A measure of success for aquarium keepers is the reproduction (and rearing) of their captive livestock. As such, at one time or the other most aquarists "try" to breed livebearing fishes, considering them to be easier goals than egg-laying species whose young start much smaller, requiring more specialized feeding and care.
In the course of keeping aquariums or any life for that matter, one gains an understanding of how the life replicates itself. Sexual reproduction, where it takes "two to tango", males and females'¦ are the general rule on our planet, with more than 99 % of all animals and plants having separate sexes. This arrangement allows for greater diversity and flexibility in space and time with the blending of genotypes within a species. Asexual reproduction modes like splitting, fission make only "carbon-copies" of individuals, with only gene-replication and "mistakes" to provide for adaptation to environmental changes.
Fishes by and large are oviparous, or "egg-laying" species, though livebearing types have arisen in disparate families, orders. Some like most sharks are called ovoviviparous, having their young develop inside them though lacking intimate connection with their mother parent. Others are viviparous, or "true" livebearers, having placenta-like structures by which the female parents nourish their young during internal development. For our purposes we'll consider the last two types of reproduction, ovoviviparity and viviparity as livebearing, versus oviparity or egg-laying.
All these reproductive strategies "work" as measured by their very existence. Consider the possibility of producing many more sex cells (eggs and sperm) and broadcasting them in the environment, versus internal fertilization, the "costs" of internal development and tremendously greater control of where and when to bear your young (though in much smaller numbers) with livebearing. Which "plan" allows for enhanced distribution? Egg-laying for sure. But at what limits do egglayers benefit in terms of loss of gametes, fry in currents and predation?
Livebearing of young is a better "reproductive game-plan" for species that live in closed spaces (like freshwater ponds, streams), in changing, possibly detrimental environments, and with species that have long reproductive cycles, late maturation, correspondingly longer replacement rates, and/or a scarcity of mating opportunities.
More than Guppies, Platies, Swordtails & Mollies:
Most everyone knows THE livebearers of the family Poeciliidae. All told livebearing fishes are represented in 54 families of fishes (forty are cartilaginous fishes; sharks and rays, though these groups include oviparous species as well). Though infrequently to never seen in aquarium use, other livebearing fishes include Old Four Legs, the Coelacanth (no longer monotypic, there is at least one new to science species in Indonesia) and thirteen other advanced bony fishes (teleost) families.
Other than the common species of the family Poeciliidae mentioned above, other families, species of livebearing fishes that a hobbyist might encounter include:
Freshwater Stingrays: Family Potamotrygonidae
For folks with very large systems, a penchant for specialized "species tank" set-ups and a wary eye for venomous (yes, they're stingrays) fishes. These species need fine sandy bottom and lots of it. Most get near or greater than two feet in diameter. Some species have been bred in captivity and even hybridized (crossed between species).
Livebearing Surf Perches: Family Embiotocidae
Of twenty three species, two can be found off of Japan and Korea, 21 occur in the eastern Pacific (on the U.S. west coast) with one, the Tule Perch (Hysterocarpus traskii) being found in freshwater... though it's only been maintained in good health with the addition of salt to its water. Public aquariums on the west coast generally display these fishes, many of which are reasonably small (several inches in length) and beautiful to warrant inclusion in coldwater marine set-ups.
Four-Eyes: Family Anablepidae
This was the first species of livebearing fish discovered; some 350 years ago. The three species of Four Eyes are bizarre fishes from a few standpoints. Their common name derives from eyes sticking part in and out of water. Along with an oval shaped cornea this allows them to see "four images" two above and two underwater. Though it has not been confirmed males and females are said to be "handed", engaging in internal fertilization either on the left or right. These fishes are found in South and Central America and have reproduced in captivity.
The family Anablepidae also includes the ten species of Jenynsia and Oxyzygonectes dovii nowadays. Jenynsia spp. Look a lot like Mosquitofishes (genera Heterandria, Gambusia of the family Poeciliidae), but like Anableps, their males have a tube-like gonopodium (male intromittent organ) associated with their anal fins, whereas those of poeciliids are independent of their anal fins. They are also said to be left or right "handed".
Goodeids: Family Goodeidae
The 40 species of 19 genera of this family are principally viviparous (two are oviparous with external fertilization). They're naturally found in Nevada to west central Mexico. Their males anal fin anterior rays are modified (shorter, crowded, semi-independent from the rest of the fin). This "primitive" gonopodium is called a pseudophallus. Their newborn young and embryos have a placenta-like structure called a trophotaeniae.
There are a few easy-going (enough) and good-looking species in this family for aquarists. About the only, and at that sporadic, one available is Ameca splendens.
Live-bearing Toothed Carps: Family Poeciliidae
With thirty genera, 293 species, this is the largest family of livebearing fishes (though the Fluviophylacine and Aplocheilichthyinae are now placed here and are not viviparous). The family's members can be found throughout the eastern U.S. to northeastern Argentina, and Africa and Madagascar. The livebearing portion, the subfamily Poeciliinae includes some all-female species (the "Amazon Molly"), whose eggs are capable of developing through stimulation by sperm of another species (without fertilization).
The livebearing toothed carps almost all appreciate hard, alkaline water, frequent partial water changes and the addition (if your other livestock can take it) of some salt to their water. If your guppies, platies, swordtails, mollies or other more exotic members of this family show "no energy", clamped fins, just hanging about in the tanks corners, look to your water quality, make a substantial water change (25%) and consider adding some salt to the system.
Halfbeaks: Family Hemirhamphidae
There are totally freshwater, brackish and marine (Atlantic, Indian and Pacific Oceans) species amongst the 12 genera and more than 100 species of halfbeaks. Looking at them, what do they remind you of? To me, the closely related flying fishes. These are surface dwelling fishes that feed on algae, zooplankton and small fishes. Most are oviparous, attaching their eggs to algae. The genera Dermogenys, Hemirhamphodon, Nomorhamphus are viviparous and freshwater, the first including the most commonly available species to aquarists, D. pusilla. Males of the livebearing species have their anal fins modified into intromittent structures called andropodia.
Storage of Sperm:
Some livebearers like the poeciliids have the ability to use some sperm, store others in packets for later batches of eggs. Others, like the goodeids must "refresh" their supply each batch of young. This ability to store and later utilize sperm has important implications. For the species, it frees up the need to locate the opposite sex in order to assure reproduction. For aquarists it relates to the need to separate individuals by sex at an early age if selective breeding is ones goal.
Biology of Reproduction:
Livebearing fishes have gestation periods of a handful of weeks to several months (cartilaginous fishes). The development and maturation of their sex organs and their products is under much the same hormonal control as other vertebrates, including ourselves. Culturists have learned that simple manipulation of environmental factors like temperature, photoperiod and metabolite control for most viviparous species. Important egg-laying species are sometimes manipulated through hormonal injection in addition.
To be clear and complete, please understand that there are many intergradations between oviparity and livebearing modes in fish species. Indeed, some egglayers have internal fertilization, and all seasoned aquarists are familiar with the myriad ways that fishes display parental care, build nests, brood their young.
Bearing young live is a useful strategy of reproduction employed by a minority of diverse families of fishes. The benefits of having larger young released live when and where the female desires seems to outweigh their far fewer numbers of young in these fishes. With sorting through and careful selection of suitable species, aquarists can capitalize on this behavior in the successful captive reproduction of viviparous fishes in captivity.
Livebearer Associations on the Internet: http://www.petsforum.com/ala/links.htm
Bond, Carl E. 1979. Biology of Fishes. W.B. Saunders Co., Philadelphia. 514pp.
Castro, Alfred D. 1999. Freshwater stingrays. The true freshwater stingray is relatively easy to keep and breeds readily in captivity. 10/99.
Coletti, Ted. 2002. Bookmark this column: Your guide to livebearer stuff on the internet. 2/02.
Coletti, Ted. 2003. Building a livebearer library; the essential reference works. FAMA 1/03.
Dawes, John. 1991. Livebearing Fishes. A Guide to Their Aquarium Care, Biology and Classification. Blandford, London. 240pp.
Fenner, Robert. 1997. A diversity of aquatic life: The freshwater stingrays of the family Potamotrygonidae. TFH 4/97.
Lambert, Derek. 1994. Livebearer world: Forgotten livebearers, the stingrays. 12/94.
Lambert, Derek. 1998. The Livebearer world. Zoogonecticus tequila- a new goodeid. TFH 6/98.
Langhammer, James K. 1988. A community with livebearers. There's more to keeping livebearers in a community than just compatibility. AFM 10/88.
Neal, Tom. 1998. Those fabulous four-eyes. TFH 3/98.
Wessel, Rusty. 2002. Four eyes in Honduras. TFH7/02.