Taxonomy & naming
Redigobius chrysosoma was first described by the prolific Dutch ichthyologist Pieter Bleeker in 1875 under the combination Lophogobius chrysosoma — a placement in one of the many staging-post genera Bleeker used while working through the enormous Indo-Pacific goby fauna. The species was later transferred to the genus Redigobius, which the Catalog of Fishes (Eschmeyer) currently recognises as the valid placement. The World Register of Marine Species (WoRMS) confirms the valid name and authorship; the type locality is within Bleeker's Indonesian material, where he collected extensively throughout the mid-nineteenth century.
Redigobius belongs to the family Oxudercidae — the 'true gobies' in the modern sense after the reshuffling of the Gobioidei. This is a critical point for understanding the fish: the true gobies are overwhelmingly marine, and the fused pelvic sucker disc is the defining synapomorphy of the group. The gudgeons and sleepers (Eleotridae) lack this fusion; they are a sister lineage, not close relatives in the usual hobbyist sense. R. chrysosoma's position within Oxudercidae sits it firmly in the 'real goby' camp. The species has also appeared in older literature under the generic synonyms Gobius and Sicydium, references that may appear in museum collection records.
Morphology
Redigobius chrysosoma is a compact, laterally compressed benthic goby reaching approximately 1.5–2 in standard length at maturity. The body is pale grey to translucent, with each scale margined in dusky brown giving the fish a fine reticulate pattern. A dark bar crosses the opercle (gill cover), and the name chrysosoma — Greek for 'golden body' — likely refers to a golden or yellowish wash on the flanks, particularly evident in breeding condition.
The most diagnostic feature is the first dorsal fin: it carries a red or orange wash overlaid by a prominent black spot near the posterior margin, the 'spotfin' of its common name. The dorsal fin count is seven spines and six soft rays; the anal fin has one spine and six soft rays — counts characteristic of the genus. The scales are ctenoid (rough-edged), typical for gobies. As in all members of Oxudercidae, the pelvic fins are fully fused into a cup-shaped sucker disc on the ventral surface — the character by which a true goby is recognised — and this disc is used to cling to hard substrate in tidally influenced water.
Habitat
Redigobius chrysosoma is a fish of the land–sea margin: it inhabits brackish mangrove estuaries, tidal river reaches, and the lower sections of fresh streams that receive tidal influence. Fishes of Australia records it from shallow, slow to moderately flowing water at depths to 16 ft, typically over a substrate of mud, sand or fine gravel with structure — submerged roots, rocks, shells and organic debris that provide cover and spawning sites.
The distribution spans the tropical western Pacific: Indonesia, the Philippines, New Guinea (including the Australian territory of Papua New Guinea), and in Australia from the Blackmore River in the Northern Territory east to Gladstone in Queensland. This is a warm coastal lowland species; the waters it inhabits are tropical, generally 75–86 °F, and slightly to moderately saline in estuarine zones though it penetrates further upriver into fully fresh water. It is not a cold-water or temperate species.
Feeding
Redigobius chrysosoma is a micro-predator, feeding in the wild on small benthic invertebrates — tiny crustaceans, copepods, chironomid larvae, worms and other organic material it finds in the substrate and among debris. Being a goby, it hunts by sight and by probing the bottom rather than by filtering or grazing; it is not an algae-eater or detritivore.
In the aquarium it should be offered a varied diet of small live and frozen foods: bloodworm, brine shrimp, daphnia, cyclops and other copepod-sized items. At under 2 in its mouth is genuinely small, so food size matters — large bloodworm may be taken, but very fine foods are more reliable. Most specimens adapt poorly to dry flake, taking it only reluctantly or not at all. A diet relying on prepared foods will lead to lost condition; live and frozen foods are effectively required for long-term health and breeding success.
Mating
Redigobius chrysosoma is a resident cave/substrate spawner — the breeding strategy typical of the non-amphidromous gobies. Because its entire reproductive cycle plays out in fresh or brackish water with no larval marine phase, it belongs to the breedable minority of gobies in the hobby, unlike the amphidromous cling-gobies (Stiphodon, Sicyopterus) whose larvae must develop at sea.
Males are territorial, each establishing and defending a small cave, crevice, shell or other enclosed site from which they court ripe females. Courtship involves fin displays and colour intensification. Females are attracted into the cave by the displaying male; sexual dimorphism is modest in this genus, but females in breeding condition typically show a rounded abdomen. A conditioned pair in a tank furnished with appropriate spawning sites will often initiate spawning without further intervention.
Breeding
Spawning follows the pattern reported across Redigobius: the female enters the chosen cave and deposits a clutch of adhesive eggs on the cave wall or ceiling, then takes no further part. The male assumes sole guard duty — defending the clutch against intruders, fanning the eggs with his fins to maintain oxygenation, and removing unfertilised eggs. Larvae hatch in approximately three to four days at tropical temperatures.
Aquarium breeding has been documented in the genus, though it is not considered easy. Success depends on providing appropriate caves or crevices (small clay pots, coconut halves, PVC pipe sections), medium-hard to slightly alkaline water — matching the species' estuarine origin rather than soft acidic tropical conditions — and moderate water movement. Fry are tiny at hatching and require micro-foods such as infusoria or rotifers before graduating to freshly hatched brine shrimp nauplii. The male's guarding behaviour means a separate rearing tank is not essential, though moving the egg-bearing surface may improve fry survival in a community setting.
In the aquarium
Redigobius chrysosoma suits a species or near-species nano tank of 10–15 US gal or more — small, but it needs structure. Furnish the tank with fine-grained substrate (sand is ideal), multiple small caves and crevices (clay pots, coconut halves, stacked rockwork), and moderate current produced by a small powerhead or internal filter. The tank must be covered; gobies are accomplished jumpers and will exploit any gap.
Water chemistry should mirror the species' estuarine origin: medium-hard to hard (GH 10–20), neutral to slightly alkaline (pH 7.0–8.0), and a small amount of aquarium salt (1–3 g per litre of marine salt mix or non-iodised sea salt) improves long-term health and is reportedly beneficial for breeding, though the fish will survive in fresh water. Temperature 75–82 °F is suitable. Males are territorial with one another; a single male with one or two females in a well-furnished tank avoids constant aggression. The fish's micro-predator diet and modest size make it incompatible with very small invertebrates but suitable with peaceful, similarly sized tankmates that will not outcompete it for food.
Conservation
The IUCN Red List assesses Redigobius chrysosoma as Least Concern (2022 assessment), reflecting its broad distribution across tropical Asia and Australia and its tolerance of disturbed, estuarine, and near-coastal habitat. It is common through much of its range and is not considered threatened at the species level.
At a local scale the usual pressures apply: mangrove clearance, estuarine pollution, altered hydrology from coastal development, and land reclamation in parts of Southeast Asia and the Philippines all degrade the low-salinity brackish fringe that this species depends on. Australia's populations in the Northern Territory and Queensland are within relatively intact coastal systems. Because it is a resident spawner without a marine larval dispersal phase, local populations may be more isolated — and therefore more vulnerable to local extirpation — than their wide distribution suggests. There is no evidence of targeted collection pressure; the species appears sporadically in the aquarium trade but is not a major import.