Taxonomy & naming
Kryptolebias caudomarginatus was described by Lothar Seegers in 1985 (some literature cites 1984; Eschmeyer's Catalog of Fishes records the accepted date as 1985) on the basis of material from the Brazilian coast. The authority is presented in parentheses, indicating that the species has since been transferred from its original genus placement into Kryptolebias. The valid combination is Kryptolebias caudomarginatus (Seegers, 1985) under the current Catalog of Fishes treatment.
The genus Kryptolebias (formerly Rivulus in part) encompasses the mangrove rivulus complex, a group of small killifishes restricted to the Atlantic mangrove belt from Florida and the Caribbean south to Brazil. The genus includes the celebrated K. marmoratus and K. hermaphroditus, which are the only known self-fertilising vertebrates; K. caudomarginatus is androdioecious — gonochoristic, meaning it reproduces through males and females — and is therefore distinct from its self-fertilising congeners in reproductive biology as well as morphology. Population genetics work published in the Journal of Fish Biology (2015) confirmed K. caudomarginatus as a valid species separate from K. marmoratus and K. ocellatus.
The specific epithet caudomarginatus combines the Latin cauda (tail) and marginatus (edged or bordered), likely referring to a coloured or distinctively patterned margin on the caudal fin. The genus name Kryptolebias derives from the Greek kryptós (hidden) and lebias (a small fish), reflecting the secretive habits of these mangrove-dwelling rivulids.
Morphology
Kryptolebias caudomarginatus is a small killifish reaching a maximum recorded total length of 2.5 in (FishBase), though most individuals encountered in the field and in the aquarium are smaller. The body form follows the typical rivulid pattern: moderately elongate and compressed, with the dorsal fin set well back — a profile well suited to slipping through root tangles and narrow crab-burrow entrances.
Colouration is not extensively documented in the literature relative to K. marmoratus, but the genus generally shows olive to brownish ground colouration with variable spotting or banding on the flanks; the specific epithet implies a distinctive tail-margin marking. As a gonochoristic species, males and females are expected to show sexual dichromatism comparable to other Kryptolebias species, with males more brightly patterned — though detailed published colour descriptions are scarce. Data on meristic counts and osteological features that separate it from K. marmoratus and K. ocellatus are primarily available in the original description and in the molecular systematics papers that validated its specific status.
Habitat
Like other members of the genus, K. caudomarginatus is a mangrove specialist. Its habitat encompasses the brackish and variable-salinity margins of Atlantic coastal mangrove systems in Brazil (states documented in the IUCN assessment include Alagoas, Rio de Janeiro, and Santa Catarina), where it occupies the same microhabitats exploited by K. marmoratus further north: decaying wood, hollow tree cavities, the water-filled burrows of land crabs, and the shallow root-tangled pools at the outer edges of mangrove stands.
These environments are characterised by wide fluctuations in salinity (from nearly fresh to full marine depending on tidal state and rainfall), high temperatures, and chronically low dissolved oxygen — conditions that mangrove rivulus species are physiologically equipped to handle. The IUCN records the habitat category as inland wetlands. It rains fishes notes that the broader mangrove rivulus group occupies both permanently aquatic sites and semi-terrestrial refuges such as hollow logs above the waterline, suggesting tolerance of emersion for short periods.
FishBase records temperatures of 68–75 °F for this species, consistent with subtropical to tropical Atlantic coastal climates. The water is typically soft to moderately hard and variably brackish.
Feeding
Kryptolebias caudomarginatus is a carnivore in the manner typical of rivulids: a surface-oriented and benthopelagic predator on small invertebrates. The mangrove environment provides an abundance of small crustaceans, insect larvae, amphipods, small worms, and other micro-invertebrates associated with decaying organic matter and root systems, and this forms the likely dietary base in the wild.
In aquarium conditions the species is described as very difficult to maintain (FishBase), which suggests sensitivity to water conditions rather than any particular feeding reluctance — captive rivulids of this group generally accept small live and frozen foods such as Artemia nauplii, Daphnia, bloodworm, and Tubifex. Feeding near or at the water surface aligns with the dorsally-set eye position and dorsal fin placement typical of the group.
Mating
Kryptolebias caudomarginatus is a gonochoristic, sexually reproducing species — males and females mate in the conventional sense, in contrast to the self-fertilising hermaphroditism of K. marmoratus and K. hermaphroditus. The Taylor (2012) ecology review of the mangrove rivulus group notes that K. caudomarginatus is androdioecious, meaning the population comprises males and females rather than functional simultaneous hermaphrodites.
Mating behaviour in the wild has not been extensively described in the published literature, but the genus as a whole uses adhesive egg placement on substrate rather than broadcast spawning. Courtship in small killifishes typically involves male display — fin spreading and colour intensification — followed by lateral alignment and a brief spawning embrace during which a small number of adhesive eggs are released. The mangrove microhabitat provides abundant attachment surfaces in the form of fine roots, submerged debris, and aquatic vegetation at the water's edge.
Breeding
Kryptolebias caudomarginatus is a non-annual killifish. It inhabits permanent (or near-permanent) water bodies and does not produce diapausing eggs buried in substrate — the annual peat-spawning strategy associated with killifishes of seasonally drying savanna pools is absent here. Instead, the species lays small batches of adhesive eggs that are deposited on fine-leaved plants, submerged roots, or spawning mops near the water surface or along the substrate, and these eggs incubate in water at ambient temperature without diapause.
It Rains Fishes records egg incubation in the broader mangrove rivulus group at approximately 14–16 days, with embryos tolerating fluctuating salinity and low-oxygen conditions consistent with the mangrove environment. Because eggs are produced in small batches over extended periods rather than a mass seasonal pulse, the breeding cycle is continuous or semi-continuous in permanently warm water. FishBase notes that the species is very difficult to maintain in aquarium conditions, which may reflect its specific requirements for water chemistry (variable salinity, warm temperatures, good aeration) rather than any intrinsic breeding difficulty once those conditions are met.
Fry care follows the non-annual pattern: once free-swimming, young fish are self-sufficient, though the adults should be separated from eggs to prevent predation. Rearing requires appropriately sized live foods.
In the aquarium
Kryptolebias caudomarginatus is rarely encountered in the hobby and FishBase explicitly notes it is very difficult to maintain in aquarium conditions. This difficulty is almost certainly rooted in its mangrove habitat requirements: the species is adapted to warm, variably brackish water with fluctuating salinity and low dissolved oxygen, parameters that do not correspond to standard freshwater aquarium maintenance.
For specialist killifish keepers willing to replicate mangrove conditions, the keys are temperature stability in the 68–75 °F range, the addition of marine salt to achieve low to moderate brackishness (a salinity of 2–5 ppt is likely appropriate, mirroring upper mangrove conditions), and thorough surface agitation or supplemental aeration to compensate for the species' tolerance of — but not preference for — hypoxic water. A small, well-filtered, species-only aquarium with tight-fitting lid, fine-leaved plants or spawning mops for egg deposition, and some cover in the form of driftwood or PVC tubes is recommended.
The AKA hobbyist records note an 'Embrapa' population label in circulation, suggesting that captive-bred individuals with a known collection origin have been distributed within the killi hobby. Obtaining captive-bred specimens from a specialist society is the recommended route to acquiring this species.
Conservation
The IUCN Red List assessment of 2023 places Kryptolebias caudomarginatus in the Least Concern (LC) category (taxon ID 186482, assessment 1813722). The assessment documents the species' native range across three Brazilian states — Alagoas, Rio de Janeiro, and Santa Catarina — spanning a substantial latitudinal range on the Atlantic coast.
Despite the LC status, the IUCN assessment identifies several active threat categories: residential and commercial development, biological resource use, human intrusions, and pollution. These are precisely the pressures bearing on Brazil's Atlantic mangrove coast, where coastal urbanisation, port development, shrimp aquaculture, and deforestation have reduced mangrove cover significantly over recent decades. Mangrove rivulus species are by nature restricted to narrow coastal wetland strips — habitat that is simultaneously the most accessible for development and the least protected in much of the region.
The IUCN notes that research into population trends is needed and that no specific conservation plans are known for the species. Given that K. caudomarginatus is rarely encountered in surveys compared to K. marmoratus, and that its distribution records are limited to three states, continued monitoring of the Brazilian Atlantic mangrove fringe is warranted. The species is not known to occur in any protected areas documented in the assessment.