some type of adult girl

biomedicalephemera:

Top left: Hippocampus sp. internal structure
Top right: Short-snouted seahorse - Hippocampus hippocampus
Center: 1. Syngnathus hippocampus [now Hippocampus hippocampus]
2. Pegasus draconis [now Eurypegasus draconis] - the Little Dragonfish (*unrelated to Syngnathidae family*)
3. Syngnathus pelagicus - the Sargassum pipefish
Bottom: Phyllopteryx taeniolatus -the Weedy Sea Dragon

Despite their remarkable appearance, seahorses are true ray-finned bony fishes (class Actinopterygii, infraclass Teleostei), along with bass, mullets, eels, salmon, and lanternfish.

Many people know of the male seahorse incubating the eggs and giving “birth” to 100-1000 offspring after they hatch, but reproduction is similar throughout the order Syngnathidae (including the seahorses, leafy and weedy sea dragons, and pipefish). There’s a persistent myth that seahorses are monogamous, but that’s not strictly true. The majority of species are serially monogamous, and remain together throughout the mating season (until the male births the babies).

Another remarkable thing about seahorses (Hippocampus spp.) is that they’re the only fish with prehensile tails - even their close relatives, the sea dragons and pipefish, don’t have this adaptation. However, since the seahorses are the only ones that swim upright, and they have the poorest locomotive skills, they need to be able to anchor themselves to the sea flora in order to not be swept away. The Guinness Book of World Records has named Hippocampus zosterae, the dwarf seahorse, the slowest fish in the world, moving less than 5 ft [150 cm] an hour.

Aside from the seahorses, the razorfish (Aeoliscus strigatus) is the only other fish to swim “upright”.

Images:
Bulletin of the United States Fish Commission, Vol 1. 1881.
Arcana; or, The Museum of Natural History. George Perry, 1811.