Taxonomy & naming
Characidium noelkempffi was described in 2026 by Leonardo Oliveira-Silva, Karina Or, Angela Zanata and Scott A. Schaefer, based on specimens collected in August 1998 by Schaefer and Damaris Rodriguez and held at the American Museum of Natural History for over two decades before being recognised as a new species. It belongs to the genus Characidium, the largest genus in the subfamily Characidiinae within the family Crenuchidae — the South American darters — a group of small, elongate fish that perch and dart along the substrate rather than swim continuously in open water like the piranhas and pacus that headline this atlas section.
The species is one of eight Characidium documented from Bolivian tributaries of the Madeira River basin, but only the third to have actually been described from Bolivian material rather than material collected elsewhere; the describing authors note that ichthyofaunal surveys of the smaller tributaries of the Iténez-Guaporé sub-basin, including the Pauserna River, remain scarce. Within the genus, C. noelkempffi is diagnosed chiefly by its colour pattern rather than any single unique structure, and the describing team's use of CT scanning to examine the Weberian apparatus — the chain of modified vertebrae that link the swim bladder to the inner ear — produced the first such imagery published for any member of the family Crenuchidae.
Morphology
Characidium noelkempffi is a small, elongate darter; the type series ranges from 1.3–2.1 in in standard length, with the holotype measuring 2 in. It is diagnosed from all but four of its many congeners by a broad, conspicuous dark midlateral stripe — at least one scale wide — that runs continuously from the rear of the gill cover to the base of the tail, markedly darker than any vertical bars present, combined with the absence of a distinct humeral blotch behind the head. Other diagnostic details include a complete (fully pored) lateral line, a well-developed adipose fin, 12 scale rows around the caudal peduncle, four scale rows above the lateral line, a bony canal branch (the parietal branch of the supraorbital canal) present on the skull roof, and a conspicuous dark spot at the base of the tail fin, set apart from the midlateral stripe.
Many specimens — apparently more often males — also show a reddish-brown hue along the borders of the paired, dorsal and caudal fin rays, a colour trait otherwise recorded in only two other Characidium species. Internally, CT scanning of two paratypes revealed a previously undescribed shell-like, leaf-shaped bony expansion on the fourth vertebra of the Weberian apparatus, a structure the describing authors could not match closely to anything reported for other Characidium.
Habitat
Characidium noelkempffi is known only from tributary streams of the Pauserna River — itself part of the Iténez-Guaporé drainage of the Madeira River, within the Amazon basin — on the Meseta Huanchaca I, an isolated sandstone-and-quartzite plateau inside Noel Kempff Mercado National Park in Santa Cruz Department, northeastern Bolivia. The plateau sits at roughly 1640–2953 ft above sea level, considerably higher than the surrounding lowlands, and the type series was collected from small streams at the mouth of tributary creeks on the plateau surface.
The describing authors argue the species' restriction to this single small drainage reflects genuine geographic isolation rather than under-sampling: streams draining off the Huanchaca plateau toward the Pauserna and Verde rivers plunge over a series of cascades, including the roughly 289 ft Arco Iris falls and the 148 ft Frederico Ahlfeld Falls, and C. noelkempffi is known only from above these barriers. It also lacks the more robust, cling-adapted pectoral fins that let some other Characidium species negotiate small waterfalls, reinforcing the case that this is a genuinely landlocked, headwater-endemic population rather than one simply missed by earlier collectors. Specific water chemistry data — temperature, pH, hardness — were not reported for the type locality.
Feeding
No dietary data were reported for Characidium noelkempffi in its description, which is a systematics paper rather than an ecological study. Based on the feeding habits documented for other Characidium darters — small, substrate-perching fish that dart forward to snap up prey rather than cruising for it in open water — C. noelkempffi is presumed to feed on small aquatic invertebrates picked from the streambed, though this has not been directly confirmed for this species.
Mating
Characidium noelkempffi shows clear sexual dichromatism, though nothing has been published on its courtship behaviour specifically. Adult males lack the dark vertical bars seen in females entirely; instead, the upper half of the body carries melanophores concentrated at the margins of the scales, producing a reticulated (net-like) pattern, and males show the reddish-brown fin pigmentation more strongly than females do. Females, by contrast, carry 11 to 16 regularly spaced dark vertical bars running from behind the head to the base of the tail. The describing authors note that a broadly similar pattern — males losing the bars that females retain — has been reported in only one other Characidium species, C. bimaculatum, though the two differ in several other characters. No bony hooks, used for mate-holding in some other Characidium, were found on the fins of either sex.
Breeding
No breeding observations, captive or wild, have been published for Characidium noelkempffi; its description is based entirely on preserved museum specimens collected in 1998. As with other Characidium, it is presumed to be an egg-scattering species with no parental care, following the pattern documented across the genus, but this has not been confirmed directly for C. noelkempffi.
In the aquarium
Characidium noelkempffi is not in the aquarium trade and, as a fish known only from a handful of museum specimens collected more than two decades before its formal description, is unlikely to appear there. Characidium as a genus are generally kept as active, undemanding bottom-dwelling companion fish that perch on the substrate and prefer current-swept water over a sandy or fine-gravel bottom, but none of that general husbandry experience has been tested against this specific, newly described and apparently very range-restricted species.
Conservation
Characidium noelkempffi has not been evaluated by the IUCN Red List, having been formally described only in 2026. Its describing authors, applying IUCN criteria directly, concluded the species qualifies as Data Deficient given how little is known of its distribution, population size and ecological requirements — it is known only from its type locality, a small tributary stream, and has not been recollected since the original 1998 survey despite the intervening 25-plus years.
The species' restriction to a single stream system does carry a measure of protection: Noel Kempff Mercado National Park is a UNESCO World Heritage Site with a strong record of conservation management, and the remoteness and limited accessibility of the Huanchaca plateau that isolated the species biologically has also kept it largely free of direct human pressure. The describing authors note, however, that a species restricted to one small headwater drainage remains inherently vulnerable to any future environmental change, and recommend further biological surveys of the more accessible reaches of the Pauserna drainage.