So what does it take to raise a Houston toad from an egg? A lot of water quality testing, algae paste, and some good ol’ fashioned TLC.
The Houston toad facility at the Houston Zoo has four sets of tadpole racks that are designed to raise eggs to tadpoles. The racks themselves are made up of four aquariums with a circulating water system that runs through a set of filters. Water quality is of upmost importance, and the water is tested for the presence of nitrogen waste products every day. Additionally, old food and debris are removed on a regular basis. The water in each rack is replaced as needed to keep it as clean as possible.
Unlike the carnivorous adults, Houston toad tadpoles are primarily vegetarians. In the wild they eat a variety of aquatic plant matter, as well as pine pollen that falls on the pond’s surface. Here at the zoo the tadpoles are fed algae wafers and an algae paste that is smeared on pieces of PVC (which sinks to the bottom where the tadpoles like to feed). Older tadpoles are fed pieces of sweet potato and bok choy for additional vitamins and minerals.
As the tadpoles grow, they start to show distinct physical characteristics that can be used to determine what stage of metamorphosis they are in. The characteristics have been broken down into specific developmental time points called Gosner stages. For example, when tadpoles start to develop a mouth they are in Gosner stage 23, Gosner stage 26 is when the hind limbs start to form, and Gosner 42 when the front limbs start to form.
We carefully monitor the tadpoles when they get close to Gosner 42. It is at this point that we collect the tadpoles from the tadpole tanks and transfer them into a separate tank with shallow water and moss. Not only are the tadpoles forming their forelimbs at this point, but they are also changing from using gills to lungs; therefore, it is very important that they have a surface that they can use to crawl up and out of the water!
We monitor the little toadlets until they completely absorb their tadpole tail. Did you know that the tail is the little toad’s first meal? Yummy! Once they absorb their tail they are carefully moved to another tank designed with shallow water and lots of moss that they can hop around on. In these tanks the toadlets are fed tiny insects called springtails. As they grow they are offered larger food items, such as baby crickets (called pinheads), fruit flies, bean beetles, and eventually crickets.
Currently, we have 6 tanks of tadpoles that we are caring for that will eventually make toads that will become members of our captive assurance colony (which means they will be parents in the future!) Our oldest tadpoles are just now growing out their back legs (Gossner 28). We’ll keep you posted on their progress!