Hans Hillewaert (CC BY-SA 4.0)
Cirolanid isopods
Family Cirolanidae (parasitic genera include Cirolana and Anilocra relatives)
Parasitic species swim out at night, latch onto sleeping fish and feed on blood and flesh; repeated attacks weaken and can kill fish one by one, and no reef-safe cure exists.
Identification
A pill-bug shaped crustacean, whitish to tan, about 5-20 mm in aquarium reports, with a segmented oval body roughly twice as long as wide, two pairs of antennae held close to the head, and disproportionately large, dark compound eyes — the big black eyes are the classic warning sign. By day it hides in sand or rock holes; at night it swims fast and may be found clamped onto a fish, or biting the keeper during rock work.
Often confused with
Harmless scavenging isopods and aegid isopods, which look very similar; even experienced keepers group them together until behaviour gives them away. Amphipods and copepods are far smaller and lack the large eyes and pill-bug build. An isopod found attached to a fish, or fish showing fresh bite wounds in the morning, settles the question in practice.
What it targets
Fish: the parasite ambushes resting fish at night, hooks on with its front claws and feeds on blood and flesh before dropping off before lights-on. Some species also scavenge, which lets them persist between fish meals. Keepers themselves get bitten during tank work.
How it spreads
Arrives in wild or maricultured live rock. Females carry young, populations establish quickly in rockwork with abundant hiding places, and they move to new tanks on shared rock.
Control (safest first)
Hand removal of every individual you can catch: night hunts with a red light and a turkey baster or net, plus tweezers removal of any isopod found attached to a fish (a brief freshwater dip made one drop off a tang immediately in a keeper report — do this only as a quick rescue for the fish in front of you, and watch for more). Tedious, and it is the core of every successful account. Bait-fish routine described by Shimek (2002), as quoted in forum discussions: remove all fish to quarantine except one easily caught, easily inspected fish, check that fish before lights-on each morning, strip any parasites with forceps, and repeat until a full month passes with none found. Run the display fishless for an extended fallow period: sources quoted in discussions suggest two to three months minimum, with the warning that scavenging species can survive four to six months without fish, so short fallows often fail. Bottle traps and stinky-water bait traps: easy to try and occasionally catch individuals, but keepers report fast, trap-wary isopods and empty traps are common — support for hand hunting rather than a solution. No reef-safe chemical treatment exists. A chitin inhibitor (Dimilin) is discussed on Reef2Reef for isopods still growing, but it threatens other crustaceans in the tank and is not a display-tank cure; do not dose a reef on forum advice alone.
Natural predators
No natural predators are listed.
Field notes
A Reef2Reef keeper who found one on a tang's fin caught the fish, gave it a freshwater dip, and watched the isopod drop off at once — then worried publicly about babies and more hiding in the rock, which is the standard sequel to a first catch. Another Reef2Reef keeper with maricultured rock tried bait traps and the stinky-water method without success, caught individuals by hand weeks apart, found one latched onto their own hand during a rescape, and reported a specimen surviving four days in a bowl of stagnant water while eating a bristleworm larva sharing the bowl. A Reef Sanctuary discussion weighed starting the new tank over against a three-month fishless fallow, with members quoting Shimek's guidance that cirolanids sit at the long end of fallow periods because many can scavenge between blood meals.