Livebearers and Killifish

Livebearers and Killifish
Family: Poeciliidae, Fundulidae
Genus: 3 or more
Most fish reproduce by spawning. This involves laying eggs and allowing their mate to find and fertilize them. Livebearers are the only fish that nurture their eggs through development and perform a live birth. Livebearers perform this using a modified anal fin called a gonopodium (found only in males).
  1. Single dorsal fin
  2. Rounded caudal fin
  3. Terminal or superior mouth
  4. Less than 6 inches (and in many cases even smaller)
  5. Males will possess a gonopodium instead of a fanlike anal fin as the females have.
largespring gambusia
Largespring Gambusia

Chad Thomas

Largespring Gambusia
(Gambusia geiseri)
Introduced to maintain mosquito populations, the largespring gambusia can be found alongside the western mosquito fish, where it usually far outnumbers it. This is mainly due to their versatility in reproduction. These fish are known to spawn year round in systems with a constant temperature, such as the San Marcos springs.
Distinguished from the western mosquitofish by a black line across the mouth, as well as a line across the keel (anal side). Similar species also lack the black spots between scales on the sides.
western mosquitofish
Western Mosquitofish

Chad Thomas

Western Moquitofish
(Gambusia affinis)
As the name implies, this fish loves eating mosquito larvae. This has made it a popular fish to stock in lakes, rivers, and reservoirs across the world. Unfortunately there was a serious and unforeseen consequence. The western mosquito fish has a tendency to eat the eggs of other fishes, and to outcompete its neighbors. This has resulted in the potential extinction of many localized populations such as the sonoran topminnow, and the gila topminnow.
The Western Mosquitofish possesses a dark bar across the eye, dorsal and caudal fins possess rows of dark spots.
texmex gambusia
Tex-Mex Gambusia

Chad Thomas

Tex-Mex Gambusia
(Gambusia speciosa)
Found only in Mexico and the Devils river, this species is considered threatened by the American Fisheries Society. Some do not consider this as a unique species and instead refer to it as a variant of the western mosquitofish, referring to it as Gambusia affinis speciosa.
The tex-mex gambusia hosts spots on dorsal and caudal fin. The dorsal rin is further back than the affinis and the anal fin is slightly smaller. Generally lighter color and smaller than other locals.
sailfin molly
Sailfin Molly

Chad Thomas

Sailfin Molly
(Poecilia latipinna)
These fish can survive extremely low oxygen water by reducing their overall consumption and drawing from the high oxygen surface film. Much like other livebearers, males are exuberant and flamboyant in order to attract a mate.
The sailfin molly should be a very easy identification. Distinct black bars are usually visible, as well as a thin black margin on the caudal fin. Males possess iridescent blue coloring and a large dorsal fin, which is placed anterior to the anal fin. Females and males will have horizontal lines made up of dots between scales on the body.
gulf killifish
Gulf Killifish

Chad Thomas

Gulf Killifish
(Fundulus grandis)
The gulf killifish is an economically valued fish, as bait fish, sold locally as cigar or bull minnows, as well as being an important predator of mosquito larvae and pupae. The gulf killifish is very different from most of its genus and is fairly unique in appearance. While this fish is native to the Rio Grande drainage, it can now be commonly found in coastal regions due to bait release.
Males have dark green and silvery bars running vertical. Females are olive above and light olive below. Dorsal fin is in line with the anal fin.

Last updated: February 11, 2019

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