Hippocampini
The Hippocampini are a tribe of small marine fishes in the subfamily Syngnathinae of the family Syngnathidae. Depending on the classification system used, it comprises either seahorses and pygmy pipehorses, or only seahorses.
Etymology
The subfamily Hippocampinae is named after the seahorse genus Hippocampus, which is derived from the Ancient Greek ἱππόκαμπος, a compound of ἵππος, "horse" and κάμπος, "sea monster". The morphologically intermediate nature of pygmy pipehorses is reflected in the name "pipehorse", a combination of the first syllable of "pipefish" and the second syllable of "seahorse". "Pygmy" is added to distinguish them from the larger pipehorses of the genus Solegnathus, which are distant relatives of the pygmy pipehorses. Other common names that have been applied to pygmy pipehorses include "bastard seahorse", "little pipehorse" and "pygmy pipedragon".Taxonomy
Genera
Based on Stiller et al :- Hippocampus Rafinesque, 1810
- Halicampus Kaup, 1856
Alternative taxonomy
- Acentronura Kaup, 1853
- Cylix Short & Trnski, 2021
- Filicampus Whitley, 1948
- Haliichthys Gray, 1859
- †Hippotropiscis Žalohar & Hitij, 2012
- Idiotropiscis Whitley, 1947
- Lissocampus Waite & Hale, 1921
- Trachyrhamphus Kaup, 1853
Systematics
In the past, due to the morphologically intermediate nature of the pygmy pipehorses between pipefishes and seahorses, the taxonomic placement of this group was contentious, and three different classifications were proposed for it:- Hippocampinae comprises both seahorses and pygmy pipehorses
- Hippocampinae includes only seahorses, pygmy pipehorses are placed into the pipefish subfamily Syngnathinae
- Hippocampinae includes only seahorses, pygmy pipehorses are placed into own subfamily
A high-resolution 2022 study sampling across all syngnathids refuted the former taxonomic treatment of Hippocampinae as its own subfamily, instead finding it to be deeply nested within the subfamily Syngnathinae, the tail-brooding pipefishes. Due to this, Hippocampinae is no longer recognized as a distinct subfamily by Eschmeyer's Catalog of Fishes. In addition, the pipefish genus Halicampus was found to be the closest relative of Hippocampus. These two genera were placed in the tribe Hippocampini, sister to the tribe Halliichthyini, containing the pygmy pipehorses. Brownstein et al found a similar taxonomy, but chose to retain the pygmy pipehorses within an expanded Hippocampini.
Evolution and fossil record
The morphology of pygmy pipehorses suggests that they are an evolutionary link between pipefishes and seahorses, and that seahorses are upright-swimming pygmy pipehorses. Molecular dating indicates that Hippocampus and Idiotropiscis diverged from a common ancestor during the Late Oligocene. During this time, tectonic events in the Indo-West Pacific resulted in the formation of shallow-water areas, which considerably changed marine habitats in this region. Particularly important was the establishment of vast seagrass meadows where there had previously been deeper water. This has led to speculation that the earliest seahorses managed to establish themselves as a new species because, unlike pygmy pipehorses, they were selectively favoured in such habitats. Not only can seahorses manoeuver exceptionally well in dense seagrass meadows, but the upright seagrass blades would have provided camouflage for their bodies and in that way improved their ability to ambush prey and avoid detection by predators. An alternative explanation for the evolution from pygmy pipehorse to seahorse is based on the finding that a vertically bent head is more efficient in capturing prey because it increases the animal's strike distance, which is considered particularly useful in tail-attached sit-and-wait predators. In that case, the evolution of an upright posture would merely be a means of maximising the angle between head and abdominal axis.There is as yet no fossil evidence for the evolution of seahorses from a pygmy pipehorse ancestor, as the fossil record of both groups is very sparse. The only pygmy pipehorse species of which fossils have been found lived in the Central Paratethys Sea during the Middle Miocene, i.e. during a time when seahorses had already evolved. In fact, the oldest known seahorse species, Hippocampus sarmaticus and H. slovenicus, were found at the same site. Independent geological confirmation of the genetic data would require finding a fossil site from the Oligocene in which seahorse-like pygmy pipehorses are present, but seahorses are not. Given the fact that Idiotropiscis is endemic to temperate Australia and the most basal seahorse lineages occur in Australia and the tropical West Pacific, these regions are the most likely candidates for such a site.