Hippopotamyrus longilateralis, Kramer & Swartz, 2010

Kramer, Bernd & Swartz, Ernst R., 2010, A new species of Slender Stonebasher within the Hippopotamyrus ansorgii complex from the Cunene River in southern Africa (Teleostei: Mormyriformes), Journal of Natural History 44 (35 - 36), pp. 2213-2242 : 2231-2233

Hippopotamyrus longilateralis sp. nov.

( Figure 1A)

Material examined

See Materials and methods section.

Type specimens

Holotype SAIAB78793 , Paratypes SAIAB 79502 , 79503 , 78778 , 78779 , 78793 , 78783 , 78787 , ZSM 38561 – ZSM 38567 .

Type locality

Cunene River, just above Epupa Falls on the Namibian / Angolan border, 17°00′07″ S, 013°14′57″ E, altitude about 600 m.

Diagnosis

pD, mean 0.4013 (range 0.3791 –0.4175) of SL; SLS, median 77 (range 74–80); PDL , mean 0.6273 (range 0.6123 –0.6499) of SL; SPc, median 20 (range 19–22); nD, median 20 (range 18–21); nA, median 24 (range 22–25); PAL, mean 0.6020 (range 0.5906 – 0.6270) of SL; CPL, mean 0.2080 (range 0.1952 –0.2202) of SL; HL, mean 0.2155 (range 0.1923 –0.2289) of SL; LSo, mean 0.4943 (range 0.4578 –0.5484) of HL; BD, mean 0.2256 (range (0.2037 –0.2451) of SL; bipolar, pentaphasic waveform of EOD pulse (bipolar main pulse, about 230 µs). Mitochondrial cytochrome b and barcode sequences (GenBank, GU453680 – GU453688 and HM 065027 – HM 065034 ).

Description

Head broadly rounded with a terminally positioned mouth, lower jaw protruding less forward than upper; body long and slender, laterally compressed. Dorsal fin origin set far back and behind that of anal fin, situated about two-thirds of standard length from snout, obliquely orientated with anterior portion of the fin higher than the posterior portion, median number of rays 20 (18–21). Anal fin opposite dorsal fin but origin further anterior and also obliquely orientated, anteriorly lower and posteriorly higher, margin broadly rounded, median number of rays 24 (22–25). Scales cycloid with reticulate striae, except in centre, extending anteriorly to operculum and pectoral fins. SPc, median 20 (19–22). Caudal peduncle slender and subcylindrical over the entire length, usually slightly less than one-fifth in SL. Tail fin with broadly rounded lobes. EOD bipolar and pentaphasic, duration of the two main phases about 230 µs but whole pulse may last up to 5 milliseconds because of weak pre- and post-potentials (25°C). Dark brown with a vertical black band from dorsal origin that is only weak in most specimens.

Colour in preservation

Dark to medium brown, often with milky-grey hue, especially where opaque ‘mormyrid skin’ (electroreceptor organ area) present (and not rubbed off the underlying skin).

Ecology

The section of the Cunene River between Ruacana Falls and Epupa Falls is a permanently flowing river with a strong current. Water level is controlled by a hydroelectric power plant at Ruacana Falls (Namib Power) and a large dam (Calueque Dam) further upstream in Angola. The Ruacana Falls was completely dry during our survey, illustrating the extreme river regulation. A major daily difference in water level was even detected at Epupa Falls (a straight line distance from Ruacana Falls of 115 km), where catches were good during lower water levels with no fish caught during high flow. Although the area around Epupa Falls is very dry, there are extensive stands of palm (Makalani, Hyphaene petersiana ) and other trees, reeds and bushes that grow on the banks, including an occasional baobab. At Ruacana, which is wetter, the arboreal and other vegetation and bird life were more abundant and varied. Crocodiles were present at both Ruacana and Epupa waterfalls, with hippopotamus reported for the former, but not seen. Most of the H. longilateralis specimens were caught along well-vegetated and steep banks of the river. They therefore possibly prefer undercut banks with structure such as roots.

Distribution

Only known from the section of the Cunene River between below Ruacana Falls downstream to above Epupa Falls, a straight-line distance of 115 km. This section of the river forms part of the Namibian/Angolan border.

Relationships

Based on our phylogenetic analysis, H. longilateralis is the sister-group of all other members of the H. ansorgii species complex. Barcode (COI) sequence of paratype specimens were done for future reference.

Etymology

The specific name longilateralis is in reference to the long lateral line.

Remarks

Referred to as the lower Cunene morph in the present paper (‘morph Ha4’ in tables with limited space); H. longilateralis is distinguished by pD and SLS highest and PDL lowest among the species complex of seven; SPc and nD also high; distinctive bipolar, pentaphasic waveform of its EOD pulse, as opposed to monopolar DC pulses generated by the Upper Zambezi and Kwando morphs and H. szaboi . The species can also be distinguished on their phylogenetic relationships with mitochondrial cytochrome b and barcode sequences. We therefore recognize the lower Cunene morph as a new species, H. longilateralis sp. nov.