Most of its closest living relatives are egg-laying, with the notable exception of the splitfin livebearers (Goodeidae).

However, a few larger species exist, with the giant killifish (F. grandissimus) and the northern studfish (F. catenatus) growing to twice the genus' average size.

Fundulus have evolved to occupy a wide range of aquatic ecosystems, including marine, estuarine, and freshwater, making it a good comparative model system for studying evolutionary divergence between marine and freshwater environments.

[2] To assist with this research, Oxford Nanopore long-read reference genomes have been sequenced for F. xenicus, F. catenatus, F. nottii, and F. olivaceus[3].

There are currently 39 recognized species in this genus:[4] The Cuban killifish (Cubanichthys cubensis, a pupfish) was formerly placed in Fundulus.