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Damselfish
There are many different damselfishes, but very few enter brackish or freshwater. They are primarily fishes of tropical marine waters around bays and reefs. Damselfish are very closely related to cichlids, and share many of their vices and virtues. They are hardy, active fish which can be easily tamed and adapt well to captive life taking most types of food readily. They are also excellent dither fish, helping nervous fish like monos to settle in. On the other hand they can be territorial and waspish in nature, and so might not work so well with shy or slow species, such as pipefish and small gobies.
A few marine damsels do seem to adjust to low salinity marine aquaria well, for example humbugs and sergeant majors. They are routinely kept around 1.018 by marine aquarists. Abudefduf saxatilis occurs in many different coastal waters and can adapt well to strongly brackish waters, but none of these marine damsels should be kept in water of specific gravity below 1.015. According to Damselfishes of the World by G R Allen (Mergus: Melle, Germany) there are three damselfishes that live in coastal freshwater streams and estuaries. These are Stegastes otophorus, Pomacentrus taeniometopon and Neopomacentrus taeniurus. A fourth genus Dischistodus is restricted to silty, coastal waters.
True brackish water and freshwater damsels are only rarely seen but for the brackish water aquarist they are a worthwhile challenge. For one thing, many damsels will breed in captivity in much the same way as cichlids, and any of these species is certainly worth a crack at. As with cichlids, many damsels are territorial, although the degree of aggressiveness varies. I would mix these fish with small open-water fish like livebearers or rainbows rather than with dwarf cichlids. Allen suggests Stegastes (as a genus) is best kept alone or in pairs; Neopomacentrus as small groups; and Pomacentrus alone or in pairs. This applies to the fishes behaviour toward other damselfish. Different looking fishes tend to be ignored, but cichlids might be too similar in shape and behaviour and might end up fighting with these damsels, so should be combined with care.
Stegastes otophorus
The freshwater gregory Stegastes otophorus is from the brackish and marine waters along the Caribbean and Panamanian coast. The salinity range is quoted as 3 to 31.5 ppt (i.e, about one-tenth to full strength marine). The photographs show a pretty fish, with a rounded shape rather like a sergeant major. The overall colour is initially a grey-brown, seemingly getting a deeper chocolate with age. The dorsal, anal and caudal fins have very attractive bright yellow edges. The juvenile seems to have a row of small blue spots along the back, and the yellow markings are not so bold. Maximum size is quoted at about 10 cm. In all, these seems a very desirable fish.
Pomacentrus taeniometopon
Pomacentrus taeniometopon is widespread throughout the tropical Indo-West Pacific region. It is said to occur in freshwater (occasionally), brackish lagoons, and shallow marine reefs with freshwater run-off. The juvenile is pretty, with a orange base to the caudal fin, a black and white eye-spot at the end of the dorsal, and some blue markings along the head and back. The lower parts are pale pink, darkening along the sides to a grey-blue. The adult is blue-black all over, with no other markings save some blue scribbles along the forehead. Rather like Neoglyphidiodon (Abudefduf) oxyodon. It may sometimes even be imported under that name. It differs from Neoglyphidiodon (Abudefduf) oxyodon (which is known as the black neon damsel) in not having the white, saddle-like band. Slightly smaller than the Stegastes otophorus.
Neopomacentrus taeniurus
The freshwater demoiselle Neopomacentrus taeniurus is found from East Africa to Australia. Appears to be an inhabitant of brackish rather than pure freshwater. I have seen this fish kept in a freshwater tank in a tropical fish store, and according to the dealer, it had been there for some months. The fish looked healthy enough but was very skittish. Possibly its nervous behaviour was related to the fact that it was the only specimen in the tank (with angels and Corydoras); but it may also have needed a ‘taste’ of salt to be really happy. In appearance similar to Stegastes otophorus but with more tapering fins and a more like a midwater blue chromis than a typical benthic damsel like a domino damsel. The edges of the caudal fins are also dark in colour. Salt-water Aquarium Fishes by Axelrod and Vorderwinkler (TFH: Jersey City, USA) includes this species as Pomacentrus taeniurus, an obsolete name. It gives a description largely matching Allen’s above, but adds that the upper half is purple and the lower half is olive. This would appear to be a description of a juvenile, changing colours when mature like Pomacentrus taeniometopon and many other damsels. Apparently breeds in fresh and brackish waters.