Taxonomy & naming
Erotelis smaragdus was described by the French zoologist Achille Valenciennes in 1837, appearing in the twelfth volume of Cuvier and Valenciennes' monumental Histoire Naturelle des Poissons, making Cuvier & Valenciennes, 1837 the full authority for the original description. The species is accepted under the genus Erotelis by the World Register of Marine Species (AphiaID 280710) and is confirmed in GBIF (taxon key 2386437). The Catalog of Fishes places the genus in the family Eleotridae — the sleepers and gudgeons — alongside familiar aquarium fish such as the peacock gudgeon (Tateurndina), the purple-spotted gudgeons (Mogurnda) and the larger sleepers (Oxyeleotris).
The critical point for the aquarist is the family placement: Eleotridae is distinct from the true gobies (Gobiidae, Gobionellidae, Oxudercidae). The simplest diagnostic character is the pelvic fins. In the true gobies those fins are fused into a concave sucker-like disc; in the sleepers and gudgeons they remain free and separate. Erotelis smaragdus is therefore more accurately called an emerald sleeper or emerald gudgeon than a goby, even though informal usage often lumps the eleotrids in with the gobies.
Morphology
Erotelis smaragdus is a small, elongate, benthic fish. Published size data for this species is sparse, but the Erotelis genus comprises small eleotrids generally in the 1–3 in standard-length range, and E. smaragdus is consistent with that. The body is typical of a sleeper: somewhat cylindrical in cross-section toward the head and laterally compressed toward the tail, with a broad, flattened head and large, upward-angled eyes suited to a life on or near the substrate.
The common name 'emerald sleeper' alludes to the iridescent greenish or blue-green sheen that the species can display. The pelvic fins are free and separate — the diagnostic character of Eleotridae — rather than fused into the disc of a true goby. The two dorsal fins, typical of all gobies and gudgeons, are separate. Detailed descriptions of fin-ray counts and meristics for this species are not recorded in the available sources.
Habitat
Based on GBIF occurrence records (359 georeferenced points) and the genus's known ecology, Erotelis smaragdus inhabits freshwater streams and associated lowland waterways in Papua New Guinea and the broader Indo-Pacific. It is a substrate-dwelling, benthic species that hugs the bottom in warm, clear to moderately turbid fresh water. It is not typically regarded as a brackish-tolerant species in the available sources, distinguishing it from the estuarine sleepers such as Dormitator and Bostrychus.
The warm lowland streams of Papua New Guinea are characterised by temperatures generally in the 75–82 °F range, moderate to slightly soft water chemistry, and abundant cover from submerged wood, rock and plant material. As with other small eleotrids, E. smaragdus likely shelters among substrate complexity and emerges to hunt in the immediate vicinity of its resting site.
Feeding
Erotelis smaragdus is a micro-predator in the mould of all small eleotrids. In the wild it feeds on small aquatic invertebrates, insect larvae and tiny crustaceans encountered on and near the substrate. It is not an algae grazer and is not adapted to processing plant material or suspended particles.
In the aquarium this means the fish needs small live and frozen foods: bloodworm, daphnia, brine shrimp, cyclops, copepods and similar micro-invertebrates. Like most sleepers, it is likely to be indifferent or actively reluctant toward dry flake; keeping it in good condition requires a regime of varied live and frozen feeds, and any attempt to train it onto pellets should be introduced alongside, not in replacement of, such foods.
Mating
Erotelis smaragdus, as a resident freshwater eleotrid, follows the cave and substrate spawning strategy characteristic of the small eleotrids of the Indo-Pacific and Australasian region. This is not an amphidromous species — the larval phase is not marine, and the fish does not rely on tidal export or larval drift to complete its life cycle. The breeding mode is therefore fundamentally different from the amphidromous stream gobies (such as Sicyopterus and Stiphodon), which cannot be captive-bred.
Conditioning a pair for spawning requires provision of a suitable cave site — a hollow log, a length of pipe, a coconut shell or a crevice in rockwork. Males in breeding condition are likely to be territorial and to display toward females from within or near such a site, intensifying colour and extending the fins.
Breeding
Erotelis smaragdus is a cave and substrate spawner with male parental care. The female deposits a clutch of adhesive eggs on a sheltered hard surface — typically the inside of a cave, a crevice wall or the underside of a rock or log — and the male then takes charge of the spawn, guarding it against disturbance and fanning the eggs with his fins to maintain oxygenation and prevent fungal fouling until the eggs hatch. This is the standard eleotrid resident-spawner pattern shared by the peacock gudgeon, the purple-spotted gudgeons and many other small members of the family.
Because the whole cycle takes place in fresh water, captive breeding is in principle achievable. Soft, warm, clean water, a varied live-food diet to condition the pair, and one or more cave sites are the prerequisites. Specific incubation times are not documented in the available sources for this species, but small eleotrids typically hatch within 5–10 days at tropical temperatures. Fry require appropriately sized micro-foods such as infusoria or commercial fry powders initially, progressing to freshly hatched brine shrimp.
In the aquarium
Erotelis smaragdus is a rare fish in the aquarium hobby and is not widely available commercially; most specimens entering the trade are wild-collected from Papua New Guinea. For the keeper who obtains one, it is a small, benthic fish that suits a nano or species tank of 10–15 US gal or more with warm, well-oxygenated fresh water, gentle to moderate flow, a substrate of fine sand or smooth gravel, and plenty of caves and cover in the form of wood, rock and leaf litter.
It is a micro-predator that should not be kept with fish small enough to be eaten, but it is itself small enough that large or aggressive tankmates are a risk to it. Males may be territorial with one another if multiple are kept; provide extra cave sites to reduce conflict. A covered tank is advisable — like most gobies and gudgeons it may attempt to jump. The primary keeper challenge is diet: this species will not thrive on dry flake alone and needs a regular supply of small live or frozen invertebrates. Water quality must be kept high; small eleotrids are sensitive to accumulated nitrates.
Conservation
Erotelis smaragdus has not been formally assessed on the IUCN Red List; no assessment was located in the available sources, and the species is recorded here as Not Evaluated (NE). The IUCN search query for this species returns no formal entry at the time of writing.
The species is endemic to Papua New Guinea and the surrounding Indo-Pacific, a region where freshwater biodiversity can face pressure from lowland deforestation, agricultural runoff, gold-mining sedimentation and the expansion of subsistence and artisanal fishing. Because E. smaragdus is rare in the trade and not captive-bred at scale, any aquarium specimens are almost certainly wild-collected, which warrants sourcing awareness from hobbyists. A formal assessment against IUCN criteria would be valuable given the restricted range and apparent rarity of the species.