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Atherinids

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The Atherinids, sometimes commonly referred to as Silversides, include the very popular hobby rainbowfishes. 

Atherinid is the common name derived from the scientific name for the Order Atheriniformes, which name comes from atherina, the Greek name for the eperlane [the European smelt fish] and the Latin forma [= shape]. Species occur worldwide in tropical and temperate marine and freshwater environments. They are typically small fish, with the largest being the jack smelt, with a head-body length of 44 centimetres (17 in). Members of the order usually have two dorsal fins, the first with flexible spines, and an anal fin with one spine at the front. The lateral line is typically weak or absent. 

Classification of the Atheriniformes has been uncertain. Nelson (2006) established what had previously been the suborder Melanotaenioidei as a distinct family, Melanotaeniidae, which includes the rainbowfishes familiar to hobbyists. This classification results in the family being monophyletic, which means the species form a clade that contains all the descendants of the closest common ancestor of the members of the group. This is now [at the time of writing] the accepted classification. 

The fish in Melanotaeniidae are characterized by having relatively large eyes and a terminal mouth. They are generally elongate in the body, and possess two separated dorsal fins, with the first consisting of flexible spines and the second having one spine followed by soft rays [Allen et.al. 1998]. The anal fin has one spine on the leading edge followed by soft rays. The pectoral fins tend to be high, and there is no lateral line. On the flanks there is a broad silvery band that is usually reflective; the scales are relatively large. The fish feed mainly on zooplankton. When spawning, they are egg scatterers, most using plants.

 Members of this family have been dated back to the lower Eocene epoch (approximately 54-34 million years ago) of the Tertiary period. All present-day species occur in New Guinea and a few of the surrounding islands and in northern and eastern Australia. There are 71 identified species in 7 genera, with several unidentified species believed to exist [Fishbase]. The genus Melanotaenia alone contains some 51 species of rainbowfish, ranging in size from less than 6 cm (2.4 inches) to the largest, Melanotaenia vanheurni, at close to 8 inches (20 cm). 

All are shoaling fish by nature, and will do best in groups in an appropriately-sized aquarium. The brighter colouration of the males will be enhanced when kept with conspecifics, and regular displays will be observed. These fish are always found in waters with thick vegetation, and the aquarium should be well planted along the rear and sides with adequate space for swimming in front; most species are active swimmers, but exceptions will be noted in the species profile. The eggs of all species are laid attached to aquatic vegetation, and hatch after 7 to 18 days depending upon species. None practice parental care of the eggs or fry, though most are not avid egg eaters as are the cyprinids by contrast. 

All species feed on insect larvae, small crustaceans, and algae--though not sufficiently to be considered an "algae eater" in the aquarium. A good diet, including some frozen and/or live foods, will also improve colouration. Aquarists must be careful not to overfeed, as these fish will greedily devour food if it is offered. 

Most species prefer basic moderately hard water, similar to livebearers; a few require soft, slightly acidic water, and this will be mentioned in the species profile. Water quality for all species is very important. These fish cannot tolerate poor maintenance, high nitrates, or fluctuating parameters. Live plants will assist in maintaining stable water conditions, along with regular partial water changes.

References: 

Allen, Gerald R., J.R. Paxton & W.N. Eschmeyer, ed. (1998), Encyclopaedia of Fishes, San Diego: Academic Press, pp. 153-156. 

Nelson, Joseph S. (2006). Fishes of the World, John Wiley & Sons, Inc.
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