Taxonomy & naming
Gobiomorphus cotidianus was formally described by the New Zealand ichthyologist Robert McDowall in 1975, and the Catalog of Fishes (Eschmeyer, CAS) — the authority for valid names — places it in the genus Gobiomorphus within the family Eleotridae. The Eleotridae are the sleepers and gudgeons, and the first point to understand about this fish is that it is not a true goby. True gobies (Gobiidae, Gobionellidae, Oxudercidae) carry their pelvic fins fused into a cupped sucker disc; eleotrids keep their pelvic fins separate, and that character distinguishes the two lineages immediately. The popular name "bully" (applied to all New Zealand Gobiomorphus) reflects local usage rather than any taxonomic affinity with the northern hemisphere bullheads.
The species name cotidianus is Latin for 'everyday' or 'common', a reference to its abundance throughout New Zealand. Earlier workers placed several of the New Zealand bullies under genera such as Eleotris, Vaimosa, and later Gobiomorphus; McDowall's 1975 revision stabilised the current arrangement. A remarkable feature of the species is that it is facultatively amphidromous — some landlocked lake populations have lost the migratory phase entirely and complete their life cycle in fresh water year-round, a distinction that is genetically confirmed but does not alter the species' validity or its genus placement.
Morphology
The common bully is a moderately elongate, benthic fish reaching up to 6 in total length, though most individuals in riverine habitat are smaller. The body is robust anteriorly, tapering toward the tail, with a broad, slightly flattened head, a large mouth, and two separate dorsal fins typical of eleotrids. Ground colour is pale brownish to olive-grey, overlaid with irregular darker blotches and mottling that provide effective camouflage against gravel and stone — the fish is cryptic and often overlooked until it moves.
The pelvic fins are separate and not fused, confirming the gudgeon lineage. Sexual dimorphism is present but subtle outside the breeding season: males in breeding condition develop a distinctly darkened head and throat, while females may appear slightly deeper-bodied when gravid. Both sexes lack the vivid colouration of some Indo-Pacific eleotrids; the common bully is a plain, well-adapted freshwater ambush predator rather than an ornamental fish.
Habitat
Gobiomorphus cotidianus is endemic to New Zealand and is the most widespread of the country's native freshwater fish. It occupies an extraordinarily broad range of freshwater habitats: cold, clear oligotrophic lakes (both littoral zones and, in large lakes, the profundal down to 230 ft depth), rivers, streams, and coastal areas close to river mouths. The species is found on both the North and South Islands and tolerates a wide temperature range — from near-freezing to approximately 72 °F — reflecting New Zealand's temperate to cool-temperate climate.
In oligotrophic lakes such as those of the South Island's high country, the common bully has been documented in two ecological stocks: a littoral stock occupying 2–82 ft depth, and a profundal stock living at 98–230 ft, a remarkable depth range for a small benthic fish. Stream populations are typically found among riffles and run habitat on gravel or cobble substrates. The species can also be present in estuarine and coastal areas, particularly in relation to the larval dispersal phase of its amphidromous lifecycle.
Feeding
The common bully is a micro-predator of benthic invertebrates. In streams and lakes it consumes amphipods, insect larvae (chironomids, mayflies, caddisflies), small crustaceans, and other invertebrates encountered on or near the substrate. Diet studies confirm seasonal and size-related variation: juveniles and smaller fish focus on amphipods and midge larvae, while larger individuals take proportionally more substantial prey.
In any aquarium setting the common bully should be offered small live and frozen foods — bloodworm, brine shrimp, daphnia, amphipods, and similar invertebrate prey. Like most eleotrids it is an ambush feeder rather than an active cruiser, and it is likely to ignore or reject dry flake food. Maintaining it on prepared diets alone typically leads to poor condition; meaty, appropriately sized live or frozen items are essential.
Mating
Gobiomorphus cotidianus is amphidromous, and its mating system reflects the demands of that lifecycle. Migratory populations spawn in winter (typically June–August in New Zealand), while landlocked non-migratory populations of the same species spawn approximately six months later, in summer — a striking example of population-level divergence in reproductive timing within a single species, documented in published ecotype studies.
Sexual maturity is reached at approximately 2.5 in total length. Males become conspicuously dark on the head and throat during the breeding season and establish territories around suitable egg-deposition sites — cavities beneath stones, overhangs, or gravel interstices along stream margins. Courtship is not well documented in detail but follows the general eleotrid pattern of male territory-holding and female attraction to the nest site.
Breeding
The common bully spawns adhesive eggs in a sheltered site — beneath a stone, in a crevice, or in similar hard substrate cover — and the male guards the clutch, fanning and tending the eggs until they hatch. This male parental care is the same broad strategy seen across many eleotrids, but the crucial difference from resident freshwater cave-spawners like Tateurndina is what happens next: the newly hatched larvae of migratory populations are tiny and pelagic, and they are swept downstream to the sea. There they spend weeks to months developing as marine plankton before the post-larval juveniles — typically 0.5–2 in — return to freshwater and migrate upriver to recruit into the adult population.
This amphidromous lifecycle means that captive breeding is effectively impossible in the home aquarium: there is no practical way to replicate the marine larval phase. Landlocked lake populations are genetically distinct non-migratory ecotypes that complete their full life cycle in fresh water, but these are rarely the populations that appear in trade, and even then captive husbandry of a cold-temperate New Zealand gudgeon is specialised work. The species is essentially wild-collected when it does appear in aquaria.
In the aquarium
The common bully is an uncommon aquarium subject — it is not commercially bred and any specimens in the trade are wild-collected, almost entirely from New Zealand. Its requirements diverge markedly from tropical community fish: it is a cool-water species that prefers temperatures between roughly 50–68 °F, and keeping it at the 75–79 °F typical of a tropical community tank would stress or kill it. A cool, well-oxygenated, clean-water aquarium with a gravel or cobble substrate and caves or overhangs to shelter under is the correct environment.
The common bully is a cryptic ambush predator and will eat any fish or invertebrate small enough to swallow, so tank-mates need to be large enough to be safe. It should be fed live or frozen invertebrate prey — bloodworm, brine shrimp, amphipods, insect larvae — as it is unlikely to accept dry food reliably. The covered tank is important, as eleotrids generally are capable jumpers. New Zealand native fish keepers maintain it successfully alongside other cool-water native species (smelt, inanga), but it is not a fish for the typical tropical community setup and its amphidromous biology means breeding is not a realistic prospect.
Conservation
The IUCN Red List assesses Gobiomorphus cotidianus as Least Concern, reflecting its status as New Zealand's most abundant and widespread native freshwater fish. It is found across a large range of habitats on both main islands and faces no species-wide threat. However, like all of New Zealand's native fish, it is subject to the broader pressures that have diminished the country's freshwater biodiversity: introduced salmonids (trout) prey on juvenile bullies in many river systems, and land-use change, sedimentation, and water quality degradation affect stream habitat quality.
The species' amphidromous lifecycle creates a vulnerability specific to migratory fish: barriers such as culverts, weirs, and dams can prevent the post-larval recruits from ascending rivers, severing gene flow between coastal and headwater populations. Restoration of fish passage on New Zealand waterways is an active conservation priority for this reason. Despite these pressures, the common bully's wide tolerance of habitat conditions and its abundance in less-modified waterways mean it is not considered at risk nationally.