The Hippocampinae are a subfamily of small marine fishes in the family Syngnathidae. Depending on the classification system used, it comprises either seahorses and pygmy pipehorses, or only seahorses. Genera Seahorses Hippocampus Rafinesque, 1810 Up to 54 species of seahorses Pygmy pipehorses Acentronura Kaup, 1853 Acentronura gracilissima (Temminck & Schlegel, 1850) (bastard seahorse) Acentronura tentaculata (Günther, 1870) (shortpouch pygmy pipehorse) Amphelikturus Parr, 1930 Amphelikturus dendriticus (T. Barbour, 1905) (pipehorse) Cylix Short & Trnski, 2021 Cylix tupareomanaia (Short & Trnski, 2021) (Manaia pygmy pipehorse) †Hippotropiscis Žalohar & Hitij, 2012 (known only from Miocene fossils) Hippotropiscis frenki Žalohar & Hitij, 2012 Idiotropiscis Whitley, 1947 Idiotropiscis australe (Waite & Hale, 1921) (southern little pipehorse) Idiotropiscis larsonae (C. E. Dawson, 1984) (Helen's pygmy pipehorse) Idiotropiscis lumnitzeri Kuiter, 2004 (Sydney's pygmy pipehorse) Description All seahorse and pygmy pipehorse species have a prehensile tail (a character shared with some other syngnathids), a fully enclosed brood pouch, a short head and snout angled ventrally from the abdominal axis, and no caudal fin. The species in the genera Acentronura, Amphelikturus and Kyonemichtys resemble pipefishes, which explains why pygmy pipehorses are sometimes grouped in the pipefish subfamily Syngnathinae. The species of Idiotropiscis are more seahorse-like in appearance in having a deeper body and discontinuous superior trunk and tail ridges. The main differences between this pygmy pipehorse genus and the seahorses is that the latter have an upright posture, and the angle of their head relative to the abdominal axis is greater. Etymology The subfamily Hippocampinae is named after the seahorse genus Hippocampus, which is derived from the Ancient Greek ἱππόκαμπος (hippokampos), 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". Systematics Due to the morphologically intermediate nature of the pygmy pipehorses between pipefishes and seahorses, the taxonomic placement of this group remains contentious, and three different classifications have been proposed for the subfamily Hippocampinae. No well-resolved phylogeny exists, making it impossible to settle this issue at the present time. Hippocampinae comprises both seahorses and pygmy pipehorses Phylogenetic analyses based on five nuclear