Taxonomy & naming
Fundulus nottii was described by the Swiss-American naturalist Louis Agassiz in 1854. Because Agassiz placed it in a genus other than Fundulus, the authority is given in parentheses following convention. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Fundulus nottii (Agassiz, 1854). The species belongs to the subgenus Zygonectes within the large and diverse genus Fundulus, a grouping that encompasses many of the starhead and flagfish-type topminnows of eastern North America.
The species has accrued several vernacular names in the literature and in regional fishery surveys: bayou topminnow is the most widely used common name in the hobby and conservation literature; southern starhead topminnow and Gulf starhead topminnow are also applied and reflect both the species' geographic focus and the distinctive iridescent spot on the top of the head that is a key recognition character across the Zygonectes group. The Fishes of Texas Project and NatureServe both record a G5 global conservation rank, indicating a globally secure and widespread species.
This fish was long listed as Fundulus notti; Eschmeyer's Catalog of Fishes now treats that name as a synonym of Fundulus nottii, the valid name followed here.
Morphology
Fundulus nottii is a slender, somewhat cylindrical topminnow with a flat-topped head and a slightly upturned mouth characteristic of a surface-feeding fish. Maximum total length is recorded as 2.4–3.1 in TL (FishBase; Fishes of Texas Project), though most individuals encountered in the field and in captivity are toward the smaller end of that range. The body is moderately compressed laterally, the back relatively straight and the belly gently curved.
Colouration in males is strongly patterned: the flanks carry a series of dark vertical bars or spot rows against an olive to straw-yellow ground, and males in breeding condition show intensified contrast with vivid spotting. Females are plainer, tending toward a more uniform olivaceous or brownish tone with reduced or absent barring, and a rounder body profile when gravid. The defining feature shared across the Zygonectes subgenus is the iridescent, metallic spot on the dorsal surface of the head — the 'starhead' — which is well-developed in both sexes but more pronounced in males.
Sexual dimorphism is thus moderate and readily apparent at close inspection: males are more vividly patterned and slightly slimmer, females are drabber and rounder in the belly when carrying eggs.
Habitat
The native range of Fundulus nottii spans the Gulf coastal plain from southeastern Louisiana east to southwestern Alabama, occupying sluggish lowland drainages that discharge to the northern Gulf of Mexico. It is a freshwater species with no significant marine or brackish tolerances, associated strongly with warm, still to slow-moving, heavily vegetated water bodies: bayous, cypress swamps, oxbow sloughs, vegetated stream margins, and the shallow weedy backwaters of larger rivers.
Water conditions in these Gulf coastal habitats tend toward the warmer end of the temperate spectrum. Optimal captive temperatures reported in husbandry accounts run 70–81 °F (70–80 °F), consistent with a subtropical lowland distribution at 30–32 °N latitude. Natural waters in this region are often lightly buffered and moderately hard; a NANFA husbandry account records pH 7.8–8.2 and approximately 550 ppm TDS under well-conditioned captive settings, suggesting the species is adapted to neutral-to-alkaline conditions rather than the acidic blackwater characteristic of many annual killies from similar latitudes. Adults favour the upper water column, hovering near the surface among emergent and floating vegetation.
Feeding
Fundulus nottii is a generalist carnivore at the surface, feeding on small aquatic and terrestrial invertebrates: insects and their larvae, small crustaceans, zooplankton, and anything else of suitable size that drifts within range at the water surface. The upturned mouth and position in the upper water column are adaptations for intercepting items in the surface film.
In captivity the fish is not fastidious. It accepts live and frozen foods such as daphnia, mosquito larvae, bloodworm, and brine shrimp readily, and will also take small dry foods — flakes and micro-pellets — once settled. Variety in the diet maintains condition and colour, and a regular component of live or frozen invertebrates is beneficial especially when conditioning fish for breeding. As with most small topminnows, the upward-facing mouth means sinking pellets may be ignored; floating or neutrally buoyant foods are preferable.
Mating
Fundulus nottii is a non-annual killifish. Unlike the annual toothcarps of South America and sub-Saharan Africa, it does not deposit eggs in bottom sediment to survive a dry season in diapause; instead it inhabits permanent or near-permanent water year-round and spawns continuously or semi-continuously through the warmer months, depositing adhesive eggs on submerged fine-leaved plants, floating vegetation, or artificial spawning mops.
Mating behaviour involves the male closely accompanying and courting a gravid female. NANFA hobbyist accounts note that the female approaches the male actively when she is laden with eggs, and the pair then move alongside one another as eggs are deposited on vegetation or mop fibres near the surface or substrate. Spawning occurs over extended periods rather than in a single event, with the female depositing small numbers of eggs at each visit to the spawning medium. Males display more intensely coloured flanks and more sharply defined markings than females; the difference is most pronounced during spawning activity.
Breeding
Breeding Fundulus nottii in the aquarium follows the standard non-annual mop-spawning protocol. A pair or small group is conditioned on live and frozen foods at 72–79 °F; the addition of fine-leaved plants such as java moss or floating spawning mops provides the egg-deposition surface. Eggs are adhesive and relatively easy to spot among the mop fibres or plant fronds. Adults should be removed from the breeding tank or the mops and plants transferred to a separate hatching vessel once a batch of eggs has been collected, as adults will consume eggs and fry.
Incubation is conducted in still, shallow water at similar temperatures. A NANFA husbandry account records hatching times of under two weeks at mid-70s °F (approximately 73–75 °F); cooler temperatures lengthen incubation. Fry emerge at roughly 0.24–0.28 in, silvery and largely transparent, and are capable of taking newly hatched brine shrimp (Artemia nauplii) from the outset — a manageable first food that is readily available. Growth is reported as slower than in some other killifish, so patience is needed before juveniles reach sale or transfer size. Multiple small spawning batches collected over several weeks are a practical approach to building up numbers.
In the aquarium
The bayou topminnow is well-suited to a species or biotope aquarium replicating Gulf coastal lowland conditions: a well-planted, soft-lit tank with sandy or fine-gravel substrate, floating plants or emergent stems, and minimal current. A cover is essential — all topminnows are accomplished jumpers and gaps in the lid will result in losses. Tank size of 16 US gal or larger is practical for a small group; the fish are not particularly aggressive toward one another at reasonable densities.
Water chemistry should sit at neutral to mildly alkaline (pH 7.0–8.2) and moderate hardness, consistent with the Gulf coastal plain chemistry documented in captive accounts. Temperature can range from around 64 °F in winter to 81 °F in summer; a natural seasonal temperature cycle stimulates normal behaviour and may promote spawning. The species is tolerant and forgiving of water chemistry within this range, making it a good introduction to native North American topminnow keeping.
Fundulus nottii is compatible with other small, similarly sized topminnows and native North American species in a biotope setup, though it will prey on fish small enough to swallow. It mixes well in the native fish aquarium hobby, where interest in indigenous species conservation drives a small but enthusiastic community of keepers. The species is occasionally available through native fish societies such as NANFA and from specialist breeders rather than the general trade.
Conservation
Fundulus nottii is assessed by the IUCN Red List as Least Concern (2012), and NatureServe assigns it a G5 global rank — globally secure with a moderate distribution across the Gulf coastal plain. It is not a conservation priority in the way of many single-spring or single-drainage endemic killifish, and no immediate population collapse is documented.
Nonetheless, the habitat it depends on — bayous, cypress swamps, vegetated sloughs, and the shallow weedy margins of lowland Gulf coastal streams — is under sustained and long-standing pressure. Wetland drainage for agriculture and development, channelisation and hydrological modification of lowland rivers, and water-quality degradation from agricultural run-off all reduce and fragment the mosaic of shallow vegetated water bodies that support this and many other Gulf coastal plain topminnows. Introduced species, including mosquitofish (Gambusia affinis and G. holbrooki), have displaced many native topminnow populations across the region by competitive exclusion and direct harassment of females. Monitoring of fine-scale population distribution and habitat connectivity remains relevant for a species whose range, while not immediately threatened at the level assessed in 2012, is embedded in one of North America's most heavily impacted freshwater landscapes.