Andrew Cannizzaro (CC BY 4.0)
Amphipods and Copepods
Order Amphipoda and subclass Copepoda (gammaridean amphipods in tanks)
Beneficial micro-crustaceans that eat detritus and algae and serve as live food for fish and corals; they never parasitise fish.
Identification
Amphipods: 5–10 mm shrimp-like animals, grey to tan, with a body curled into a comma or C shape and flattened from side to side, scurrying over rock, sand, and macroalgae, mostly at night. Copepods: far smaller at 1–3 mm, seen as tiny white specks on glass and rock that move in short jerky hops. Both swarm happily in refugiums and mature rockwork.
Often confused with
Cirolanid isopods, the fish-parasitic look-alike that latches onto fish at night and feeds on blood. The curled-versus-flat tell separates them: an amphipod rests curled on its side in a C, while a cirolanid is a flat, straight, oval pill-bug shape flattened top-to-bottom, with disproportionately large black eyes on top of the head and a fan-like tail. Copepods are an order of magnitude too small to be cirolanids. If the bug curls, it belongs to the cleanup crew.
Natural predators
No natural predators are listed.
Field notes
The base of the reef-tank food web: pods convert detritus and film algae into live food that mandarins, wrasses, and corals hunt all day. A glass covered in pods usually means the tank is maturing well, not infested.