#Mollienisia sphenops complex

[aquariuminfobureau]

Fish keepers, and even the professionally written texts intended for consumption by aquarists, are prone to what philosophers call faulty generalization. Specifically I am now considering the livebearers, and the commonplace assertion, nowadays, that they require hard and alkaline waters in order to thrive. This fact makes these fishes appealing to people in areas with such a water supply, not least because such water is by nature more buffered against attempts to make it more soft and acidic, than is soft and acidic water when you want to make it harder and more alkaline.

In principle, a livebearer is any fish that does not spawn externally. But to aquarists it refers to 'livebearing toothcarp' or those cyprinodontoid fishes with such a life history. These fish are not a natural descent group, independent of those retaining the external spawning habit; which is to say, livebearing toothcarp' are polyphyletic. However the most common such toothcarp' in the aquarium are a natural or monophyletic group, among the poecilid subclade. Aquarists who do not have a special hobbyist interest in 'wild livebearers', are almost always referring to the mollies, guppies, platies, and swordtails.

Mollies in the wild are fishes of estuaries and even shallow seas, and in laboratory conditions, they grow better in brackish conditions than in freshwater, and not well at a low pH. Natural selection surely shapes the tolerances of wild molly populations, as in laboratory studies, shortfin mollies (Mollienisia sphenops complex, including the black molly) tolerate up to 8 ppt, and introduced mollies of this type, have colonised marine environments without further human assistance, in the Gulf of Thailand. Also laboratory studies indicate that the pH when keeping black mollies, a donesticated morph of shortfin molly, should not drop lower than 7.

The other popular mollies, the sailfin mollies (M. latipinna complex), are naturally present in marine environments in the wild, and experimentally tolerate salinities of even 70 ppt or above, and do best at 10 to. 25 ppt. The ecological preference of shortfin and sailfin mollies, for different levels of salinity, holds not only in their natural range, but also where both are introduced in the Gulf of Thailand. Shortfin mollies have colonised the inner waters of the Gulf of Thailand, where salinity is reduced, and sailfin mollies have colonised further. As with shortfin mollies, they breed and grow normally at a pH value above 7, and thus are not suitable for the softwater aquarium either.

Guppies (Lebistes reticulata) might be close evolutionary allies of the mollies, yet they are also very different animals, as they lack the special herbivorous and saltwater specializations of their molly kin. Guppies are adaptable fishes that do best at a pH that is circumneutral, and although they can tolerate slightly brackish environments, they are typically absent from them in their natural range, being thus limited to the 'interface' between fresh and brackish water conditions. Guppies actually can thrive in waters that are on the soft and acidic side, but only when the parameters are nicely close to neutral.

Still more distantly related are the platies and swordtails of the genus Xiphophorus (X. maculatus and X. helleri) that inhabit freshwaters upriver of estuaries. Although these fishes possess some physiological tolerance of low salinities, up to 10 ppt, they prefer different habitats than do tbe similarly sized mollies. Like them, however, they prefer a pH and hardness above those of our softwater aquariums. Both platies (X. maculatus complex) and swordtails (X. helleri complex) should be maintained at a pH value above 7.

The 'wild livebearers' as a whole, tend also to prefer circumneutral to hard and alkaline water, though some species are especially adaptable and tolerant. Among these are a few species popularly known as the mosquitofishes, although confusingly a number of species in different genera have been referred to by that name, sometimes even the guppy. Usually in aquaria, two species of Gambusia are indicated, either G. affinis or G. holbrooki. These species are easily confused, and the aquarium trade rarely recognizes the difference.

The adaptability of these species to variables of water parameters, has often made them very successful outside of their native range, once they have been introduced to new, suitable localities by the hand of man. Gambusia sp. may be found in waters with a pH as low as 5 and as high as 10, but usually it is at least 6. The temperature is usually 25 degrees, a good match for our tropical tanks, but they may be maintained between 15 and 30 degrees centigrade. Also, they are able to handle a salinity that is up to 16 ppt.

The overlapping appellation of millionsfish is also used for a number of livebearer species, including again the guppy. Another is the dusky millionsfish or caudo, Phalloceros caudimaculatus, further known as the speckled mosquitofish in fact, and also as the one-spot livebearer. Like Gambusia sp. this little toothcarp has established new populations far outside its natural range, assisted by its inherent and broad environmental tolerances, and human efforts to control biting mosquito populations. Based on the collection records that exist for P. caudimaculatus populations, they naturalise best where the temperature is 16-22 degrees centigrade, and the pH is 6.5 to 8.

From North America hails another small poecilids species, Heterandia formosa, known as the least killifish and midget livebearer, as well as the dwarf topminnow. The latter being a North American colloquialism referring to the killifishes. H. formosa inhabits the still waters of pools, and slow moving streams, in association with dense vegetation. The species is reported from localities with a pH of 4.5, but between 5 and 7.5 is probably better. This species feeds normally, when the temperature is between 11 and 32 degrees centigrade.

Some other livebearers may be obtained that will do fine at a pH lower than 7. Two species that may be found together in the same habitat, are the merry widow, Phallichthys amates, and the knife livebearer, Alfaro cultratus. Both species are naturally present where the pH may be as low as around 6.5. Both fishes inhabit slow-flowing streams, creeks, and other backwater to stagnant waters, around vegetation, and usually over a mud bottom. Knife livebearers have overlapping but different habitat requirements, than does the sympatric merry widow, as they are commonest in forest pools and small creeks, and also in river edge habitats.