Taxonomy & naming
Guyanancistrus megastictus was formally described by Fisch-Muller, Mol & Covain in 2018 in PLoS ONE (volume 13, number 1, article e0189789), as part of an integrative revision of the genus Guyanancistrus with particular emphasis on the Guyanancistrus brevispinis species complex. The holotype (MNHN 2002–3508, 2.5 in SL) and a single paratype were collected on 25 October 2002 by P. Keith and P. Gaucher at Crique Alama, a tributary of Crique Saranou in the Maroni River Basin, Massif du Mitaraka, French Guiana (approximately 2°18'08"N, 54°32'00"W). The species was caught by poison fishing in a small forest stream.
The Catalog of Fishes (Eschmeyer, CAS) recognises Guyanancistrus megastictus as a valid species under this authorship.
The genus Guyanancistrus is placed in the subfamily Hypostominae, tribe Ancistrini, within the family Loricariidae. Phylogeographic analysis by Fisch-Muller and colleagues suggests that Guyanancistrus is of Amazonian origin, with ancestors likely dispersing from the Jari River area westward toward the Maroni Basin, consistent with known biogeographic patterns across the Guiana Shield.
The specific epithet megastictus is formed from the Ancient Greek mega (large) and stictos (spotted), referring directly to the species' most conspicuous morphological character: very large, roundish spots on the body and fins, each covering more than a single lateral dermal plate.
Morphology
Guyanancistrus megastictus has a maximum reported standard length of 2.5 in SL — a small species even by loricariid standards. Fin formula: two dorsal spines and seven soft dorsal rays; six anal soft rays. Body shape is described as elongated.
The defining diagnostic character is the presence of very large, roundish pale spots on the body and fins, each spot covering more than one lateral dermal plate. This distinguishes the species from all three known congeners: Guyanancistrus brevispinis, G. nassauensis, and G. tenuis all lack spots of this size and pattern. G. tenuis has spots that do not span four plates, and in that species the spots tend toward straighter posterior bands; it is also separated by a lower count of plates bordering the supraoccipital (2–3 in G. tenuis vs. 4 in G. megastictus).
Beyond the spot pattern, detailed morphometrics are not extensively reported in the available sources, reflecting the very small type series (two specimens). The body armour and general body plan follow the loricariid norm: ventrally flattened, fully plated, with a ventrally positioned oral disc adapted for adhering to hard substrates.
Habitat
Both known specimens of Guyanancistrus megastictus were collected from Crique Alama, a small, shallow forest stream in the Massif du Mitaraka, the most remote highland massif in French Guiana, in the upper Maroni River basin. The collection site was described as a shallow (8–23.5 in water depth), mainly sandy reach of a small forest stream, caught by poison fishing.
The Massif du Mitaraka lies in extreme south-western French Guiana near the Suriname and Brazil borders and was at the time of collection among the least biologically surveyed areas of French Guiana. The Maroni River drains northward to the Atlantic coast, forming much of the French Guiana–Suriname border.
No water chemistry data (temperature, pH, conductivity) were reported for the type-locality collection. As a forested Guiana Shield stream draining granitic substrate at moderate elevation, the habitat would be expected to be soft, low-conductivity, and mildly acidic to neutral — consistent with the general character of Guiana Shield blackwater and clearwater streams.
Feeding
No dietary observations for Guyanancistrus megastictus exist. Based on the biology of the genus and its position in tribe Ancistrini, it is expected to graze epilithic algae, diatoms, and biofilm from rock and hard substrate surfaces in the stream. The sandy bottom substrate noted at the collection site suggests some capacity for feeding on organic matter associated with sandy or mixed-substrate environments as well.
In the aquarium, close relatives within Ancistrini are typically maintained on algae wafers, spirulina-based sinking pellets, and fresh vegetable matter. There are no aquarium husbandry records for this species.
Mating
No mating observations for Guyanancistrus megastictus are available. The species has never been reported in the ornamental fish trade, and no captive specimens are known to exist.
Given the extremely limited known material — two specimens from a single collection event — nothing can be said with confidence about sexual dimorphism, breeding condition, or courtship behaviour in this species specifically. Ancistrini in general exhibit male territorial behaviour over crevice or cave sites, with males developing odontodes as secondary sexual characters.
Breeding
No breeding records for Guyanancistrus megastictus exist, in either the wild or in captivity. The species is not commercially collected and is not present in hobbyist databases.
With only two specimens ever documented, the sex of the known material has not been reported, and clutch size, spawning behaviour, and parental care specifics remain entirely unknown for this taxon.
In the aquarium
Guyanancistrus megastictus has never been reported in the ornamental fish trade and is not available to hobbyists. It was not commercially collected at the time of its formal description in 2018, and the authors note that no aquarium trade data exist for the species.
The collection locality in the Massif du Mitaraka is among the most remote and least accessible areas of French Guiana. Unless the species is found to occur more broadly across the Maroni drainage or adjacent Guiana Shield river systems, it is unlikely to enter the trade in the foreseeable future.
Any hobbyist or researcher who encounters specimens should prioritise documenting locality data and water chemistry, both of which are currently absent from the record.
Conservation
The IUCN Red List assessed Guyanancistrus megastictus as Data Deficient (DD) on 21 October 2020. The assessment notes the species' very restricted known range — confined to the type locality, Crique Alama in the Massif du Mitaraka — and the minimal sampling on which knowledge of the species rests: only two specimens have ever been collected.
The IUCN assessors concluded that insufficient data are available to determine extinction risk. The remote location of the type locality, within a poorly sampled highland massif, means that the true extent of the species' range is unknown. The habitat — a small, shallow forest stream in an area with minimal anthropogenic disturbance at the time of collection — does not face immediately obvious threats, but the very small number of known specimens makes any population-level inference impossible.
Further collecting expeditions to the Massif du Mitaraka and adjacent Maroni drainage tributaries are needed before a substantive threat assessment can be made.