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Livebearers
Although the ability to produce live young (rather than eggs) is found in many groups of fishes, the family Poecilidae is so specialised for this mode of reproduction that the group is usually referred to simply as the ‘livebearers’. Females are generally larger than the males, and often quite drab in colouration. The males have bright colours which they use to attract the females. When they mate, the male uses the specially modified anal fin, which is shaped like a tube, to squirt sperm into the female. In many cases the female can use the sperm up a bit at a time, allowing her to produce a series of broods after just one mating.
Livebearers do not form lasting pairs; after mating the male immediately seeks out new females. For this reason it is generally considered best to either the fish in single sex tanks except for mating, or else to ensure females outnumber by two to one or more. Otherwise the persistent attentions of the males stress the pregnant females and can cause miscarriages.
Limias, Limia spp.
Several species of Limia occur in brackish water, usually in slow-moving streams and pools. They are primarily algae-eating fish though they also eat insect larvae and other small animals. In aquaria they are hardy, easy to breed, non-aggressive and generally very easy to keep. Limia nigrofasciata is known as the humpbacked limia on account of the deep body that mature males develop. They also have speckled black dorsal fins. The females are more streamlined in shape. Both sexes are yellowy-brown in colour with vertical black bands and yellow fins. Ideally suited to low salinity brackish water aquaria around SG 1.005.
Guppies, Poecilia reticulata
All guppies do very well in low salinity brackish water aquaria, and ‘feeder guppies’ and wild-caught guppies can be slowly acclimated across a few weeks to salinities as high as that of normal seawater. Fancy guppies are less tolerant, and often do not do well above SG 1.010.
Platies and Swordtails, Xiphophorus spp.
Swordtails rarely occur in brackish water in the wild, and platies hardly at all in their natural range, but both will adapt to slightly saline conditions up to about SG 1.005.
Mosquitofish Gambusia spp.
Mosquitofish such as Gambusia holbrooki and Gambusia affinis are tolerant of brackish water at least as saline as SG 1.005 and likely to SG 1.010 with careful acclimation. Lab specimens of Gambusia affinis have been acclimated to full seawater when adjusted to such conditions over a course of three days. But their value as aquarium fish is limited because they are not particularly colourful and are also very aggressive and nippy. Recommended for advanced aquarists interested in infrequently kept livebearers rather than community fish.
Pike livebearers Belonesox belizanus
Belonesox belizianus are very salt tolerant and will adapt to any brackish water aquarium. They are peaceful towards fish too large to be swallowed (for example adult scats and monos) but they will attempt to eat anything smaller than they are, including their own kind. The main problem for the aquarist is that they are reluctant to take anything other than live foods. Wild-caught specimens normal only accept live fish, large insects and river shrimps. Tank-bred specimens may be more amenable to frozen foods: check with the retailer or breeder.