Taxonomy & naming
Procatopus similis was described by the German ichthyologist Ernst Ahl in 1927. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Procatopus similis Ahl, 1927, placing it in the genus Procatopus within the family Poeciliidae (subfamily Procatopodinae), the African lampeye killifishes. Two junior synonyms are recognised by the IUCN assessment: Procatopus glaucicaudis Clausen, 1959, and Procatopus lacustris Trewavas, 1974 — both collapsed into P. similis under current taxonomy.
The genus name Procatopus is a compound of the Greek pro- ('before' or 'in front of') and katops ('seen from below'), an allusion to the ventral position of the anal fin and the fish's elongated lower profile. The specific epithet similis is Latin for 'resembling' or 'similar', likely reflecting Ahl's observation that the species resembles related lampeyes in the genus.
The species shows notable colour variation across its range, and the American Killifish Association records multiple named population strains from collection localities including Muyuka, Mundemba, Moliwe, Kumba, and Idenau in Cameroon. These are maintained as separate aquarium strains by hobbyists but are not accorded subspecific rank under current Catalog of Fishes treatment.
Morphology
Procatopus similis is a slender, laterally compressed killifish reaching a recorded maximum of 2.5 in total length (FishBase), with most adults closer to 2 in. The body is elongated and fusiform, typical of the Procatopodinae. Meristic counts from FishBase give 9–13 dorsal soft rays, 15–19 anal rays, and 27–31 mid-longitudinal scales — proportions consistent with a fast, open-water schooling fish.
The diagnostic feature of all lampeyes is a well-developed iridescent patch at the upper margin of the iris, produced by a specialised reflective light organ. In P. similis this patch glows blue-green in subdued or angled light and is conspicuous in both sexes, though slightly more developed in males.
Male coloration is variable and population-dependent. The base body colour is light olive to light grey, with light blue iridescent reflections along the flanks. A reddish-brown stripe may be present along the dorsal midline. The unpaired fins — dorsal, anal, and caudal — span a wide palette from light blue-grey to yellow, orange, or red, often bearing red spots or stripes and a dark red outer margin. The caudal fin base shows blue reflecting pigment. Females are semi-translucent grey or pale olive with subdued coloration, though both sexes share the characteristic orange reflective upper iris. Sexual dimorphism is moderate; males are more intensely coloured and typically slightly slimmer than ripe females.
Habitat
This species inhabits small rivers and brooks on the coastal plain of the Gulf of Guinea, primarily in the streaming sections of watercourses flowing through sedimentary-soil terrain. Its range extends from Nigeria southward through Cameroon (both west and east of the Cameroon Highlands) to the Lokundje River, with a separate population on Bioko Island, Equatorial Guinea. The dominant vegetation context is coastal-plain rainforest and secondary forest, with some occupation of streams in or adjacent to secondary growth.
FishBase records the water-chemistry envelope as pH 6.0–8.0, hardness 5–12 dH, and temperature 75–79 °F — clear, moderately soft to moderately hard, and warm but not hot. The streams are typically well-oxygenated and often shaded by forest canopy, with substrates of sand and sedimentary silt. The IUCN assessment notes the species is 'not a seasonal killifish', meaning it inhabits permanent water bodies rather than the temporary pools associated with annual killies.
The benthopelagic designation in FishBase reflects the species' use of the water column from near the bottom up through the midwater, where it schools in the shelter of submerged vegetation, roots, and woody debris — the same microhabitat in which spawning takes place.
Feeding
Procatopus similis is an insectivore in the wild, as is typical of the African lampeye killifishes. Small invertebrates — aquatic insect larvae, microcrustaceans, and surface-fallen insects — form the core of its diet in the clear-water forest streams it inhabits.
In the aquarium the species accepts a range of small live and frozen foods readily: Daphnia, Artemia nauplii and adults, bloodworm (chironomid larvae), tubifex, and mosquito larvae are all taken. Dried micro-pellets and fine flake are also accepted, though a varied diet with a strong live or frozen component keeps the fish in best colour and condition. The small mouth imposes a practical size limit on food items; anything much above 0.5 in is beyond its gape.
Mating
Procatopus similis is a non-annual, continuous egg-layer that spawns among submerged woody debris and fine plant matter in permanent-water streams. Like other Procatopus species, it deposits small numbers of adhesive eggs over an extended period rather than producing a large clutch at once — the spawning mode of a long-lived fish exploiting stable habitat rather than racing to reproduce before a temporary pool dries.
The lampeye light organ plays a role in social signalling and mate recognition. In dim, forest-shaded water the iridescent iris patch is highly visible to conspecifics, and males display to females and rival males by orienting and tilting the head to catch available light. Fin coloration — in P. similis ranging from blue-grey to red depending on the population — reinforces individual identity and male quality. Males are not markedly aggressive toward each other by killifish standards, and groups with a numerical advantage of females over males function well in both wild and aquarium settings.
Breeding
Spawning in P. similis follows the pattern typical of the Procatopodinae: pairs or small groups deposit individual or small batches of adhesive eggs among fine-leaved vegetation, Java moss, submerged roots, or fibrous spawning mops, and then leave them without further care. Eggs hatch after approximately 10–14 days at typical maintenance temperatures of 75–79 °F. Fry are small at hatching and require appropriately sized first foods — infusoria, paramecia, or commercial fry powder — before graduating to newly hatched Artemia nauplii.
Because the species spawns continuously over weeks and months rather than in discrete seasonal events, a productive pair or trio can yield a steady trickle of fry throughout the year. Eggs and fry are not actively consumed by the parents under normal conditions, but the fry are small enough that they may be outcompeted for food in a densely stocked tank. A dedicated breeding or rearing vessel with dense surface cover or spawning mops simplifies egg collection. The species is considered one of the less demanding members of the Procatopodinae to breed in captivity, though it rewards attentive water quality and a steady diet of varied live foods. Breeding.mode for this species: Non-annual / continuous egg-layer among vegetation and woody substrate.
In the aquarium
Procatopus similis is well-established in the ornamental killifish hobby, particularly among members of the American Killifish Association, which lists the species with population strains from several Cameroonian localities. It is not commonly encountered in the general trade but is reliably available through specialist society fish rooms and killi breeders.
The fish is a schooling species and does best in groups of six or more. A well-planted aquarium of 15–20 US gal or larger, with subdued lighting, dark substrate, and floating plants to diffuse surface light, suits it well and shows its coloration to best advantage. The lampeye patch is most visible in angled or dim light, so avoiding bright overhead illumination is worthwhile. Tankmates should be small and peaceful; robust or boisterous species stress the fish and suppress colour. Other small lampeyes, small African characins, or dwarf Corydoras species from overlapping ranges can work in a biotope setup.
Water conditions in keeping with the habitat — pH 6.0–8.0, hardness 5–12 dH, temperature 75–79 °F, clean and well-oxygenated — are the maintenance target. The species is reported as moderately hardy once established, though some sources note initial settling-in can take care; good water quality and a varied diet are the main requirements. Lifespan is several years under good conditions, long for a killifish, reflecting its non-annual origins in permanent water.
Conservation
The IUCN Red List assessed Procatopus similis as Least Concern in September 2019 (published 2020), with the justification that it holds a wide distribution across coastal Cameroon, Nigeria, and Bioko Island and faces no known widespread major threat. Population trend is listed as unknown.
The assessment identifies localised threats to the species and its habitat: oil exploration and associated pollution in the Niger Delta affect Nigerian populations; logging degrades streamside forest and increases sedimentation; and domestic, agricultural, and industrial pollution affects water quality in the coastal plain rivers. These pressures are real but are judged not to be operating at a scale that threatens the species as a whole at this assessment point.
The species' reliance on clear-water, forest-stream habitat means it is sensitive to catchment-level deforestation, which increases runoff, sedimentation, and water temperature beyond the ranges the fish tolerates. The Lokundje River populations at the southern edge of the range and the island population on Bioko are geographically isolated and potentially more vulnerable to local disturbance than the assessment's range-wide view captures. Continued monitoring of population status across the range — particularly in Nigeria, where Niger Delta pressure is greatest — is warranted.