Melanotaenia wilsoni, Hammer & Allen & Martin & Adams & Unmack, 2019

Hammer, Michael P., Allen, Gerald R., Martin, Keith C., Adams, Mark & Unmack, Peter J., 2019, Two new species of dwarf rainbowfishes (Atheriniformes: Melanotaeniidae) from northern Australia and southern New Guinea, Zootaxa 4701 (3), pp. 201-234 : 224-230

Melanotaenia wilsoni sp. nov.

Little Rainbowfish

(figs 5 & 8; tables 4–5)

Melanotaenia maccullochi non Ogilby, 1915 — Larson & Martin (1989): 82; Armstrong (1995): 410 (inset image, Burton Creek); Allen (1995) (in part): 57; McGuigan et al. (2000) (in part): 715; Unmack (2001) (in part): 1061; Allen et al. (2002) (in part): 152; Pusey et al. (2004) (in part): 243; Allen et al. (2008) (in part): 92; Tappin (2010) (in part): 282 (image Burton Creek); Cook et al. (2012) (in part): 265; Larson et al. (2013): 60; Schmida (2015) (in part): 86 (images Burton Creek); Pusey et al. (2017) (in part): 39 (point data distribution map).

Melanotaenia sp. NT — Unmack et al. (2013): 19; Graf et al. (2015): 878.

Material examined (50 specimens, 17.2–33.3 mm SL) .

Holotype: NTM S.17776-001, male, 26.1 mm SL, Sandy Creek, Reynolds River system, Litchfield National Park (13° 15.21ʹS, 130° 44.42ʹE), Northern Territory, 0.5 m, seine, M. Hammer & N. Litjens, 25 August 2014 .

Paratypes: AMS I.49002-001, 5 specimens, 18.9–22.4 mm SL, Petherick Swamp, Burton Creek (13° 05.7ʹS, 130° 39.3ʹE), Northern Territory, 0.3 m, electrofisher, M. Hammer & D. Wilson, 23 June 2012 ; NTM S.12595-001, male, 33.3 mm SL, Burton Creek, Finniss River system (13° 05.8ʹS, 130° 39.4ʹE), Northern Territory, 0.3 m, scoop net, D. Wilson, October 1988 ; NTM S.13836-001, 10 specimens, 19.5–30.1 mm SL, Burton Creek (captive stock) (13° 05.7ʹS, 130° 39.3ʹE), Northern Territory, 0.3 m, scoop net, D. Wilson, September 1993 ; NTM S.17402-007, 17 specimens, 17.2–29.9 mm SL, Petherick Swamp, Burton Creek (13° 05.7ʹS, 130° 39.3ʹE), Northern Territory, 0.3 m, electrofisher, M. Hammer & D. Wilson, 23 June 2012 ; NTM S.17776-002, 3 specimens, 26.0– 27.6 mm SL, same data as holotype ; QM I.40977, 5 specimens, 21.3–24.8 mm SL, Petherick Swamp, Burton Creek (13° 05.7ʹS, 130° 39.3ʹE), Northern Territory, 0.3 m, electrofisher, M. Hammer & D. Wilson, 23 June 2012 ; WAM P.34930-001, 4 specimens, 23.7–26.7 mm SL, Petherick Swamp, Burton Creek (13° 05.7ʹS, 130° 39.3ʹE), Northern Territory, 0.3 m, electrofisher, M. Hammer & D. Wilson, 23 June 2012 ; WAM P.34931-001, 3 specimens, 23.2–25.9 mm SL, same data as holotype.

Diagnosis: A species of Melanotaenia with well-contrasted dark lateral body stripes, distinguished by the following combination of characters: dorsal rays IV–VI, I,7–9 (mode 8); total anal rays I,13–16 (mode 14); total procurrent caudal rays 11–13 (mode 12); lateral scales 30–32 (mode 31); cheek scales usually 8–9; total gill rakers on first arch 14–16; vertebrae 31–33 (mode 32); body deep, greatest depth 3.1–3.6 (mean 3.3) in SL and wide, greatest width 1.9–2.9 (mean 2.3) in HL; maxilla relatively short 4.0–4.7 (mean 4.3) in HL; long predorsal distance 1.9–2.1 (mean 2.1) in SL; colour in life brown to whitish body with a series of black lateral stripes the middle being the most pronounced having a dark purple hue; fins pale with diffuse patterns, most pronounced in nuptial males.

Description: Based on 30 specimens, 19.3–33.3 mm SL; counts and measurements that appear in parentheses refer to the minimum and maximum values for paratypes and other specimens where different from the holotype. Dorsal rays V (IV–VI) + I,7 (7–10; median 8); total anal rays I,14 (13–16, median 14); pectoral rays 11 (11–13); pelvic rays I,5; branched caudal rays 15; total procurrent caudal rays 12, 6 + 6 (11–13, 5–6 + 5–6, rarely 7); lateral scales 30 (30–32, median 31); transverse scales 10 (9–10); predorsal scales 16 (14–16); prepelvic scales 14 (13–16); cheek scales 8 (7–10, median 9); circumpeduncular scales 14 (13–14); gill rakers on first branchial arch 2+13 (2–3 + 12–13), total gill rakers on first arch 15 (14–16); total vertebrae 32 (31–33, median 32), precaudal vertebrae 19 (18–20, median 18). Full meristic counts are shown in Table 4 .

Head length 3.5 (3.2–3.8) in SL; body depth 3.2 (3.1–3.6, mean 3.3) in SL; greatest width of body 2.2 (1.9–2.9, mean 2.3) in HL; snout length 4.1 (3.3–4.7, mean 4.0) in HL; eye diameter 2.6 (2.3–3.0) in HL; bony interorbital width 2.7 (2.4–3.0) in HL; depth of caudal peduncle 2.4 (2.2–2.6) in HL; length of caudal peduncle 1.7 (1.4–1.9) in HL.

Jaws about equal, oblique, premaxilla with an abrupt bend between the anterior horizontal portion and lateral part; maxilla ends slightly in front of level of anterior edge of pupil; maxillary length 4.4 (4.0–4.7, mean 4.3) in HL; lips thin; teeth villiform to narrowly conical with slightly curved tips, extending on to outer surface of lips; teeth of upper jaw in about 2–3 irregular rows anteriorly, reduced to single row posteriorly where clearly exposed when mouth closed; teeth in lower jaw in about 3–4 irregular rows anteriorly, reduced to a single row posteriorly.

Scales of body cycloid, relatively large, and arranged in regular horizontal rows; scales of midlateral row with small rounded to vertically elongate, shallow pit at centre, beginning at about 10 th– 13 th scale; row of small, truncated scales along bases of dorsal and anal fins, forming a low sheath on anal fin; no scales on membranous portions of fins except 3–6 transverse rows of small scales basally on caudal fin and cluster of several small scales (posteriormost largest) medially at base of pelvic fins; scale margins smooth or with slight crenulations; predorsal scales extending forward to about middle of interorbital space; preopercle (cheek) covered with 8 (7–10) scales; main body of operculum covered with 17 (12–17) scales and with single row of about 4–5 additional small scales on anterior limb (i.e. interopercular directly below lower edge of preopercle).

Predorsal distance 2.0 (1.9–2.1, mean 2.1) in SL (46.6–51.2, mean 48.4 as a % of SL); preanal distance 1.8 (1.6–1.9 in SL); prepelvic distance 2.4 (2.2–2.6) in SL; length of second dorsal fin base 6.3 (5.0–6.3, mean 5.8) in SL; length of anal fin base 3.6 (3.1–3.8) in SL.

First dorsal fin origin clearly anterior to level of anal-fin origin, frequently by a distance about equal to pupil width; longest spine (second to fourth) of first dorsal fin 1.9 (1.5–2.0) in HL, its adpressed tip reaching base of second dorsal-fin spine; longest (first) ray of second dorsal fin 2.0 (1.7–2.2) in HL, adpressed posterior rays extending about one-half length of caudal peduncle in male and female; longest (middle or about 6 th to 9th) anal rays 2.3 (1.8–2.5) in HL; pelvic fin tips when adpressed slightly anterior to anal-fin origin; length of pelvic fins 2.1 (1.6–2.1) in HL; length of pectoral fins 1.6 (1.4–1.7) in HL; length of caudal fin 1.4 (0.9–1.4 in HL); caudal fin moderately forked, caudal concavity 6.8 (3.3–8.4) in HL. A summary of proportional measurements is shown in Table 5 .

Colour in life: Generally yellow-brownish on upper half of body, lower half of body white; seven or eight black body stripes, the middle is full and thickest with a dark purple hue, the second stripe out from the middle both dorsally and ventrally is also full and black in colour, other stripes tend to be broken and black in colour; head darker brown, lips brown sometimes with a hint of orange; small square shaped orange to red mark on the upper operculum; iris silver to orangish; fins pale, with distinct zonation in the dorsal and anal fins formed by a thick greyish to black band that can appear to cut diagonally across the fin beginning submarginal at the anterior part of the fin, there is a corresponding thick to thin area of light yellow colour above the band with often a whitish to light blue fin margin, and inside the black band is typically an area of pale yellow to orange with a line of scattered spots or blotches close to the fin base; an area of orange can be present on the breast into the pelvic fins, brighter in males (figs 5 & 8). Nuptial males tend to develop an overall whitish body sheen and brighter white-yellow on the fins with the orange on the breast intensifying, mature females a bronze colour with intensified purple mid-lateral body stripe; otherwise sexual dimorphism is slight. A faint, rarely bright, white to yellow (sometimes pale orange) nape stripe can be ‘flashed’ by courting and sparring males.

Colour in alcohol: Generally light brown to yellowish tan with 7–8 dark brown stripes on side of body as described above; fins mainly translucent with dark bands on basal and outer portions of dorsal and anal fins sometimes evident ( Fig. 8 ).

Comparisons: Readily distinguished from both other Australian members of the maccullochi species group based on the pattern and colour of body stripes (midlateral stripe most prominent and purple), other fin colour differences (pattern and intensity), small maximum size, and the combination of fewer lateral scales (usually less than 32 v. usually 32 or more), fewer vertebrae (31– 33 v. usually 33+), fewer total procurrent caudal rays (11–12 v. 12 –15), fewer anal-fin rays (usually 14–15 v. 15 +), fewer check scales (usually 8– 9 v. usually 9+) and on average a smaller maxilla and snout. Additionally, M. wilsoni sp. nov. differs from M. maccullochi s.s. in having a shorter second dorsal fin base and from M. sahulensis sp. nov. in having a shorter anal-fin base, and is unique in that the adpressed first dorsal fin of males extends only to the base of the second dorsal-fin spine.

Distribution and habitat: Museum records are displayed in Figure 1 , with M. wilsoni sp. nov. native to tropical northern Australia, south of Darwin in the Northern Territory. The known range is highly restricted (extent of occurrence 26 km 2) being recorded in three small streams draining west off the Tabletop Range, namely Burton Creek (Finniss River system) and Tolmer and Sandy creeks (Reynolds River system). Habitat is shallow (<50 cm) comprising small Pandanus lined spring fed creeks over sandy soils with strong lateral connectivity to densely vegetated peat swamp and side-channel areas. Swamp and channel habitat can be seasonal or retract to refuge holes ( Fig. 9 ). Abundance of fish is typically very low, adults being found scattered along stream margins and sometimes concentrated in small permanent side pools, and juveniles in seasonally flooded areas. Water transparency ranges from clear (streams) to heavily stained, with indicative water quality parameters in the dry season including pH 6.0–6.5 (noted also as 5.0: Armstrong 1995), temperature 23–24°C, water conductivity 15–30 µScm-1 and general hardness and carbonate hardness <10 ppm. Co-occurs with regionally ubiquitous Melanotaenia nigrans (Richardson, 1843) and Mogurnda mogurnda (Richardson, 1844) and other swamp dwelling fish including Pseudomugil gertrudae , Oxyeleotris nullipora , Porochilus obbesi Weber, 1913 , Denariusa bandata Whitley, 1948 and Ophisternon gutturale (Richardson, 1845) . The maximum size recorded for wild fish is a 29.9 mm SL female (36.2 mm TL: NTM S.17402-007), with captive specimens growing slightly larger to at least 33.3 mm SL (40 mm TL: NTM S.12595-001). This makes M. wilsoni sp. nov the smallest growing of all known species in the genus. Little else is known on the biology of the species other than records of spawning and behaviour in captivity, where it is an extremely shy fish, which is suggested to be an adaption to shallow habitat ( Armstrong 1995).

Remarks: This species has been recognised as a distinct dwarf form since its discovery in the 1980s, with strong genetic divergence noted in several more recent mtDNA studies ( McGuigan et al. 2000; Cook et al. 2012; Unmack et al. 2013) and now supported with nuclear genetic markers.

Etymology: The new species is named for David Wilson who was the first person to document the presence of the small isolated population in the Northern Territory, and has worked tirelessly and generously to promote the sustainable use and conservation of native aquatic animals, especially rainbowfishes.

Figures

FIGURE 1. Location map of the Australia and New Guinea region showing Melanotaenia maccullochi s.l. genetic study collection sites (see Table 1) and locations of other museum material examined.

FIGURE 8. Aquarium and specimen images of Melanotaenia wilsoni sp. nov. below Tabletop Range Northern Territory (A) male displaying to female, Burton Creek, Finniss River system (captive bred), (B) male courting female showing white body sheen, Burton Creek (captive bred), (C) wild fish in a field tank, Burton Creek, (D) 24.7 mm SL male, Burton Creek (QM I.40977), (E) 33.3 mm SL male, Burton Creek, wild fish reared in captivity (NTM S.12595-001), and (F) 26.1 mm SL male holotype, Sandy Creek, Reynolds River system, Litchfield National Park (NTM S.17402-007). Aquarium photos G. Schmida, other photos MPH.

FIGURE 9. Comparative radiographs of M. maccullochi s.l. species complex rainbowfishes (male fish from Figure 5): (A) Melanotaenia maccullochi s.s. 30.0 mm male, Boggy Creek, Trinity Inlet, Cairns, Queensland (NTM S.18178-001); (B) Melanotaenia sahulensis sp. nov. 38.2 mm male, Skull Creek, Cape York, Queensland, captive stock (NTM S.18153-001); and (C) Melanotaenia wilsoni sp. nov. 26.1 mm male holotype, Sandy Creek, Reynolds River system, Litchfield National Park, Northern Territory (NTM S.17776-001).