Taxonomy & naming
Dicrossus gladicauda was described in 2008 by the German ichthyologists Ingo Schindler and Wolfgang Staeck, in the journal Vertebrate Zoology, from material Staeck collected in February 2006. It was the third valid species recognized in the genus, after Dicrossus maculatus Steindachner, 1875 (the genus's type species) and Dicrossus filamentosus (Ladiges, 1958). The holotype, an adult male of 1.5 in standard length, is held at the Museum für Tierkunde Dresden (the Senckenberg Natural History Collections Dresden) under register MTD F 31312, with a series of paratypes from the same locality. The fish was already familiar to European hobbyists before it had a name, traded under the provisional labels Dicrossus sp. "Obenschwert" and Dicrossus sp. "Top Sword" — both alluding to the male's sword-like upper tail lobe. Dicrossus belongs to the eartheater subfamily Geophaginae; Kullander's 1998 classification placed it in a tribe Crenicarini alongside Crenicara, Biotoecus and Mazarunia, but later cladistic work (López-Fernández et al. 2005) failed to recover that group as monophyletic. What is consistently supported is that Dicrossus and Crenicara are sister genera, united by the unusual serrated posterior margin of the preopercle — a rare character among Neotropical cichlids that the new species also shows. The genus name was at one time treated as a synonym of Crenicara before Kullander revalidated it in 1990. An early printing of the Catalog of Fishes carried the epithet misspelled as "glaudicauda," but the correct original spelling is gladicauda.
Morphology
This is a true dwarf cichlid: the type males measured 1.5 in standard length and females 1–1.5 in, and FishBase records a maximum of about 1.5 in SL. The body is elongate and only moderately compressed, with a long, narrowing head (head length about 29–31% of standard length) and a narrow, subterminal mouth in which the upper jaw is slightly prognathous, more so in males. The single diagnostic feature — the one that names the fish — is the caudal fin of adult males. Its upper (dorsal) lobe is drawn out into a long filamentous streamer formed by the elongation of caudal rays D3 and D4, while the lower lobe stays rounded and the middle is truncate or slightly concave, producing a conspicuously asymmetrical, sword-tailed tail. Its closest relative, Dicrossus filamentosus, instead has a symmetrical lyrate tail with a streamer on both lobes; in Dicrossus gladicauda only the top trails. Females and juveniles have a plain rounded-to-truncate tail. The body wears the genus's checkerboard livery: two longitudinal rows of squarish black blotches — six or seven flank spots along the midline and a matching row of dorsal spots along the back — set against a yellowish ground that is tan to greyish above and white below. There is no dark lateral stripe and no suborbital stripe, only a short preorbital bar through the eye. Males in life are vividly finned — the dorsal, anal and caudal fins carry red spots and streaks, a narrow red margin and a bright blue submarginal band, and the pelvic fins are pointed and filamentous — while females are comparatively plain, their fins hyaline and largely free of the red and blue. The sexual dimorphism is marked and unmistakable in adults.
Habitat
Dicrossus gladicauda is known from the drainage of the lower Río Atabapo, a blackwater tributary that joins the Orinoco at San Fernando de Atabapo on the Colombia–Venezuela frontier; the type locality is the Caño Jigua, a small western tributary of the lower Atabapo south of that town (roughly 3°47′N, 67°38′W). The IUCN records the species from both Colombia and Venezuela within this upper-Orinoco system, and aquarium exports have been attributed more loosely to the wider Orinoco drainage. The water is a textbook Amazonian–Orinocan blackwater: clear but tea-stained by tannins, strongly acidic and exceptionally soft. The describers measured the type locality at pH 4.4, an electrical conductivity of just 10 µS/cm, total and temporary hardness below 1 °dH, and a water temperature of 76 °F — among the softest, most ion-poor water any cichlid inhabits. The fish were taken along the banks of the rivulet in extremely shallow water, between roughly 10 and 19.5 in deep, either within a layer of dead leaves blanketing the bank-side bottom or among submerged terrestrial vegetation flooded at the margin. The associated fish fauna at the site reads like a blackwater roll-call — the cardinal tetra Paracheirodon axelrodi, Hemigrammus bleheri, Nannostomus eques, Copella meinkeni and the dwarf cichlid Apistogramma uaupesi shared the shallow leaf-litter zone, while Mesonauta insignis and Biotodoma wavrini occupied the deeper water just offshore.
Feeding
No formal gut-content study has been published for Dicrossus gladicauda, and its diet must be inferred from its microhabitat, its anatomy and the habits of its close relatives. It is a small benthic micro-predator of the leaf-litter zone: its dentition consists of fine, pointed, mostly unicuspid teeth on the jaws and a modest triangular lower pharyngeal plate, the equipment of a fish that picks tiny invertebrates rather than one that crushes hard prey or rasps algae. FishBase places it at a trophic level near 3.2, consistent with a micro-carnivore. In the wild it would forage over and within the submerged leaf litter and flooded vegetation where it was collected, taking aquatic insect larvae, microcrustaceans and other small invertebrates from the substrate and the water immediately above it. Captive observations of the genus bear this out — checkerboard cichlids feed readily on small live and frozen foods and tend to ignore anything they cannot pick delicately from the bottom or mid-water. Within its community it is a minor mesopredator of the shallow bank zone, itself small enough to be prey for the larger characins and cichlids sharing the creek.
Mating
The describers observed reproduction in aquaria and reported a social system of polygyny built on a male's defended ground. A male holds a comparatively large territory that contains several potential spawning sites, and each of those sites can become the focus of a smaller sub-territory held by a separate female — so a single dominant male may court and spawn with more than one female arrayed across his patch. This harem-like arrangement is typical of the open-substrate checkerboard cichlids and of many dwarf geophagines generally. Courtship and pair contact centre on the chosen spawning surface, but the bond is brief and lopsided: once eggs are laid the female turns on the male and drives him energetically away from the immediate vicinity of the clutch, ending his role at the nest. The pre-spawning phase is thus a male holding and advertising territory while ripe females settle into and defend discrete spawning stations within it.
Breeding
Dicrossus gladicauda is an open substrate spawner with exclusively maternal brood care, the pattern Schindler and Staeck documented under aquarium conditions. Like other open brooders the female deposits her eggs on a horizontal surface; the preferred sites are the upper face of a stout plant leaf or a piece of driftwood rather than a cave. After spawning the female assumes sole care of the clutch and drives the male away, although he may still contribute indirectly by patrolling and defending the wider territory against predators. At 81 °F the eggs hatch about three days after spawning, and the larvae become free-swimming roughly six days after that. No wild clutch has been counted, and the describers did not publish an egg number; for the genus, congeners such as Dicrossus filamentosus typically lay only small clutches of a few dozen to around a hundred eggs, in keeping with the diminutive size of these fish. The combination of single-female care, small broods, and a requirement for warm, extremely soft, acidic blackwater makes this a demanding fish to spawn and rear, and it has remained an uncommon subject even among specialist dwarf-cichlid keepers.
In the aquarium
Dicrossus gladicauda is a specialist's fish, not a beginner's, and it is scarce in the trade — it appears only sporadically, often under its old hobby names "Obenschwert" or "Top Sword." The non-negotiable requirement is water chemistry. This species comes from water of pH 4.4 and about 10 µS/cm conductivity, and like its better-known relative Dicrossus filamentosus it does not thrive — and will rarely if ever breed — in hard, alkaline, or mineral-rich water. Serious keepers run it in extremely soft, acidic blackwater: reverse-osmosis or rainwater remineralized only lightly, pH held in the rough range of 4.5–6.0, conductivity kept low, and the water darkened and acidified with botanicals such as catappa (Indian almond) leaves, alder cones and peat filtration. A bed of leaf litter over fine sand is not merely decorative — it recreates the exact microhabitat the fish was collected from and gives it cover and foraging ground. Temperature should sit in the warm tropical band of roughly 79–84 °F; the 76 °F measured at the type locality reflects a single field reading rather than an upper limit, and the species is kept warmer in captivity.
A modest tank suits it well given its size — a planted, dimly lit aquarium with plenty of structure (driftwood, leaf litter, fine-leaved plants, broad leaves or flat wood as spawning surfaces) for a pair or a small harem of one male to several females. As with other Dicrossus, keeping more than one mature male in a small space invites relentless aggression, so a single male is the safe configuration unless the tank is large and well broken up. Tankmates should be small and peaceful blackwater companions of the kind it lives with in the wild: pencilfish, small tetras such as cardinals or rummynose, Nannostomus, hatchetfish and the like; boisterous or hard-water fish are unsuitable. Feed small live and frozen foods — Artemia nauplii, microworm, daphnia, cyclops, finely chopped bloodworm — which it takes far more willingly than dry food. The usual pitfalls are predictable: water that is too hard or too alkaline, too cold, or insufficiently clean. These are sensitive soft-water fish, and stable, well-maintained, ion-poor acidic conditions are the whole game with this species.
Conservation
Dicrossus gladicauda is one of comparatively few cichlids to carry a formal threatened listing. The IUCN Red List assesses it as Endangered (EN), under criterion B1ab(iii), assessed on 23 October 2020 by L. Mesa-Salazar and published in 2022, with the population trend recorded as unknown. The Endangered status follows from the species' tiny known range — it is documented only from the lower Río Atabapo drainage shared by Colombia and Venezuela — combined with a continuing decline in the quality of its habitat. The Red List flags mining and quarrying and industrial or military effluents as the principal threats; gold mining and its mercury and sediment pollution are pervasive pressures across the upper-Orinoco and Guiana-Shield blackwaters this fish depends on, and any degradation of water quality is acute for a species adapted to such extraordinarily soft, acidic, clean conditions. The fish carries no CITES listing. It is collected in small numbers for the specialist aquarium trade; given the narrow range and the listing, captive-bred stock and careful sourcing matter more here than for most dwarf cichlids. FishBase reflects the current IUCN listing, while the Cichlid Room Companion's older species page still records it as not evaluated — the live Red List entry is the authoritative status.