| Syngnathiformes Temporal range:
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|---|---|
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| Trumpetfish (Aulostomus maculatus: Aulostomidae), head in natural pose | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| (unranked): | Acanthomorpha |
| Superorder: | Acanthopterygii |
| Clade: | Percomorpha |
| Order: | Syngnathiformes |
| Type species | |
| Syngnathus acus | |
| Families | |
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See text | |
The Syngnathiformes /ˈsɪŋ(ɡ)nəθɪfɔːrmiːz/ are an order of ray-finned fishes that includes the sea moths, trumpetfishes and seahorses.[2]
These fishes have elongated, narrow, bodies surrounded by a series of bony rings, and small, tubular mouths. The shape of their mouth—in at least syngnathids—allows for the ingestion of prey at close range via suction.[3] Several groups of Syngnathiformes live among seaweed and swim with their bodies aligned vertically, to blend in with the stems. The most defining characteristic of this order is their reverse sexual system. In this order, males conduct in specialized brooding and rearing of the embryos. The males house eggs in an osmoregulated pouch or adhere eggs to their tail until the eggs reach maturity.
The name Syngnathiformes means "conjoined-jaws". It is derived from Ancient Greek syn (συν, "together") + gnathos (γνάθος, "jaw"). The ending for fish orders "-formes" is derived from Latin and indicates "of similar form".
The earliest known syngnathiform is Gasteroramphosus from the late Cretaceous (either Santonian or Campanian) of Italy, which is similar in form to Marcroramphosus but which has some characters which are suggestive of a relation to Gasterosteoidei.[4] However, most recent studies have reaffirmed it being a syngnathiform.[1][5] The second oldest syngnathiform is the syngnathoid Eekaulostomus from the early Paleocene (Danian) of Mexico.[1] Many fossil syngnathiform families are known from the Paleogene.[6]
In some models, these fishes are placed as the suborder Syngnathoidei of the order Gasterosteiformes together with the sticklebacks and their relatives.[7] Better supported by the evidence now available is the traditional belief[8] that they are better considered separate orders, and indeed among the Acanthopterygii, they might not be particularly close relatives at all.[9]
In addition, the Pegasidae (dragonfishes and sea moths) are variously placed with the pipefish or the stickleback lineage. While the placement in Syngnathiformes seems to be correct for the latter, the former is possibly an actinopterygian order of its own.[9] Following the convention of the major fish classification organizations (Fish Base, ITIS, Encyclopedia of Life), the Indostomidae are currently placed in the Gasterosteiformes.
Morphological traits uniting the flying gurnards (Dactylopteridae) and the Syngnathiformes have long been noted.[10] Most authors, however, placed them with the Scorpaeniformes. However, DNA sequence data quite consistently support the belief that the latter are paraphyletic with the Gasterosteiformes sensu lato. As it seems, flying gurnards are particularly close to Aulostomidae and Fistulariidae, and probably should be included with these.[9]
The order as set out in the 5th Edition of Fishes of the World is classified as follows:[11]
Other authorities are of the view that without the inclusion of other taxa within Syngnathiformes then the order is paraphyletic. This wider order consists of a "long snouted" clade and a benthic clade and this classification is:[12][13]
In their study Longo et al (2017) found that there were short distances between the groupings on the Syngnathiform phylogenetic tree and this supported a hypothesis that there had been a rapid but ancient radiation in the basal Syngnathiformes.[13]
The following fossil families are known:[6]
