Although freshwater invertebrates play a significant ecological role in their natural habitat, they are not popular with aquarists, and this is a great shame. They are represented by two main groups: the mollusks and the crustaceans.
• Ampullaria sp., like this golden variety, are one of the most effective gastropodsagainst algae.
Bivalves commonly known as freshwater mussels can be found in some tropical waters. Although they differ as regards their anatomy, they are in fact distant cousins of the mussels on our coasts. They live buried, or partially buried, in the sediment. They eat by filtering micro-algae or vegetal plankton dissolved in the water. This can be a useful attribute in an aquarium, as it helps contain the proliferation of this type of algae. On the other hand, if there is not enough food, they will eventually die - which is not always noticeable at first, and therefore entails a risk of pollution from their dead bodies lingering in the water. Bivalves therefore have a somewhat limited appeal to fish-keepers, especially as few tropical species are available in the aquarium trade. As regards the gastropods, some species are considered pests -physas, for instance -while others, such as the Planorbidae, Ampullaria, and Malaysian snails, play a positive ecological role, above all by feeding on unwanted algae. In good conditions, they proliferate rapidly; they can be removed by hand or by trapping them with a leaf of lettuce or boiled spinach: if you put one of these in a tank at night, the next morning it will be covered in snails. If a population of gastropods disappears from an aquarium, this is probably a sign of imbalance.
In freshwater tanks, these are mainly represented by several species of shrimps, although it is occasionally possible to find a tropical crawfish, and even small crabs. As crustaceans' carapaces mainly consist of calcium carbonate (CaCO3), it is advisable to keep them in hard water, and some crabs can even live in brackish water. Carnivorous fish are obviously out of the question as roommates. Small species of fish are not normally at risk from crustaceans, but a weakened or sick fish may be captured and devoured. Crustaceans' role in the wild as environmental regulators can be reproduced in captivity.
Whatever species you may choose, all crustaceans feel more at home if they are pro-
Shrimps from the Macrobrachium genus are becoming increasingly common in freshwater aquariums.
vided with shelters, especially during the molt. If they are fed properly, this process will occur regularly, but the animal is vulnerable while it is forming a new carapace. It is during this critical period in their life-cycle that crustaceans usually attack each other.
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