seatrench:

Pygmy Seahorses are tiny fish that have evolved to closely match their coral host’s appearance.  Adult Pygmy Seahorses usually measure less than 2cm in height.  Most species never leave their host coral,  feeding on minuscule plankton that drifts by,  but there are several free-swimming species.  Some dwarf species of seahorse are called “pygmy”,  but a true Pygmy Seahorse has a single gill opening and broods their young on the trunk.

(source)