Fundulus is a genus of topminnows and killifishes native to North America, with the atlas covering 36 species found from Canada down through the United States and into Mexico, plus a small number of records in the Bahamas, Bermuda, and Cuba. Adult sizes generally run from about 2 in to 8 in depending on species. Unlike many of the egg-laying killifishes prized in the hobby, Fundulus are not annual fish tied to seasonal ponds — most species live in permanent fresh, brackish, or even coastal saltwater habitats, from sluggish streams and marshes to tidal creeks and salt flats, and several, such as Fundulus heteroclitus (the mummichog), are classic estuarine fish tolerant of a very wide range of salinity and temperature. The genus as a whole shows the typical killifish body plan: a blunt head, an upturned mouth suited to picking food from the surface, and a stout, torpedo-shaped body.
Within the aquarium hobby, Fundulus attracts a smaller, more specialist following than tropical killifish genera — species like Fundulus chrysotus (the golden topminnow) and Fundulus diaphanus (the banded killifish) are kept by native-fish and cold-water enthusiasts, often in unheated setups that mirror their temperate North American origins. Fundulus heteroclitus in particular is a workhorse of physiology and toxicology research thanks to its extreme tolerance for changing salinity and temperature. Conservation status varies widely across the genus: most species are secure, but the atlas records an Extinct species, Fundulus albolineatus, and several narrow-range species with Vulnerable, Endangered, or Critically Endangered status, generally tied to isolated spring systems or small coastal drainages.