl alcohol are largely colorless except for the head and abdomen. The iris, opercle, and subopercle are silvery, as is the peritoneum. The straw-coloured pale brown or tan body itself is nearly devoid of pigment cells. There are virtually no melanophores on the side of the body, predorsal surface, and ventral surface. Paratypes. – ZRC 50700, 32 ex.: 18.3–22.5 mm, collected with holotype (including 9: 20.0– 21.4 mm stained with alizarin but not cleared); CAS 224372 , 13 : 19.4–21.0 mm, collected with holotype (including 3: 20.0, 21.0 and 21.0 mm, cleared and stained with alcian) .

Diagnosis. – Minyclupeoides dentibranchialus apparently is a very small species (largest specimen collected 22.5 mm; a 21.5 mm female is gravid). It apparently is most closely related to the Southeast Asian pellonulin genus Clupeoides . It differs from the only species of Clupeoides in the Mekong Basin, the much larger and fully-scaled C. borneensis , in having abdominal scutes 9–10+4–6=13–16 (vs. 12+6–9=18– 21); gill rakers on lower limb of first gill arch 13 (vs. 18–

THE RAFFLES BULLETIN OF ZOOLOGY 2008

The dorsal part of the body posterior to the dorsal fin has a row or two of about ten melanophores. Even the external dorsal surface of the head overlying the brain has no melanophores. The few patches of pigment cells over the brain are intracranial. The largest concentration of melanophores occurs on the caudal fin, particularly on the upper lobe and on the innermost ray or two of the lower lobe. The upper procurrent caudal rays have scattered melanophores. Melanophores extend along the length of the uppermost (simple) principal caudal fin ray, and on the distal fourth or third of the upper lobe branched rays. The paired fins are nearly or quite devoid of melanophores. The base of the anal fin, particularly posteriorly, has a few large melanophores.

Gill arches. – In Minyclupeoides , as perhaps in all Pellonulinae , the gill rakers (derived from primitive branchial toothplates) are armed with tiny teeth. The first gill arch has 7+1+13 rakers on the outer face of the first epibranchial and first ceratobranchial ( Fig. 2 ). These are most elongate toward the angle of the arch. Each gill raker is armed with up to 20 or so small conical teeth, about equally distributed on the leading and trailing edges of the raker. There are no gill rakers on the inner face of the first gill arch, and the only teeth on the first gill arch are on the gill rakers. The second gill arch has 6/1/9 gill rakers on the outer face and one on the inner. These rakers are less elongate than those on the first gill arch, but are similar toothed. The ventral face of the first epibranchial armed with about a dozen small conical teeth similar to those on the gill rakers. These teeth articulate directly with the epibranchial. The third gill arch has 2/1/9 gill rakers on the outer face and none on the inner; these rakers apparently have no teeth. The fourth arch has 2/1/7 gill rakers on the outer face and 9 on the inner face of the ceratobranchial; these rakers are toothless. The fifth branchial arch has 9 toothless gill rakers on the outer face; lower pharyngeal tooth plate with 8 small scattered conical teeth.

Etymology. – The species or trivial name dentibranchialus is from the Latin “dent”, tooth, and “branchus”, gills, in reference to the heavily denticulate gill rakers on the gill arches.

Figures

Fig. 1. Minyclupeoides dentibranchialus new species: Composite drawing of several specimens superficially stained with alizarin. Inset above left: humeral scale with lateral line pore; inset above right: upper procurrent caudal scale-like sheath (median structure) with first upper procurrent caudal fin ray (paired structure).

Fig. 2. Minyclupeoides dentibranchialus new species: Jaw bones (left); first gill arch with denticulate gill rakers (right).