Some species cope well with conditions where there are large variations in salinity. In the case of others, juveniles start their lives in fresh water before descending to estuaries, where they complete their development; once grown to adulthood, they live permanently in the sea. It is vital to keep such fish in special aquariums where the salinity is carefully adapted to suit their metabolisms. You will find several species available on the market; some possess really remarkable features and are regarded by aquarists as curiosities. However, few hobbyists think of actually keeping a brackish water aquarium, though this can be just as exciting as a freshwater or marine tank. Some of the species described below will coexist quite happily in the controlled environment of an aquarium.
Monodactylus argenteus (Monodactylidae)
The mono begins its life in fresh water, with the juveniles migrating to brackish regions as they mature. The adults are true sea-dwellers. This species requires a large tank with enormous amounts of space for group swimming. A timid species, it prefers live prey or fresh food. A near relative, M. sebae (the fingerfish), is partially herbivorous. When in poor health, or when the water quality is unsuitable, the coloration of both species darkens; normally, the silvery background predominates. Size: 15-18 cm.
Brachygobius xanthozona (Gobiidae)
The bumblebee fish comes from the coastal regions of South-East Asia. Its pelvic fins are modified into a sucker, which allows the fish to attach itself to a support and resist the current. It feeds on small, live prey and rarely breeds in captivity. Size: 5 cm. •
Periophthalmus sp. (Gobiidae)
The genus Periopthalmus contains some very bizarre fish which can leave the water and move around using their pectoral fins. They are found in all the world's tropical regions, except America, living in the sandy areas of estuaries. Caring for them in captivity requires an aquaterrarium rather than a simple aquarium; they need a bank of sand, with a gentle slope allowing them to emerge from the water, which should not exceed 15 cm or so in depth. Captive breeding is very rare. The two species most commonly available commercially are P. barbarus (blotched mudskipper) and P. papilio (butterfly mudskipper). Size: 10-12 cm.
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