The Japanese seahorse reaches a maximum length of 8.0 cm, is usually dark brown and has a relatively long tail, a ridgelike coronet and flattened spines.

H. mohnikei has a planktonic stage in the first six months of life, after which they settle, preferably in Zostera sea grass beds or estuaries, and begin to breed.

[6] After the planktonic stage, they prefer to settle in Zostera sea grass beds and estuaries.

[7] H. mohnikei reaches a maximum length of 8.0 cm and is usually dark brown all over, but may be mottled.

[9] H. mohnikei have completely developed dorsal, anal, and pectoral fins even as juveniles of 6.0 mm long.

Juveniles 6.0–26.4 mm total length also have a tiny caudal fin with just two rays.