Melanotaenia lacunosa

Species Summary
Adult male Melanotaenia lacunosa are bronze-yellow to bluish grey on the upper back, grading to bluish white (anteriorly) or yellowish (posteriorly) on the lower half of side; bluish grey mid-lateral stripe from pectoral region to caudal-fin base, widest portion on caudal peduncle where it covers about two horizontal scale rows; pale yellow stripe immediately below, about one-third width of dark mid-lateral stripe; most of scales above anal fin base with variable amount of bluish grey colouration; head blue greyish with silvery stripe behind eye extending across upper operculum where punctuated with diffuse red or orange spot; dorsal, anal, pelvic and caudal fins pale yellowish to reddish-white; caudal fin translucent bluish; pelvic fins whitish; pectoral fins translucent.

Melanotaenia lacunosa is a member of the "Goldiei" rainbowfish group that share a wide range of morphological and meristic features and lack notable differences, as would be expected for closely related species. Due to their great similarity, the species belonging to the "Goldiei" group are most reliably distinguished on the basis of genetic differences, colouration, and their geographically isolated distribution. The challenge with the "Goldiei" species group is that while they all have strong genetic differences and some clear colouration differences, morphological and meristic differences are minimal in most cases. Males of M. lacunosa have a distinctive, broad yellowish area immediately above the dark mid-lateral stripe and narrower pale yellow stripe immediately below it, as well as yellow-margined scales on the middle of the lower side.

Distribution & Habitat
Melanotaenia lacunosa is known only from streams in the vicinity of Mbuta (sometimes spelled Mbutu) Basin near Etna Bay in West Papua. The holotype and most of the paratypes were obtained in the main portion of the Basin by Gerry Allen in 1997 via helicopter access from a temporary exploration camp maintained by Freeport Indonesia Mining Company. Additional specimens were collected in 2013 from nearby streams on a recently constructed road linking the Mbuta Basin with Kayu Merah Bay. Mbuta Basin consists of an oval-shaped basin, approximately 9 km long and 3.2 km wide, along a NE-SE axis. Its southeastern end is only 3.5 km from Etna Bay via a narrow valley flanked by 800-900 m high ridges. The valley gradually ascends from the bay to a 200 m high pass before opening into Mbuta Basin, which lies at an elevation of 170 m above sea level. The basin is often represented on published maps and charts as a lake, but in 1997 it was marshland surrounded by low mountains. Much of the basin floor was covered with 3-4 m tall grass with numerous small ponds, creeks, and at least one small river. Drainage is apparently subterranean, which is typical of other lakes in this limestone region. There is no apparent outflow to nearby Etna Bay. The basin is uninhabited by humans, but supports large numbers of wild pig and deer. Contrary to conditions in 1997, the basin was inundated in December 2013 with widespread lacustrine conditions, therefore indicative of widely fluctuating conditions, depending on local rainfall. The type specimens collected in 1997 were captured with rotenone in a 2-3 m-wide creek with depths to about 2 m. The collection was made over a 50 m-long section immediately above its confluence with a small turbid river. The water was very clear, but darkly stained (tea-coloured), with relatively fast flow through forest that almost formed a closed canopy. The bottom consisted mainly of mud with occasional rocks and log debris with sparse aquatic vegetation. A water temperature of 25.8°C and pH of 6.4 were recorded. Other rainbowfish inhabitants of the creek included Pelangia mbutaensis. Melanotaenia lacunosa was the most abundant fish collected at this site. An attempt to reach this location in 2013 was thwarted due to high water levels when the site of the former stream was flooded by a large lake.

Remarks
Melanotaenia lacunosa was named "lacunosa" (Latin: "full of lakes and ponds") with reference to the many small ponds and larger lakes found in the Mbuta Basin.

Literature
Allen G.R., P.J. Unmack and R.K. Hadiaty (2016) The Goldiei group of rainbowfishes (Melanotaeniidae) from the Birds Neck Region of New Guinea (Papua and West Papua Provinces, Indonesia) with descriptions of five new species and recognition of Melanotaenia dumasi Weber. aqua, International Journal of Ichthyology 22(1): 1-32.

Adrian R. Tappin
January, 2016