Hippocampus sindonis (known in English as Dhiho’s or Shiho’s Seahorse) is a species of seahorse known only from the waters around Japan. The maximum recorded adult height is 8 cm. Total head length/snout length ratio is around 3.0 (2.8 to 3.3). The body includes 10 trunk rings and usually 37 tail rings (36 to 38). The dorsal fin is supported by 2 trunk rings and 1 tail ring. There are typically 12 dorsal fin rays (11 to 15) and 12 pectoral fin rays (12 to 14). There is a single prominent round-tipped cheek spine and a prominent double eye spine (front spine shorter than back one). (Lourie et al. 2004)

Seahorses of this species have often been misidentified as H. coronatus (which, among other differences, has more tail rings) and H. mohnikei (which, among other differences, has 11 trunk rings, more tail rings, and double [low] cheek spines) (Lourie et al. 2004).

Although little is known about the biology of this particular species, for seahorses in general, it is the male, rather than the female, that becomes pregnant. The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the male’s pouch. The pouch acts like the womb of a female mammal, complete with a placental fluid that bathes the eggs, and provides nutrients and oxygen to the developing embryos while removing waste products. See Project Seahorse for more information about the fascinating biology of seahorses.