These fishes have generally elongate, narrow bodies surrounded by a series of bony rings, with small, tubular mouths. The shape of their mouth—at least, in syngnathids—allows for the ingestion of prey at close range via suction.[3] Many species of Syngnathiformes also employ strategic camouflage (such as cryptic coloration and overall physical form) to hunt successfully and gain closer access to prey, as well as to protect themselves from larger predators. Several groups, for example, live among seaweed, not only swimming with their bodies aligned vertically (to blend in with the floating plant matter) but have also developed physical features that mimic the seaweed. The pygmy seahorses are among the smallest of all syngnathids, with most being so tiny—and mimicking the specific coral they spend their lives on—that they were only recently described by scientists.
The most defining characteristic of Syngnathiformes is their reproductive and sexual system, in which syngnathid males become "pregnant" and carry the embryonic fry. The males house the fertilized eggs in an osmo-regulated brood pouch, or (in some species) adhere them to their tail, until the eggs reach maturity.
The name Syngnathiformes means "conjoined-jaws". It is derived from Ancient Greeksyn (συν, "together") + gnathos (γνάθος, "jaw"). The ending for 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]
Systematics and taxonomy
In some models, these fishes are placed as the suborderSyngnathoidei 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]
^ abcKawahara Ryouka, Miya Masaki, Mabuchi Kohji, Lavoue Sébastien, Inoue Jun G., Satoh Takashi P., Kawaguchi Akira, Nishida Mutsumi (2008). "Interrelationships of the 11 gasterosteiform families (sticklebacks, pipefishes, and their relatives): A new perspective based on mitogenome sequences from 75 higher teleosts". Mol. Phylogenet. Evol.46 (1): 224–236. doi:10.1016/j.ympev.2007.07.009. PMID17709262.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Pietsch, T.W. (1978): Evolutionary relationships of the sea moths (Teleostei: Pegasidae) with a classification of gasterosteiform families. Copeia1978(3): 517–529. JPEG abstract and first page text
McAllister D.E. (1968). "Evolution of branchiostegals and classification of teleostome fishes". Bulletin of the National Museum of Canada, Ottawa. 221: 1–239.
