Atheriniformes, also known as the silversides, is an order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of ray-finned fish that includes the Old World silversides and several less-familiar families, including the unusual Phallostethidae

Phallostethidae

The Priapium fishes are a family, Phallostethidae, of atheriniform fish native to South-east Asia.They are small fish, no more than in length, with partially translucent bodies. They are found in fresh and brackish water from Thailand to the Philippines. They are named for a muscular organ found...

. They are found worldwide in tropical and temperate marine and freshwater environments.

Description

Atheriniforms are generally elongate and silvery in colour, although exceptions do exist. They are typically small fish, with the largest being the jacksmelt, with a head-body length of 44 centimetres (17.3 in), and the smallest species, such as the Bangkok minnow, being only 2 centimetre (0.78740157480315 in) in adult length.

Members of the order usually have two dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

s, the first with flexible spines, and an anal fin with one spine at the front. The lateral line

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 is typically weak or absent. Atheriniform larvae share several characteristics; the gut is unusually short, there is a single row of melanophores along the back, and the fin rays do not become evident until some time after hatching. They scatter their eggs widely, with most species attaching them to aquatic plants.

Taxonomy

Classification of the atheriniforms is uncertain, with the best evidence for monophyly

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 in the larval characteristics mentioned below. Their closest relatives are thought to be the Caprinodontiformes.

Following Nelson (2006), the family Melanotaeniidae includes the subfamilies Bedotiinae, Melanotaeniinae, Pseudomugilinae, and Telmatherininae, to demonstrate their monophyly

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

. However, in a 2004 study, a different classification scheme classifies the families Bedotiidae, Melanotaeniidae, and Pseudomugilidae (also include Telmatherinine genera) in a suborder Melanotaenioidei. Thus, the number of families in Atheriniformes varies from author to author.

Classification under Nelson, 2006:

Order Atheriniformes

• Suborder Atherinopsoidei
  • Family Atherinopsidae (Neotropical silversides)
    • Subfamily Atherinopsinae
      • Tribe Atherinopsini
      • Tribe Sorgentinini
    • Subfamily Menidiinae
      • Tribe Menidiini
      • Tribe Membradini
• Suborder Atherinoidei
  • Infraorder Notocheiroida
    • Family Notocheiridae
      Notocheiridae
      The surf silversides are a small family, Notocheiridae, of salt water atheriniform fish found in tropical and temperate seas.They are small fish, typically less than in length, with silver stripes and a keel-shaped underside...
      
       (surf silversides)
  • Infraorder Atherinoida
    • Family Melanotaeniidae
      • Subfamily Bedotiinae
        Bedotiidae
        Bedotiidae is a family of fish known as the Madagascar rainbowfish, Madagascan rainbowfish, or Malagasy rainbowfish due to their endemism to Madagascar. It includes two genera, Bedotia and Rheocles.-Anatomy and morphology:...
        
         (Madagascar rainbowfishes)
      • Subfamily Melanotaeniinae (rainbowfishes)
      • Subfamily Pseudomugilinae
        Pseudomugilidae
        The Blue-eyes are the family of atheriniform fish, the Pseudomugilidae, related to the rainbowfishes. They inhabit fresh and brackish water in Australia and New Guinea. Blue-eyes are small fish, typically no more than in length...
        
         (blue-eyes)
      • Subfamily Telmatherininae
        Telmatherinidae
        The Sail-fin silversides are a family of atheriniform fish, the Telmatherinidae, inhabiting fresh and brackish water . All but one species are restricted to the island of Sulawesi, and most are found solely in the Malili Lakes of that island....
        
         (sail-fin silversides)
    • Family Atherionidae (silversides)
    • Family Phallostethidae
      Phallostethidae
      The Priapium fishes are a family, Phallostethidae, of atheriniform fish native to South-east Asia.They are small fish, no more than in length, with partially translucent bodies. They are found in fresh and brackish water from Thailand to the Philippines. They are named for a muscular organ found...
      
       (priapiumfishes)
    • Family Dentatherinidae (Mercer's Tusked Silverside)
    • Family Atherinidae (Old World silversides)
      • Subfamily Atherinomorinae
      • Subfamily Craterocephalinae
      • Subfamily Atherininae