Taxonomy & naming
Kulkarni described Horaichthys setnai in 1940 from creeks near Mumbai, in a genus of its own. Parenti's 2008 revision places it in Adrianichthyidae, in a lineage separate from Oryzias. The generic name honours the Indian ichthyologist S.L. Hora and the species name honours Setna.
Morphology
A very small, slender fish; FishBase gives a maximum of 1 in TL. Males have the anal fin rearranged into a gonopodium-like clasping organ, and females differ in having a normal fin. The pelvic fins are also modified in males.
Habitat
FishBase reports it from estuaries and quiet creeks and backwaters, with a range from near the Gulf of Kutch to Trivandrum in Kerala, and notes that it can live temporarily in freshwater. It tolerates a wide range of salinity. FishBase gives a pH range of 7.5 to 8.5 and a temperature range of 24 to 29 C.
Feeding
Takes small zooplankton and other minute invertebrates at the surface. FishBase adds that it eats mosquito larvae and has been used as a larvicidal fish in malaria-affected areas.
Mating
Males use their modified anal fin to transfer sperm packets to the female; fertilisation is internal, so courtship is a close-range clasping rather than a pair spawning over plants.
Breeding
Spermatophores are passed from male to female, and the fertilised eggs are laid one by one or in small numbers over a wide area rather than in a single spot, attached by threads to floating or submerged material. FishBase says it breeds easily through the year, with a peak in July and August. Clutch size and incubation time have not been published.
In the aquarium
Rarely kept. If attempted, brackish water, fine live foods such as brine shrimp nauplii, and a quiet, heavily planted tank suit a fish this small. Tankmates larger than a few millimetres of mouth gape would eat it.
Conservation
The IUCN Red List assessed Horaichthys setnai as Least Concern in 2010. Its habitat of coastal creeks near a very large city is exposed to development and pollution.