Taxonomy & naming
Sicyopus exallisquamulus was described by Watson in 1996. It belongs to the family Oxudercidae and to the subfamily Sicydiinae, the cling-gobies — a lineage of small, stream-dwelling true gobies centred on the Indo-Pacific. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name, and the genus Sicyopus has at various times been placed in or synonymised with allied sicydiine genera; the 1996 description established exallisquamulus within Sicyopus.
The species name unpacks as follows: ex- (Latin, without or lacking), alli- (possibly from Greek allos, other or different), and squamulus (Latin diminutive of squama, scale) — together conveying the sense of having scales that differ from or lack the typical form, interpreted by early reviewers as "without square scales." Among its closest relatives in the genus are other small sicydiines adapted to fast insular and coastal streams across the Indo-Pacific.
Gobies as a whole are overwhelmingly marine — the majority of the roughly 2,000 species in the order Gobiiformes live in the sea or in brackish estuaries. The freshwater sicydiines are a striking exception: a tribe of reef-bordering stream specialists whose adults have moved far inland while retaining a marine larval phase.
Morphology
Sicyopus exallisquamulus is a small goby, falling within the typical Sicyopus size range of roughly 1–3 in standard length; precise maximum size data are sparse for this species, and published material specifically measuring adults is limited. The body plan is slender and cylindrical in the fashion of a rheophilic stream goby, suited to clinging to and manoeuvring among smooth rocks in fast current.
The diagnostic character of all true gobies is the pelvic girdle: the two pelvic fins are fused along the midline into a concave, cup-shaped sucker disc that the fish uses to anchor itself against strong flow and even to climb damp rock faces. This disc distinguishes true gobies (Gobiidae, Gobionellidae, Oxudercidae) from the superficially similar sleepers and gudgeons (Eleotridae), which keep their pelvic fins separate. Squamation characters noted in the original description differ from the typical sicydiine pattern, which is the morphological basis of the species epithet.
Habitat
The genus Sicyopus is distributed across insular and coastal streams of the Indo-Pacific, and the biology of S. exallisquamulus mirrors the wider genus ecology. Adults occupy fast-flowing, clear, highly oxygenated upland freshwater streams, typically over exposed bedrock, cobble or boulder substrates where current is strong. The water is cool to warm depending on altitude and latitude, well-oxygenated, and generally low in nutrients — the antithesis of the soft, still, tannin-stained blackwater associated with many hobbyist fish.
The specific type locality and confirmed country distribution of S. exallisquamulus are not resolved in currently accessible literature, and the pre-crawl research returned no locality data beyond the genus-level Indo-Pacific range. The rheophilic habitat — swift, clear, rocky stream — is consistent across the Sicydiinae and almost certainly applies here. Temperature and pH specifics for the species are not documented in accessible sources; cool-to-warm tropical stream conditions (approximately 68–81 °F) and near-neutral to slightly alkaline pH are typical for the group.
Feeding
Within the Sicydiinae, feeding strategies divide broadly between the algae-grazing genera (Stiphodon, Sicyopterus) and the more carnivorous genera of which Sicyopus and Lentipes are the clearest examples. Sicyopus species are carnivorous micro-predators, taking small aquatic invertebrates, insect larvae, chironomid midges, aufwuchs fauna and similar small animal prey from the stream substrate and water column.
This carnivorous orientation is important for aquarists: unlike the algae-grazing sicydiines, a Sicyopus is not met by providing a well-lit tank and a mature biofilm. It requires live or frozen invertebrate foods — small bloodworm, daphnia, baby brine shrimp, and similar — and should not be expected to thrive on dry flake alone. In strong-flowing, clear-water aquaria, the same prey items and natural foraging behaviour would apply.
Mating
The reproductive strategy of S. exallisquamulus follows the amphidromous pattern established for the Sicydiinae. In fresh water, males are territorial and defend a spawning site — typically a smooth stone, rock face or overhang — where courtship and egg deposition take place. The male courts ripe females with characteristic lateral displays and fin erections.
Once a female is drawn to the spawning site, she deposits adhesive eggs directly on the rock surface, which the male then fertilises. From this point the male assumes sole parental responsibility for the clutch, guarding and fanning the eggs. Unlike many substrate-spawning freshwater fish, however, the purpose of the guarding phase is brief: it covers only the embryonic development to hatching, at which point the larvae are released and the marine part of the cycle begins.
Breeding
Breeding in Sicyopus exallisquamulus is amphidromous and cannot be replicated in a home aquarium. The male guards adhesive eggs on a rock or stone in fresh water until hatching, but the tiny, undeveloped larvae that emerge are immediately swept downstream by the current toward the estuary and sea. Once at sea, they spend weeks to months as marine plankton, developing through a series of larval stages in saltwater before metamorphosing into post-larvae.
These post-larvae — often called "glass gobies" at this stage — then recruit back to coastal fresh water and migrate upstream. Sicydiinae post-larvae are among the most remarkable climbers in the fish world: they use their fused pelvic sucker disc to scale wet rock faces and even climb active waterfalls, reaching upland streams far from the coast. The entire cycle requires both fresh water (for adult life and spawning) and the open sea (for larval development), making captive breeding biologically impossible. All S. exallisquamulus in the aquarium trade are wild-collected.
In the aquarium
Sicyopus exallisquamulus is a rare fish in the hobby, available only occasionally as wild-caught specimens imported from the Indo-Pacific. Providing appropriate care demands replicating the fast, clear, highly oxygenated stream conditions the species requires: a strong circulation pump or powerhead producing significant current, excellent filtration, regular large water changes, and a rocky substrate with smooth stones and hiding places. The tank must be covered — gobies are accomplished jumpers and a gap in the lid means a dead fish.
Because this is a carnivorous sicydiine, the diet should centre on live and frozen invertebrate foods: small bloodworm, daphnia, brine shrimp and similar. The species does not graze algae and cannot be maintained long-term on dry foods alone. Tank mates, if any, should be chosen carefully: Sicyopus males are territorial, and the tank must provide adequate line-of-sight breaks and multiple retreats. The combination of rarity, demanding water conditions, and wild-caught-only availability places this fish firmly in the specialist category; it is not suitable for general community aquaria or beginners.
Conservation
Sicyopus exallisquamulus has not been formally assessed on the IUCN Red List and carries a status of Not Evaluated. Many small-range Indo-Pacific stream gobies in the Sicydiinae remain unassessed or data-deficient; the constraints of their island or coastal-stream distributions and the limited taxonomic attention paid to the group mean that formal threat assessments are slow to follow description.
As an amphidromous species dependent on both intact freshwater streams and accessible marine nursery habitat, S. exallisquamulus faces the characteristic pressures of its ecology: watershed deforestation, damming, and stream diversion that block upstream migration routes are the primary threats to sicydiine gobies across the Indo-Pacific. Because the species is wild-collected and cannot be captive-bred, any aquarium trade demand translates directly to collection pressure on wild populations. Data on population sizes and trends are not available for this species, and caution is warranted given the narrow ecological requirements of the group.