A tour through the aquarium: The Potosi Pupfish

As you enter the Aquarium at Bristol the first tank you meet is a display tank containing a few rather pretty little blue fish. These are male Potosi pupfish, a species of killifish sadly extinct in the wild. Originating in Mexico, from El Potosi in Nuevo Leon, they are examples of a group of species found all over the south west of North America, from Nevada down into Mexico, which are relicts of a very different environment to that experienced today. The group we have at Bristol were originally from London Zoo, and we and London are the only zoos on ISIS to have these species, with a total population of around 400 fish, although there are a few private breeders in Spain, Mexico and the US who also keep them.

During glacial periods, the interior of North America experienced much higher rainfall, and melting ice from further north also raised water levels. As a result, vast wetlands developed in areas which are now grassland or desert, and the common ancestor of the Potosi pupfish (and its relatives) was widespread. With the end of the last glaciation water levels dropped, and where small ponds survived (even in Death Valley), so did the pupfish. Isolation is a potent generator of new species, and today every pool will have its own unique local variety.

Unfortunately, what 10,000 years of climate change has done can easily be destroyed by modern agriculture. Water abstraction across the south west has lowered water tables, drying out the ponds, and fertilizer and pesticide pollution has affected water quality. Pupfish are pretty tough, surviving where no other species can, but several forms have already become extinct, and a species that only lives in a pond 10 metres across is perpetually vulnerable to any passing accident in any event.

Many species of pupfish have already been bred in aquaria, and they are for the most part quite easy. They mostly like quite alkaline, even saline, water, and are adapted to the extreme temperature changes in their shallow desert pools. In the winter months the fish are cooled down to around 150, and in the spring when they are warmed up the males will encourage the females to spawn on the mats of algae that cover the floors of the pools. The eggs are quite large and tough, as is standard with the killifish, and are at least partially drought resistant, which has enabled them on occasion to be transported between ponds on the feet of waterfowl. When the eggs hatch they feed on mosquito larvae and other small invertebrates, along with a fair amount of algae as well. In an aquarium it is best to have only a single male in a breeding tank, as they are very territorial.

A small fish which is straightforward to breed would not seem to present many problems for a conservation breeding programme, but there is a particular problem with many rapidly breeding, short lived species, which is that they are very plastic in their physical form (the phenotype) depending on their environment, and genetic changes can happen very quickly as well. The result is that after a few generations in a captive environment, with a different diet, different water composition, different temperatures and temperature ranges, amongst other variables, you may well wind up with a species that no longer resembles its ancestor in significant ways. With more longer-lived species the process takes longer, but it still occurs. The challenge for the forthcoming century is not merely how you preserve a species in captivity, but how you ensure that the multitude of adaptations, not merely in physical appearance, but in physiology, reproductive ecology, psychology and so forth are also conserved.

In the case of the pupfish, those species in the US have often been maintained in outdoor concrete ponds close to their original springs. This can be quite successful, but there is an inevitable change in the environment all the same, and the rescued species may still appear at least slightly different from their ancestors after a few years. I believe that consideration of details of the habitat, diet, and even risk of predators is going to be the next major step forward in captive husbandry in zoos, especially for smaller vertebrates and invertebrates where the risk of ‘unnatural selection’ is probably most severe. I would be interested in what the readers think.


Next week: livebearers.