Hydrichthys sarcotretis is a species of colonial marine hydrozoans now included in the family Pandeidae.

H. sarcotretis is a species that exhibits hyperparasitism by attaching itself to a copepod, itself the parasite of a fish.

In H. sarcotretis, parasitism is taken a stage further when the hydrozoan attaches itself to the copepod Cardiodectes medusaeus.

This is itself an ectoparasite of the Northern lampfish (Stenobrachius leucopsarus) in the family Myctophidae.

Both male and female fish do not reproduce and seem to grow faster when attacked by the copepod and it seems to have a negligible energy demand from them.