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The Syngnathidae is a family of fish which includes , , and seadragons ( Phycodurus and Phyllopteryx). The family name comes from Ancient Greek ( sún), meaning "together", and ( gnáthos), meaning "jaw", referring to the fused jaw that the entire family have in common.
Uniquely, after syngnathid females lay their eggs, the male then fertilizes and male pregnancy during incubation, using one of several methods. Male seahorses have a specialized ventral brood pouch to carry the embryos, male sea dragons attach the eggs to their tails, and male pipefish may do either, depending on their species. The most fundamental difference between the different lineages of the family Syngnathidae is the location of male brood pouch. The two locations are on the tail (Urophori) and on the abdomen (Gastrophori). There is also variation in Syngnathid pouch complexity with brood pouches ranging from simple ventral gluing areas to fully enclosed pouches. as In species with more developed, enclosed pouches it has been demonstrated that males directly provide their brood with not only nutrients but also immunity to pathogens. Syngnathids with more developed brood pouches are also known to be able to partially or completely abort a brood from a female with low fitness.
A wide variety of mate choice and mating competition has been observed in Syngnathidae. For example, hippocampus kuda exhibits conventional sex roles of males competing for female access while Corythoichthys haematopterus is completely sex role reversed. Most conventional sex role syngnathids are monogamous whereas sex role reversed species mostly exhibit polygamous behavior.
Seahorses and pipefish also have a unique feeding mechanism, known as elastic recoil feeding. Although the mechanism is not well understood, seahorses and pipefish appear to have the ability to store energy from contraction of their epaxial muscles (used in upward head rotation), which they then release, resulting in extremely fast head rotation to accelerate their mouths towards unsuspecting prey.
Syngnathidae was historically divided into two major lineages based on brood pouch location: Neophinae (located on the trunk) and Syngnathinae (located on the tail). Genome sequencing shows a parallel increase in brood pouch complexity in both Neophinae and Syngnathinae. Some species may have also independently evolved to have trunk brooding phenotypes, separate from the Neophinae. One example of this convergent evolution arises in ( Hippocampus bargibanti, Hippocampus denise, Hippocampus pontohi). Pygmy seahorses are very small (about 1–2 cm tall) trunk brooders, phylogenetically surrounded by tail brooders. It is likely that the pygmy seahorse once had their brood pouch on their tail. The brood pouch may have moved locations when there was strong a correlated selection for a prehensile tail and diminutive size, resulting in a very small, trunk brooding organism.
Viviparity and male-pregnancy in Syngnathidae have a complex evolutionary history with many independent origins of similar traits. Early members of the family developed traits to limit the presence of deleterious mutations, allowing for more rapid evolution. The advantage of a more controlled and protected embryonic development seemed to be enough to enact evolutionary development throughout Syngnathidae to varying degrees.
In species with the most complex brood pouch systems, many traits (behavioral, physiological, morphological, and immunological) must have co‑evolved to allow for male pregnancy, driven by the increase of the fitness of those individuals’ offspring. The evolution of these traits resulted in a sex-role reversal in which females may exhibit competitive behavior for a mate.
Recent research, especially whole-genome sequencing, has allowed for greatly improved understanding of the evolutionary history of Syngnathidae, but there is still a need for further development in the field. Further investigations into the genetic mechanisms and selective motivation for the evolution of these traits in Syngnathidae may provide insight into the evolution of pregnancy separate from the female reproductive system.
The earliest syngnathids are known from the Eocene of Monte Bolca, Italy.
Classification
In the past, this family was divided into two subfamilies: the elongated Syngnathinae, containing pipefish & seadragons, and the coiled Hippocampinae, containing seahorses and their presumed relatives. However, more recent genetic studies have found these divisions to be paraphyletic, with the seahorses being deeply nested within the pipefish, and the major division within the family being tail-brooding vs. trunk-brooding species. Due to this, the family has been re-circumscribed with its two major subfamilies being Nerophinae (trunk-brooders) and Syngnathinae (tail-brooders, including seahorses and seadragons). The split between these two families is thought to have occurred during the Paleocene.
Taxonomy
The following classification is based on Eschmeyer's Catalog of Fishes, with tribal divisions based on Stiller et al (2022):
Fossil taxa
Images
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