Tomeurus

{{Short description|Genus of fishes}}

{{Speciesbox

| image = Tomeurus gracilis.jpg

| image_caption =

| display_parents = 3

| grandparent_authority = Eigenmann, 1912

| parent_authority = C. H. Eigenmann, 1909

| taxon = Tomeurus gracilis

| authority = C. H. Eigenmann, 1909

}}

Tomeurus gracilis is a species of poeciliid fish native to South America, in Brazil, Surinam and Venezuela. This species grows to a length of {{convert|3.3|cm|in}} TL.{{Fishbase|Tomereus|gracilis|month=August|year=2019}} It is the only known member of its genus.{{Cof record|genid=5536|title=Tomeurus|access-date=10 November 2019}} This species is classified in the tribe Cnesterodontini in the 5th edition of Fishes of the World,{{cite book |title=Fishes of the World |edition=5th |author1=J. S. Nelson |author2=T. C. Grande |author3=M. V. H. Wilson |year=2016 |pages=379 |publisher=Wiley |isbn= 978-1-118-34233-6 |url=https://sites.google.com/site/fotw5th/ }} but more recent classifications place it as the only member of the subfamily Tomeurinae, the facultative viviparous poeciliids.{{cite journal | author1 = Paulo Henrique Franco Lucinda | author2 = Roberto E. Reis | year = 2005 | title = Systematics of the subfamily Poeciliinae Bonaparte (Cyprinodontiformes: Poeciliidae), with an emphasis on the tribe Cnesterodontini Hubbs | journal = Neotropical Ichthyology | volume = 3 | issue = 1| pages = 1–60 | doi = 10.1590/S1679-62252005000100001 | doi-access = free }}{{Cite web |title=CAS - Eschmeyer's Catalog of Fishes |url=https://researcharchive.calacademy.org/research/ichthyology/catalog/fishcatmain.asp |access-date=2025-03-15 |website=researcharchive.calacademy.org |language=en}}

This is a sociable fish which lives in small schools consisting of several dozens of fishes. It is found in muddy creeks or along the sandy-muddy edges of shallow estuaries. It has been observed remaining still just below the surface but it prefers to hide beneath aquatic plants. Fertilisation is internal and, uniquely among the Poeciliinae, the females may then lay eggs placing them individually onto aquatic plants. They can also be facultively ovoviviparous.