Bellapiscis medius, the twister, is a triplefin fish of the family Tripterygiidae, commonly found around the coast of New Zealand.

[3] This species is the only triplefin that as an adult lives in intertidal zone habitat.

[4] It can be observed in tidal pools and has adapted to survive the changes in temperature, oxygen saturation and levels of water acidity that accompany living in that habitat.

[4] The abundance of this fish in tidal pools decreases during the winter months.

[5] B. medius feeds on small crustaceans including barnacles, molluscs including snails, and amphipods.